autobiographical memory research studies

Highly Superior Autobiographical Memory (HSAM): A Systematic Review

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• Published: 23 February 2024

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• Jessica Talbot   ORCID: orcid.org/0009-0009-1606-0204 1 ,
• Gianmarco Convertino 1 ,
• Matteo De Marco 2 ,
• Annalena Venneri 2 , 3 &
• Giuliana Mazzoni 1 , 4  

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Individuals possessing a Highly Superior Autobiographical Memory (HSAM) demonstrate an exceptional ability to recall their own past, excelling most when dates from their lifetime are used as retrieval cues. Fully understanding how neurocognitive mechanisms support exceptional memory could lead to benefits in areas of healthcare in which memory plays a central role and in legal fields reliant on witnesses’ memories. Predominantly due to the rareness of the phenomenon, existing HSAM literature is highly heterogenous in its methodologies used. Therefore, following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, we performed the first systematic review on this topic, to collate the existing behavioural, neuroanatomical, and functional HSAM data. Results from the 20 experimental selected studies revealed that HSAM is categorised by rapidly retrieved, detailed and accurate autobiographical memories, and appears to avoid the normal aging process. Functional neuroimaging studies showed HSAM retrieval seems characterised by an intense overactivation of the usual autobiographical memory network, including posterior visual areas (e.g., the precuneus). Structural neuroanatomical differences do not appear to characterise HSAM, but altered hippocampal resting-state connectivity was commonly observed. We discuss theories of HSAM in relation to autobiographical encoding, consolidation, and retrieval, and suggest future directions for this research.

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Introduction

Highly Superior Autobiographical Memory (HSAM) is a rare form of exceptional memory characterised by an enhanced ability to remember autobiographical content (LePort et al., 2012 ; Patihis et al., 2013 ). Internal or external cues, including dates from one’s life span (e.g., 1st January 1999) can elicit HSAM individuals to access specific memories from nearly every day of their past (Gibson et al., 2022 ; Parker et al., 2006 ). The skill also involves a remarkable ability to locate memories temporally; participants can accurately and confidently report exact time-related details (e.g., day of the week) of events of their own life and public events of which they have a personal recall (Ford et al., 2022 ; Parker et al., 2006 ). HSAM is exclusive to autobiographical memory (ABM), and retrieval is accurate (Ally et al., 2013 ) and extensively detailed (LePort et al., 2016 ).

Parker et al. ( 2006 ) reported the first case of a woman, given the pseudonym “AJ”, with near perfect ABM, though as far back at the nineteenth century an individual was described possessing similar memory traits (Henkle, 1871 ). The seminal 2006 study coined the term “hyperthymesia”, referring to the Greek word for remembering (thymesis). At 34 years old, AJ wrote to researchers in California describing a “non-stop, uncontrollable and totally exhausting” ability to remember. When researchers invited her to the laboratory she excelled at numerous standardised and ad hoc ABM tasks, effortlessly providing clear and verifiable memories in response to dates. Since AJ, almost one hundred more individuals have been identified possessing a hyper memory, and the term has been redefined to its more commonly used label of HSAM which reflects its specificity in memory type (Patihis, 2015 ).

Unlike other forms of exceptional memory, such as Memory Athletes (Dresler et al., 2017 ), those with HSAM do not utilise deliberate mnemonic techniques (e.g., method of loci) to support encoding or retrieval of information (LePort et al., 2012 ; Santangelo et al., 2021 ). Instead, memories are described as entering one’s mind in an automatic way (Mazzoni et al., 2019 ), and are retained regardless of perceived importance or emotional saliency (Santangelo et al., 2018 ). The enhanced ability typically manifests during late childhood (De Marco et al., 2021 ), though for some individuals their ability to remember in excess reportedly begins at 5 years old (Patihis, 2015 ). The seemingly spontaneous and heightened nature of HSAM makes it a particularly fascinating cognitive phenomenon.

For decades, scientists have investigated the complexity of human memory, but the exact mechanisms of different subtypes are not yet fully understood (Santangelo et al., 2022 ). HSAM provides a unique angle to explore ABM with potential applications benefitting health and legal contexts. Memory typically involves vast amounts of unintentional forgetting (Maxcey et al., 2019 ) and is susceptible to age-related cognitive decline (Wright et al., 2021 ), neurodegeneration, or clinical abnormalities, including mild cognitive impairment or Alzheimer’s Disease (Venneri et al., 2011 ). Extreme memory impairments negatively impact longevity (Rhodius-Meester et al., 2018 ), quality of life (Burks et al., 2021 ), and increase financial burden on healthcare services (Dauphinot et al., 2022 ). Similarly, misremembering, false memories, or forgetfulness can implicate settings reliant on personal testimonies throughout the justice process (Conway, 2012 ). Ultimately, by ascertaining neural processes responsible for near-perfect memory, strategies could be implemented to improve normal memory, or to overcome issues of flawed memory.

Due to the small population, HSAM research remains relatively scarce. However, researchers generally share the same overarching goals: to understand what people with HSAM are capable of and how they are capable of superior memory. The existing studies have utilised a broad range of paradigms to measure memory and cognitive functioning in HSAM. To our knowledge, no review has systematically organised the available data. We attempt to address this gap and achieve the following objectives. Firstly, we seek to summarise the defining characteristics of HSAM, by collating knowledge from neuropsychological, neuroanatomical, and functional neuroimaging assessments. Secondly, we theorise what this data tells us about the mechanisms supporting HSAM and discuss future directions for this area.

Methodology

This systematic review adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Moher, 2009 ). The protocol was preregistered, details of which can be viewed at https://www.crd.york.ac.uk/PROSPERO/ (ID: CRD42022312854).

Eligibility Criteria

Full-text articles that reported group or single-cases possessing HSAM or hyperthymesia were selected for this systematic review. No restrictions were made regarding race, age, or sex. Samples that were inadequately screened for HSAM, or who possessed ‘normal’ or dysfunctional memory (e.g., severely deficient autobiographical memory (SDAM)), were excluded. Book chapters, non-English language, non-peer-reviewed, or non-experimental articles were excluded.

Information Sources and Search Strategy

The first author ran the first of three systematic online literature searches, initially spanning the 1st of January 2006–17th January 2022 on international databases: Web of Science, Scopus, PubMed, Ovid Medline, EBSCO host, and ProQuest. Multiple databases were chosen as this allows literature searches to be thorough (Bramer et al., 2017 ) and is recommended by gold-standard systematic review guidelines (Lefebvre et al., 2022 ). Start date was chosen because the HSAM phenomenon was first described that year (Parker et al., 2006 ). To identify HSAM studies, a search string was devised using advanced search techniques, such as Boolean operators (e.g., OR) and truncations (e.g., hyperthym*, autobiograp*), in the title and abstract fields. The search strings included the following words: (“superior” OR “exceptional” OR “extraordinary” OR “savant” OR “hyperthym*”) AND (“autobiograph*” OR “personal” OR “hyperthym*”) AND (“memor*” OR “retriev*” OR “recall*” OR “recogn*” OR “encod*” OR “rememb*” OR “mnem*” OR “mnes*” OR “recollect*) (Supplementary Materials). Similar strings were used across databases but adapted for each search engine’s specifications. Search strings were rerun approximately six months (1st of January 2006–23rd August 2022) and one year later (1st of January 2006–1st February 2023), to update the pool of eligible manuscripts with the most recent publications. Reference lists from included articles were screened for additional suitable articles. A total of 11,516 results were identified using the strategies described.

Selection Process

The entire selection process was completed independently by the first and second authors using the predetermined eligibility criteria. Duplicates were removed manually, then titles and abstracts screened. Suitable full-text articles were downloaded and assessed for inclusion. If inclusion agreement could not be reached, the senior researcher, and fifth author, was consulted to make the final decision, using the same eligibility criteria previously described.

Data Collection

An extraction template was designed on Microsoft Excel, and a pilot of three studies was performed to test appropriateness. Adjustments were made to improve the template design; then key information was extracted by the first and second authors independently. All responses were compared for accuracy verification. Data extracted included article information (location, study aims, and methods) and HSAM population details (sample size, sex, age, handedness, case abbreviation, clinical information, and HSAM screen). In addition, the following results were extracted: behaviour results (task name, purpose of task, and main findings), main structural and functional neuroimaging results (implicated brain areas, neuroimaging task details, controls, and neural activations). If information was missing from a study, it was decided that the first author would contact the relevant corresponding author, requesting the information. After two weeks if no response was received, the information was left as missing. No data in this review was acquired in this way.

Quality Assessment

Methodological quality of included studies was measured using a modified version of the Downs and Black Quality Assessment checklist (Downs & Black, 1998 ). Quality assessment was completed independently by the first and second author, then results compared to ensure consistency. As some studies are single-case, certain questions are not applicable, therefore percentage scores were chosen to assess quality of papers. Quality levels were as follows: Excellent Quality  ≥ 75%, Moderate Quality 50% to 74%, Low Quality 25% to 49%, and Poor quality  ≤ 25%.

Synthesis Methods

Outcome measures and statistical analyses implemented in included studies are highly varied, therefore, a meta-analysis was not performed to synthesise findings. The variability discovered was beyond the limits described in the literature (Ioannidis et al., 2008 ). For synthesis of findings, extracted information was used to group results into categories based on the methodologies implemented. Categories were as follows: behavioural, structural magnetic resonance imaging (MRI), task-based functional magnetic resonance imaging (fMRI), and resting-state fMRI results. Studies appear in multiple categories when several methodologies were used. Once categorised, results were presented in table form using Microsoft Word for formatting, alongside written descriptions in the result sections.

Study Selection

The initial search identified 3853 results. 2476 duplicate records were manually removed, and the remaining 1377 records were independently screened based on their title and abstract. Next, full-texts of fifty-eight records were independently assessed for eligibility and seventeen articles were included. An additional two searches were run approximately six months and one year later, following the same study selection process, and three additional papers were identified. Twenty full-text articles are included in this review. The PRISMA flow diagram illustrating the initial study selection process can be seen in Fig.  1 . Several studies appeared to meet inclusion criteria by reporting participants with enhanced cognitive abilities (e.g., Cook Maher et al., 2017 ; De Marco et al., 2015 ; Mella et al., 2021 ). However, full-text assessments revealed that the exceptional traits described were distinct from ABM, and therefore, they were excluded.

Study selection process (PRISMA flow chart)

Study Characteristics and HSAM Participant Information

Table 1 displays study characteristics, including geographical location of researchers, main objectives, and methodologies used. The United States of America (USA) is the country with the most publications on HSAM; thirteen studies received contributions from institutions located there (Ally et al., 2013 ; Brandt & Bakker, 2018 ; Daviddi et al., 2022b ; Frithsen et al., 2018 ; LePort et al., 2012 , 2016 , 2017 ; Levine et al., 2019 , 2021 ; Parker et al., 2006 ; Patihis, 2015 ; Patihis et al., 2013 ; Santangelo et al., 2018 ). Eight of the twenty included studies are single case (Ally et al., 2013 ; Brandt & Bakker, 2018 ; De Marco et al., 2021 ; Ford et al., 2022 ; Gibson et al., 2022 ; Mazzoni et al., 2019 ; Parker et al., 2006 ; Santangelo et al., 2021 ). Case abbreviations (e.g., ‘RS’) are in Table  1 and will be used throughout this review.

HSAM participant characteristics (not controls) are presented in Table  2 . Participants’ age varied between nineteen (LePort et al., 2016 ) and 80 years old (Santangelo et al., 2021 ). More males than females have been reported with HSAM, although exact numbers of each sex cannot be determined due to lack of clarity about participants appearing in multiple studies. Right handedness was more commonly reported than left-handedness in HSAM (Brandt & Bakker, 2018 ; De Marco et al., 2021 ; Ford et al., 2022 ; Gibson et al., 2022 ; LePort et al., 2012 ; Mazzoni et al., 2019 ; Parker et al., 2006 ; Patihis, 2015 ). Parker et al. ( 2006 ) described anomalous hand dominance in AJ; despite stating she was right-handed, photographs showed her playing with her left-hand during childhood and she worked from left-to-right on tasks normally performed in reverse by right-handed participants. Of the studies that reported current occupation, no HSAM participants were consistently employed. Reasons for this varied; MM was occupationally disabled (Brandt & Bakker, 2018 ), AJ was a mother (Parker et al., 2006 ), and BB was a student (De Marco et al., 2021 ; Mazzoni et al., 2019 ). Footnote 1

The most frequently used HSAM screening tools were the Public Events Quiz (PEQ), followed by the Random Dates Quiz (synonymously 10 Dates Quiz). Two studies used the Hull Memory Screening Questionnaire (HMSQ) (De Marco et al., 2021 ; Mazzoni et al., 2019 ). Three studies did not explicitly state which tasks were used for HSAM screening; however, the included HSAM participants completed in depth neuropsychological assessments (Ally et al., 2013 ; Brandt & Bakker, 2018 ; Parker et al., 2006 ).

Clinical profiles of HSAM participants are highly heterogenous. Many participants have obsessive compulsive tendencies (Ford et al., 2022 ; Gibson et al., 2022 ; LePort et al., 2012 , 2016 ; Parker et al., 2006 ; Santangelo et al., 2018 ). More specifically, LePort et al. ( 2012 ) used the short form version of the Leyton Obsessional Inventory (LOI) to measure obsessional tendencies. 81.18% of their HSAM sample reported traits such as hoarding items or being avoidant of germs. Later, LePort et al. ( 2016 ) administered the long-form version of the LOI to produce a symptom score. The symptom scores of the HSAM sample (M = 31.75, SD = 11.02) were normalised using z -scores to an OCD population’s normative data (M = 33.3, SD = 7.7). HSAM scores were found to be indistinguishable from the OCD population. Personality Assessment Inventory data obtained by Santangelo et al. ( 2018 ) showed that for the “obsessive–compulsive” subscale, the overall mean HSAM score was in the 92nd percentile relating to obsessive and compulsive symptoms. Elevated psychological trait anxiety scores (Patihis, 2015 ), and presence of anxiety conditions (Brandt & Bakker, 2018 ; Gibson et al., 2022 ; Parker et al., 2006 ) have also been reported. However, BB (De Marco et al., 2021 ; Mazzoni et al., 2019 ) and GC (Santangelo et al., 2021 ) showed no clinical traits. Group studies have shown that HSAM participants are not within the clinical depression range (LePort et al., 2012 ), but single-cases have reported current depression diagnoses (Brandt & Bakker, 2018 ; Ford et al., 2022 ; Gibson et al., 2022 ) and previous depressive periods (Parker et al., 2006 ).

Risk of bias was assessed using a modified version of the Downs and Black Quality Assessment checklist (Downs & Black, 1998 ). Results are presented in Table  3 . Fourteen studies were considered “excellent quality” and six “moderate quality”.

Main Behavioural Results

Main behavioural results are summarised below (see Supplementary Materials for a more detailed list). All twenty HSAM studies reported enhanced ABM performance. When dates were retrieval cues, test–retest reliability was perfect (Ally et al., 2013 ; Parker et al., 2006 ) and verifiable detail accuracy was exceptional (98% accuracy) (Mazzoni et al., 2019 ). GC passed HSAM assessment at 75 and 80 years old (Santangelo et al., 2021 ). In fact, PEQ performance improved (approximately 12%) between timepoints and memories remained high in episodic details. In ad hoc tasks designed to assess semantic and ABM, RS performed significantly better than matched controls (Ford et al., 2022 ). Gibson et al. ( 2022 ) hypothesised enhanced past ABM may coincide with enhanced future thinking (i.e., a capacity to disengage from the present and mentally project oneself into the future to imagine hypothetical scenarios) (see D’Argembeau et al., 2010 ). When single words were used as cues to simulate a future autobiographical event (Adapted Autobiographical Interview), RS described more detailed events than controls. However, in future thinking tasks not related to one’s own experiences (Narrative Scene Construction – Cinderella and Cookie Theft), performance was comparable to controls, and RS repeated herself more.

HSAM individuals were found to have an enhanced performance for some tasks that did not measure ABM. On a measure of associative memory, LePort et al. ( 2012 ) found that HSAM individuals had superior Names to Faces task performance, compared with controls. This finding was confirmed later by significantly higher HSAM Face-Name-Occupations Task scores (LePort et al., 2017 ). Despite these results, on other tasks which involve aspects of associative memory (e.g., the three-phase story), researchers did not find that the HSAM group were superior. Enhanced or excellent performance was found for olfactory functioning (Parker et al., 2006 ), celebrity face recognition (Brandt & Bakker, 2018 ), word recognition (Parker et al., 2006 ), and narrative abilities as measured by the Script Generation Task (LePort et al., 2017 ). “Absorption” and “fantasy proneness” personality traits, measured by the Tellegen Absorption Scale and Creative Experience Questionnaire, respectively (Patihis, 2015 ), were significantly higher than controls.

On some measures of ABM performance, HSAM and controls were comparable. During the Meta Test (i.e., to quantify retrieval of the whole testing experience), the experimenter asked participants questions about their life (e.g., “How was your weekend?”). Participants provided responses and the experimenter also offered a story in return (e.g., a story about seeing a gun on campus) (LePort et al., 2017 ). One week and one month later, participants were tested on their memory of these responses. Whilst HSAM participants excelled at the personal recollections, their recall for the experimenters’ anecdotes was entirely analogous with the control population. In a Dates task, HSAM participants provided higher quantity and quality of memories for remote time periods (1 month, 1 year, and 10 years from memory testing) (LePort et al., 2016 ) but comparable responses to controls for dates 1 week from testing. When completing the Autobiographical Interview, RS was comparable to controls during free recall of specific events from time periods of her life (e.g., adolescence) and provided fewer external elements (i.e., semantic details not specific to events) during a single event from Early Adulthood (Gibson et al., 2022 ).

LePort et al. ( 2017 ) administered the Three Phase Story to explore memory retrieval for a story that induced negative emotional arousal. When exposed to emotional stimuli HSAM participants did not recall more than controls. Similarly, HSAM participants were no better than controls at predicting how emotional they would feel at an upcoming political election (Levine et al., 2019 ) or remembering their emotions three weeks (Levine et al., 2019 ), or six months post-election (Levine et al., 2021 ). HSAM participants reported feeling high arousal emotions as frequently as controls (Patihis, 2015 ). Other cognitive domains that were associated with performance that was not statistically different from controls or normative scores in HSAM included verbal (LePort et al., 2012 , 2017 ; Parker et al., 2006 ), prospective (Brandt & Bakker, 2018 ; Gibson et al., 2022 ), and semantic memory (Parker et al., 2006 ). Language (Gibson et al., 2022 ; Mazzoni et al., 2019 ; Parker et al., 2006 ), mental imagery (LePort et al., 2017 ), and creative (Daviddi et al., 2022b ) or critical (Patihis, 2015 ) thinking were also not statistically different to controls or normative scores. Questionnaires indicated sleep was not altered in HSAM (Patihis, 2015 ). These results indicate that for people with HSAM their cognitive skills for non-autobiographical tasks are well within the range of normality. These conclusions require accepting the null hypothesis; it would therefore be highly beneficial to calculate the Bayes factor. Due to lack of relevant information in the published articles this was not feasible.

HSAM individuals’ intelligence was generally in the normal range (Ally et al., 2013 ; Brandt & Bakker, 2018 ; Gibson et al., 2022 ; Parker et al., 2006 ; Patihis, 2015 ). BB demonstrated overall intelligence in the 90th percentile (Mazzoni et al., 2019 ). Five studies administered Digit Span tasks to assess attention and working memory in HSAM; three found average results (Daviddi et al., 2022b ; Gibson et al., 2022 ; LePort et al., 2012 ), and the remaining reported scores better than reference controls (Mazzoni et al., 2019 ; Parker et al., 2006 ). Visual memory and visuospatial abilities varied between studies; above average performance was found in the Visual Memory Index (Parker et al., 2006 ) and Visual Reproduction subtests of the Wechsler Memory Scale—Revised (Mazzoni et al., 2019 ). Conversely, performance was average for tests such as Visual Reproduction (LePort et al., 2012 ), Visual Patterns, and the Progressive Silhouettes task (LePort et al., 2017 ). When administering the Wechsler Memory Scale (WMS) to assess memory performance, two studies found average overall performance (Ally et al., 2013 ; Brandt & Bakker, 2018 ) and one study (LePort et al., 2012 ) found average performance for Logical Memory (recognition). Conversely, AJ scored near maximum for the WMS General Memory Index (Parker et al., 2006 ) and HSAM participants were superior at Logical Memory (free recall) (LePort et al., 2012 ). HSAM individuals were as susceptible as controls to false memories in the Deese, Roediger, and McDermott (DRM) task and on the non-existent news footage paradigm and had slightly more overall false memories during the Misinformation task (Patihis et al., 2013 ).

Areas of cognitive weakness also varied. AJ showed impairments in tasks assessing aspects of executive functioning, including shifting measured by the Wisconsin Card Sorting Test (Parker et al., 2006 ). Other studies, which used tasks like the classic Stroop, found executive functioning elements (e.g., interference, initiation, and inhibition) to be intact (Gibson et al., 2022 ; Mazzoni et al., 2019 ). A motor speed impairment was reported for AJ (Parker et al., 2006 ), but scores were average for RS (Gibson et al., 2022 ). HSAM individuals have lower flexible thinking scores, particularly relating to “tolerance for ambiguity” (Patihis, 2015 ). Cognitive and affective empathy was reported as normal in HSAM (Patihis, 2015 ), assessed with the Empathy Questionnaire and Basic Empathy Scale scores, respectively. RS was deficient in her ability to “comprehend emotions of others” (Gibson et al., 2022 ). HSAM individuals had a lower response bias criterion, indicating a more liberal tendency to report items as previously seen (Frithsen et al., 2018 ).

Structural Neuroimaging Findings

Main MRI findings and details of controls are summarised in Table  4 . Four studies (Ally et al., 2013 ; Brandt & Bakker, 2018 ; LePort et al., 2012 ; Mazzoni et al., 2019 ) found anatomical differences spanning both hemispheres in HSAM participants. In the left hemisphere, MRI data from BB (Mazzoni et al., 2019 ) revealed significantly bigger grey-matter volumes compared with controls from an occipitotemporal cluster extending to the posterior hippocampus. Brandt and Bakker ( 2018 ) found an increase in the bilateral temporopolar cortex total volume in MM, and Ally et al. ( 2013 ) reported a bigger subcortical volume of the right amygdala in HK. Diffusion Tensor Imaging analysis performed by LePort et al. ( 2012 ) revealed increased anisotropy (i.e., indicative of better signal conduction) bilaterally in the forceps major, parahippocampal gyrus and intraparietal sulcus in HSAM participants (vs. controls). This was accompanied by increased anisotropy in the left uncinate fasciculus and right lingual gyrus. Tensor based morphometry (TBM) analysis showed bigger volumes in the left posterior insula, whilst voxel-based morphometry-grey matter (VBM-GM) analysis revealed bigger size of right hemisphere structures (e.g., posterior pallidum) in HSAM participants.

Conversely, in MM, the right perirhinal cortex was smaller in total size (Brandt & Bakker, 2018 ) compared with control data. Ally et al. ( 2013 ) found significant reductions in size of bilateral subcortical structures (i.e., thalamus, caudate, putamen, pallidum, hippocampus, and the left amygdala) for HK. Lower grey and white matter concentrations for HSAM participants were found when VBM-GM and voxel-based morphometry-white matter analysis was performed to assess the anterior and middle temporal gyrus, bilaterally (LePort et al., 2012 ). In the same study, reduced grey matter relative concentrations were discovered by VBM-GM in the vicinity of the bilateral intraparietal sulcus.

The remaining structural neuroimaging studies found no significant structural differences (Ford et al., 2022 ; Gibson et al., 2022 ; Santangelo et al., 2021 ). Two studies (Daviddi et al., 2022a ; De Marco et al., 2021 ) obtained MRI data to confirm absence of neurostructural abnormalities that may explain between group resting-state connectivity differences and found no significant anatomical differences.

Overall, the statistical effects observed in the included neuroanatomical data indicate an involvement of subcortical nuclei and mediotemporal-limbic and temporo-occipital regions in HSAM. Moreover, the collated evidence shows an asymmetric trend suggesting that structural alterations may be linked to functional hemispheric specialisation.

Resting-State Functional Connectivity Results

Results from studies reporting resting-state fMRI data are presented in Table  5 . All studies (Ally et al., 2013 ; Brandt & Bakker, 2018 ; Daviddi et al., 2022a ; De Marco et al., 2021 ) reported between group differences in resting-state functional connectivity, and each investigated region-to-region connectivity. One study explored within network functional connectivity (Ally et al., 2013 ) and another analysed large scale brain networks and used graph theory approaches to process data (De Marco et al., 2021 ).

Three studies found greater hippocampus connectivity with other brain areas. Brandt and Bakker ( 2018 ) found the left hippocampus had greater connectivity with left hemisphere regions (inferior prefrontal cortex, inferior frontal gyrus–pars opercularis, premotor cortex, and retrosplenial cingulate cortex) and the bilateral dorsolateral prefrontal cortex, in HSAM vs. controls. Ally et al. ( 2013 ) found the right amygdala had increased functional connectivity with the right hippocampus, and Daviddi et al. ( 2022a ) reported the right anterior hippocampus and right posterior hippocampus had greater connectivity with the left fusiform gyrus and bilateral inferior temporal gyrus, respectively.

Contrastingly, weaker hippocampus resting-state connectivity was found in HSAM participants vs. controls. Less left hippocampus connectivity was reported with the left posterior entorhinal cortex and bilateral perirhinal cortex (Brandt & Bakker, 2018 ). Weaker anterior left hippocampus connectivity was found with the left middle frontal gyrus, supramarginal gyrus, inferior precentral gyrus, and right superior frontal gyrus (Daviddi et al., 2022a ). The posterior left hippocampus had less connectivity with the left inferior frontal gyrus and middle cingulate cortex. The right anterior hippocampus had weaker functional connectivity bilaterally with the insula, temporoparietal junction and anterior cingulate cortex (Daviddi et al., 2022a ). Lower levels of resting-state functional connectivity were found in the posterior cingulate and ventral precuneus network (Ally et al., 2013 ).

Resting-state connectivity related to the cerebellum yields significant between-study variability in HSAM vs. controls. Ally et al. ( 2013 ) found no difference between within-network cerebellum connectivity in the seed-based cerebellar connectivity network analysis. Contrastingly, De Marco et al. ( 2021 ) found the right orbitofrontal cortex had more connectivity with the right cerebellar lobule IX. Graph theory analysis in the same study found the right cerebellar lobule IX had higher levels of betweenness centrality in BB than controls. Higher levels of betweenness centrality (a measure of pathway-related relevance assumed by a region) were also reported in the right temporal pole and right orbitofrontal cortex. The left globus pallidus was found to have lower levels of local efficiency and clustering coefficient (i.e., two metrics indicative of local integration).

Stronger connectivity was found in patterns of inter-regional connectivity (e.g., between left lingual gyrus and right Heschl’s gyrus) (De Marco et al., 2021 ) and in the postcentral and thalamic networks (Ally et al., 2013 ) of HSAM participants. Large-scale brain network analysis revealed default-mode network regions bilaterally (superior temporal gyrus and inferior parietal lobule) and ABM network areas (e.g., left superior and inferior temporal gyrus) were more significantly expressed in BB than controls (De Marco et al., 2021 ).

To summarise, these results indicate significant alterations to mediotemporal, limbic, and prefrontal neural pathways that typically support high-order cognitive abilities such as language or speech processing and memory. As the methodologies deployed in these studies are quite diverse, there is considerable variability in the emerging pattern of findings. It should also be noted that, whilst the differences observed in this section could be linked to superior memory; they could also be associated with idiosyncrasies of neurofunctional architecture.

Task-Based fMRI Results

Table 6 presents details of the fMRI tasks that have been used to explore HSAM retrieval and provides the related main behavioural results. In two studies ABM cues prompted participants to remember the “first” or “last time” they experienced an event (Santangelo et al., 2018 , 2020 ). In two studies, dates from the participants life were used as memory cues (Mazzoni et al., 2019 ; Santangelo et al., 2021 ). Participants reported when a memory was initially accessed and, again, when they “elaborated” (Mazzoni et al., 2019 ) or “relived” (Santangelo et al., 2018 , 2020 , 2021 ) this memory. Table 7 presents the main neural activations recorded by task-based fMRI studies.

Access is the moment a memory is reported to surface to consciousness. The temporoparietal junction, ventromedial prefrontal cortex, and dorsomedial prefrontal cortex on the left side were found to be selectively activated during HSAM access (Santangelo et al., 2018 ). Running a cvMANOVA, on the same data set, Santangelo et al. ( 2020 ) obtained a significant relationship between “pattern distinctness” (D measure) and older memories compared with newest memories for HSAM participants in the left ventromedial prefrontal cortex but not in left dorsomedial prefrontal cortex or the left hippocampus. During access, compared with recorded brain activity when no memory was being recalled, BB recruited left-sided (e.g., middle frontal gyrus, precuneus, and lingual gyrus) and posterior (e.g., cerebellum) brain areas (Mazzoni et al., 2019 ).

When comparing access vs. elaboration, significant activation was detected in the precuneus in both BB and GC (Mazzoni et al., 2019 ; Santangelo et al., 2021 ). On the left-side increases in activation were found in the thalamus and frontal (middle frontal gyrus), temporal (middle and inferior temporal gyrus), limbic (posterior cingulate cortex), and occipital lobes (cuneus) in GC (Santangelo et al., 2021 ). In BB the cuneus (right) and middle frontal gyrus (bilaterally) were also significantly activated (Mazzoni et al., 2019 ). BB additionally recruited many right-sided brain areas, including the superior parietal lobule.

Memory Elaboration

Elaboration-reliving is when a memory is remembered in its entirety. Reliving showed no detectable significant increases in activation, compared with access in GC (Santangelo et al., 2021 ). Compared with the no memory condition, elaboration resulted in increases in activation in the right precuneus and several left hemisphere structures (e.g., inferior frontal gyrus, inferior parietal lobule) in BB (Mazzoni et al., 2019 ). When elaboration was compared with access, BB displayed greater activation in structures across both hemispheres. Regions showing increases in activation were in the right hemisphere (e.g., superior temporal gyrus, inferior parietal lobule), bilaterally in the frontal lobe (i.e., middle and superior frontal gyri), and in the left hemisphere in the frontal (inferior frontal gyrus, medial frontal gyrus), parietal (postcentral gyrus, angular gyrus), temporal (middle temporal gyrus), and limbic lobes (posterior cingulate gyrus, anterior cingulate gyrus).

Overall Retrieval

HSAM individuals had greater neural activity than controls during overall retrieval in areas including, the bilateral angular gyrus and ventromedial prefrontal cortex, right dorsolateral prefrontal cortex and left temporoparietal junction (Santangelo et al., 2018 ). Mazzoni et al. ( 2019 ) found overall memory retrieval, compared with a no memory condition, recruited left side brain areas (e.g., the precuneus, cuneus, frontal gyrus, and temporal gyrus), and the bilateral middle temporal gyrus.

Behaviour Results

HSAM participants were excellent at retrieving memories using date cues (Mazzoni et al., 2019 ; Santangelo et al., 2021 ). Post-scanner verification of dates showed 100% accuracy in verifiable events (Santangelo et al., 2021 ). BB’s mean time to access (1816 ms) and elaborate (11,725 ms) memories was very fast (Mazzoni et al., 2019 ). HSAM groups were faster at accessing ABMs than controls for non-date cues and provided more detailed post-scanner descriptions (Santangelo et al., 2018 ). The same study found no between group-differences for self-report measures of emotional intensity or reliving rating for ABMs.

Overall, collated data from the limited number of functional neuroimaging studies reveal an involvement of a wide range of cortical areas and across all lobes during memory retrieval, in individuals that possess HSAM. Such widespread activity was observed in all stages of memory retrieval (i.e., access and elaboration-reliving), and behaviourally, speed of retrieval was very fast (< 2 s).

To our knowledge, this is the first systematic review on HSAM. The goal of this work was to collate the knowledge acquired from existing literature and to summarise what is currently known about the behavioural and neural basis of exceptional ABM. Fully understanding how exceptional memory functions could provide an alternative viewpoint to the study of human memory, that is more traditionally based on memory deficits (e.g., Cole et al., 2015 ; Rathbone et al., 2009 ).

The collated data presented in this systematic review leads to some interesting interpretations of how neurocognitive systems may sustain HSAM. Firstly, HSAM individuals and controls were found to be comparable for both number and quality of details described for autobiographical events dated closest to testing (LePort et al., 2016 ). This finding suggests that acquisition of information is not quantitatively or qualitatively enhanced, allowing us to claim that encoding processes in HSAM may be similar to the cognitive and metacognitive mechanisms of the normal population. It seems logical then that memory enhancement must occur in later memory stages (i.e., consolidation and retrieval) and these behavioural data encourages investigators to direct their attention to the forgetting processes in HSAM, that appear to be not in line with the expected pattern as in the classic Ebbinghaus’ forgetting curve (Ebbinghaus, 1885 ; Radvansky et al., 2022 ). For dates dating back one month or more from testing, HSAM individuals are vastly superior in their autobiographical recall (LePort et al., 2016 ), suggesting enhanced consolidation underlies the capacity to retain personal information. Following this line of reasoning, it seems HSAM individuals are not necessarily able to remember everything but instead, are unable to forget personal experiences. However, it must be noted that if additional measures during retrieval were recorded (e.g., response times or brain activations), between group differences may become evident at less than one week, contradicting the theory that HSAM involves normal encoding abilities. Furthermore, the aforementioned study (LePort et al., 2016 ), as with most that investigate ABM, assumed that all recollected ABMs described by participant’s were accurate, despite having no means to verify any claims. Perhaps even after one day, individuals with HSAM have more accurate memory representations. To address this uncertainty, wearable cameras during ABM encoding could be utilised in the future, allowing for accuracy of personal elements to be objectively measured, and the nature of HSAM processes to be better understood.

Moreover, the HSAM pass rate for the highly difficult PEQ also supports heightened consolidation, as it relies on dates of famous events (e.g., a date of a World Cup Final match) as memory cues (LePort et al., 2012 ; Santangelo et al., 2021 ). The perceived cultural importance of selected PEQ stimuli increases the likelihood that these events were initially encoded by people and thus is deemed a strong measure of whether individuals have retained or forgotten such information. Where those with normal memory have been shown to score close to zero on this task, individuals with HSAM must score a minimum of 50% (LePort et al., 2016 ) and some participants have been found to score over 90% accuracy (Talbot et al., 2022 ). Of course, the assumption that public events are known to all participants may not be fully warranted, and it might be useful to develop assessment tools, like the HMSQ (Mazzoni et al., 2019 ) that are more specifically tailored to individuals, to avoid failing to identify an exceptional case.

From an interpretational viewpoint, resting-state functional connectivity data highlighted in this review suggests that HSAM individuals may have better consolidation skills. Higher betweenness centrality and increased resting-state functional connectivity of the right orbitofrontal cortex and right lobule IX of the cerebellum were observed in BB (De Marco et al., 2021 ). The orbitofrontal cortex is believed to interact with the hippocampus during the formation of long-term memories (Ramus et al., 2007 ) and therefore could contribute to heightened consolidation in HSAM. In fact, greater resting-state connectivity of the hippocampus with other cortical and limbic structures (and cerebellar structures in some cases) was found to be the most consistent difference in HSAM (vs. controls) in this review (Ally et al., 2013 ; Brandt & Bakker, 2018 ; Daviddi et al., 2022a ), and these differences could explain why forgetting is reduced in HSAM. According to the systems model of how memories are consolidated (see Squire et al., 2015 ), the hippocampus directs reorganisation of information to regions of the neocortex. This process is believed to transform a memory from labile to a more permanent memory trace that is eventually no longer dependent on the hippocampus; a stronger resting-state connectivity of the hippocampus with several other structures in HSAM could amplify this process. Crucially, whether this interpretation is true cannot be concluded in a study that lacks any kind of behavioural measures. Indeed, the risk that reverse inferences pose when interpreting neuroimaging data is well described (Poldrack, 2011 ). Future research should consider ways to test this explanation empirically.

Only one study (De Marco et al., 2021 ) has investigated graph theory-informed metrics of functional connectivity. This approach is complementary to the typical map’s representative of statistical modelling of regional signal; graph-theory indices can inform, amongst others, about computational centrality, integration, and segregation of regions via a pathway-based elaboration of correlational measures. Contrastingly to fMRI data, structural neuroimaging studies have shown no neuroanatomical differences in the size of the hippocampus related to HSAM (Ford et al., 2022 ; Gibson et al., 2022 ; Santangelo et al., 2021 ). However, structural differences were generally found to be highly inconsistent across the included studies. Furthermore, one study (Ally et al., 2013 ) used a blind participant, lowering the degree of confidence in the conclusions that can be drawn from neuroanatomical data.

Further potential insight on why information is better consolidated in HSAM comes from the only available resting-state group study (Daviddi et al., 2022a ). Disrupted functional connectivity was observed in HSAM participants between the hippocampus and saliency network related brain regions (anterior cingulate cortex and bilateral insula). The authors refer to the salience network as “a core hub” that allows the detection of relevant stimuli present in the external environment, resulting in goal-directed behaviours as an outcome (Uddin, 2014 ). Furthermore, they observed decreased connectivity between the hippocampi and ventral frontoparietal regions (e.g., temporoparietal junction) that they describe as contributing to “deployment of attentional resources” (Corbetta et al., 2008 ). Enhanced functional connectivity of the hippocampus with sensory regions (e.g., inferior temporal cortex) was also observed. Taken collectively, the authors speculate that these findings could suggest that HSAM individuals are less able to discriminate or choose salient information, and this leads to greater encoding and consolidation of sensory information, regardless of how relevant it might be. As the authors clearly acknowledge themselves, this is an inference that cannot be confirmed using a resting-state study design, and thus they recommend further studies with behavioural measures. As memory consolidation is believed to occur partly during sleep (Cairney et al., 2014 ; Walker & Stickgold, 2004 ), we may expect that HSAM is linked with better sleep. However, Patihis ( 2015 ) found no significantly better patterns of sleep in HSAM individuals on self-reports measures of sleep quality (e.g., time taken to fall asleep). To ensure there are truly no meaningful differences in consolidation during sleep, additional research should consider using overnight physiological monitoring measures, such as a polysomnography, in the study of HSAM.

Based on the evidence that we have presented thus far, it appears a reasonable postulation that HSAM could be characterised by enhanced consolidation. However, the results of our systematic review reveal a lack of empirical evidence to explain how this enhanced consolidation may occur. The current HSAM literature cannot adequately explain the neurobiological processes that underlie this stage of memory. Looking to the wider literature, for several decades, research on both animals and humans has supported that emotional arousal contributes to consolidation (see McGaugh, 2013 ; McGaugh & Roozendaal, 2008 ). Hormones that are mediated by the amygdala and are released in response to stress (e.g., adrenaline and corticosterone) are thought to be involved in modulating whether or not a memory is retained (McGaugh, 2013 ). Perhaps then, enhanced remembering in HSAM is a result of a highly specialised activation of these modulatory systems. Similarly, HSAM individuals may share genetic or epigenetic markers that alter their memory ability. To our knowledge, no research exists that has explored these possibilities, though both could be promising avenues to explore in the future when working to understand better how HSAM occurs. Another possibility is that HSAM is linked to specialised aspects related to neurotransmission. Recent advancements in neuromolecular imaging, namely, the development of positron emission tomography (PET) radioligands that can pass the blood brain barrier, have allowed researchers to visualise the topography associated with specific aspects of neurotransmission. PET tracers have since been utilised in research and clinical studies spanning numerous disciplines (e.g., neurology and psychiatry) (Kilbourn, 2021 ). As an example, tracers have been created that are specific to D1 and D2 dopaminergic receptors (for a review see Kilbourn, 2021 ), and recently, it has been hypothesised that regional balance of D1 and D2 receptors is linked to cognitive functioning (for a review see Matzel & Sauce, 2023 ). Whether dopaminergic (or other types of) receptors contribute to cognitive functioning in HSAM is currently unknown, but using in vivo imaging techniques, like these, could provide direct insight into the biological mechanisms supporting superior memory. These suggestions, some of which we will be testing in our laboratory, are just a few possibilities to understand if and how consolidation is enhanced in HSAM.

Previously, some researchers (LePort et al., 2016 ) have hypothesised that underlying clinical conditions, such as obsessive–compulsive disorder (OCD), are prerequisites of HSAM that increase consolidation of memories through repetitive and habitual retrieval-practice (LePort et al., 2017 ). The OCD hypothesis derives from studies that have found that many HSAM participants have symptoms in line with OCD (e.g., LePort et al., 2012 ). Deliberate strategies, such as distributed practice or practice retrieval (see Schwartz et al., 2011 ), can strengthen memory, and thus, it is reasonable to infer that rehearsal and rumination over an autobiographical event could also help preserve this type of memory (LePort et al., 2016 ). According to this view, HSAM individuals are highly interested in their own personal memories, think about them frequently, and thus become excellent at remembering them. Footnote 2 Consistent with this, research has shown that HSAM individuals enjoy thinking about their memories, reflecting on events whilst stuck in traffic (LePort et al., 2016 ) or whilst blow-drying their hair Footnote 3 (Rodriguez McRobbie, 2017 ). Similarly, HSAM participants in our own laboratory have reported fears around forgetting information, and other individuals in the literature have stated that understanding the importance of remembering served as a turning point for which their own memory began to excel (Rodriguez McRobbie, 2017 ). The attachment to personal memories could explain why HSAM individuals were not enhanced at recalling memories less related to themselves in the Meta Test (LePort et al., 2017 ) or were found to be unable to remember what an interviewer was wearing after having sat for hours in front of them (Rodriguez McRobbie, 2017 ). Of course, the time required to rehearse every day of one’s life would be far too excessive for explicit rehearsal to be the sole process responsible for superior memory. The authors (LePort et al., 2016 ) acknowledge that HSAM individuals do not spend as much time practicing as other groups of individuals with enhanced memory, such as memory champions (Foer, 2011 ). Therefore, they suggest that passive rumination could underlie the strengthening of ABM. Our systematic review partially contradicts the OCD hypothesis. In fact, many of the participants identified did not have OCD symptomatology (Brandt & Bakker, 2018 ; De Marco et al., 2021 ; Mazzoni et al., 2019 ; Santangelo et al., 2021 ). In addition, in participants with high OCD scores, no correlation was found between higher Leyton Obsessional Inventory (LOI) scores (i.e., self-reported obsessional symptom scores) and faster memory access (Santangelo et al., 2018 ). Thus, our review highlights that more research is needed to clarify how a cognitive enhancement state and a clinical trait might interact in HSAM.

Palombo et al. ( 2018 ) have previously theorised that specialised memory consolidation in HSAM could be a result of an enhanced self-reference effect. The self-reference effect states that information involving the self will be better remembered (e.g., Betz & Skowronski, 1997 ; Klein, 2012 ), and this is thought to be due to easier integration of this information with pre-existing schematic representations of oneself (Burden et al., 2021 ; Conway, 2005 ; Conway & Pleydell-Pearce, 2000 ). Recent studies provide additional support to this theory: RS provided descriptions richer in semantic and episodic details for future events involving herself than matched controls, but she was comparable to controls for scenarios unrelated to herself (Gibson et al., 2022 ). In another recent study, RS excelled at semantic tasks, seemingly by attaching personal information to them (Ford et al., 2022 ). For example, her ability to recall the Harry Potter books word for word appears to be associated with a related ABM from the time she read it. In a similar vein, descriptions from HSAM individuals emphasise the fact that their memories are highly personal (Parker et al., 2006 ), and using the Meta Test, LePort et al. ( 2017 ) showed that people with HSAM only excelled at recalling memories related to themselves. This task also successfully demonstrated that HSAM individuals do not incidentally encode everything, providing additional support for the previously discussed theory that normal encoding processes are linked to exceptional ABM.

Our review found strong evidence that HSAM involves extraordinary retrieval. fMRI evidence shows HSAM involves, in part, an intense overactivation of common brain regions belonging to the ABM network (Maguire, 2002 ; Svoboda et al., 2006 ), including many temporoparietal and prefrontal areas (Mazzoni et al., 2019 ; Santangelo et al., 2018 , 2021 ). Compared with controls, more than twice as many brain regions are activated during retrieval in HSAM (Santangelo et al., 2018 ), and this may explain why access and elaboration of autobiographical material is extremely quick (see Conway et al., 2003 ; Conway & Loveday, 2010 for normal populations). Daselaar et al. ( 2007 ) previously mapped ABM retrieval in healthy individuals, distinguishing access and elaboration. They found initial access is predominantly supported by anterior structures, including frontal and temporal brain areas (e.g., right prefrontal cortex) and later elaboration recruits posterior areas (e.g., visual areas and precuneus). In contrast, neural activation and functional connectivity data in this review appear to suggest an early recruitment of posterior areas during HSAM access (Mazzoni et al., 2019 ; Santangelo et al., 2018 , 2021 ). These include the precuneus that is thought to play a role in visual imagery (Ahmed et al., 2018 ) and in retrieval of memories that are considered true (Addis et al., 2004 ). Interestingly, this is the exact opposite pattern observed in a SDAM (i.e., a syndrome involving an incapability of reliving personal events) sample (Palombo et al., 2018 ); SDAM individuals showed a reduction in activation in the right precuneus during an ABM retrieval task. Taken together, these findings suggest that it is the level of neural activity in deputed areas rather than recruitment of novel brain structures that may support exceptional retrieval. Similarly, unlike the reported shift of neural activation from left-to-right between the brain hemispheres during access and elaboration (Conway et al., 2003 ), BB showed widespread and bilateral activations during memory elaboration. Unlike the findings of other studies (Conway et al., 2001 ), BB’s left hemisphere remained activated during elaboration, both anteriorly and posteriorly (Mazzoni et al., 2019 ).

An additional point for discussion is that if HSAM is simply an enhancement of normal ABM, it would also be susceptible to false memories (Wade et al., 2007 ). Widely replicated data has demonstrated that distortions can influence both initial encoding (e.g., Findley, 2012 ) and post-encoding of memories (Mazzoni & Memon, 2003 ; for a discussion, see Mazzoni & Vannucci, 2007 ), leading to the presence of false memories. False memories are often said to be the product of the reconstructive nature of memory processes (Conway & Loveday, 2015 ) and are characterised by several “sins” (Schacter, 1999 , 2021 ). Patihis et al. ( 2013 ) found a comparable frequency of false memories between HSAM individuals and controls, a finding that suggests that HSAM people may have an extraordinary strength of personal memories, supported by otherwise ordinary cognitive abilities. An important limitation of the Patihis et al. ( 2013 ) study should be noted before reaching any conclusion: the false memory assessments that were used (e.g., neutral single words in the DRM) were not autobiographical in nature. Our review evidences that it is clear that only ABM is exceptional in HSAM (Brandt & Bakker, 2018 ; Daviddi et al., 2022b ; Frithsen et al., 2018 ; Gibson et al., 2022 ; LePort et al., 2012 , 2017 ; Levine et al., 2019 , 2021 ; Parker et al., 2006 ); it might still be possible, therefore, that when appropriate stimuli (i.e., personal events) are presented, fewer false memories are obtained compared with controls.

Our review did find some evidence of unique processing in HSAM. Mazzoni et al. ( 2019 ) suggested that ABM access can bypass the hippocampus. Many authors, however, have suggested that the hippocampus is essential to ABM (Maguire, 2002 ; Svoboda et al., 2006 ), particularly during access (Daselaar et al., 2007 ), and during retrieval of specific and general memories (e.g., Addis et al., 2004 ). Evidence from a task-based fMRI study investigating ABM retrieval revealed that for BB a broad range of left hemisphere brain areas are activated during access, including the ventrolateral prefrontal cortex (Mazzoni et al., 2019 ), that is thought to be related to the semantic contribution to an ABM (Jacques, 2012 ). The authors suggest that in HSAM an ABM becomes “ semanticised ” and, as demonstrated by Santangelo et al. ( 2018 ), semantic memory retrieval is significantly quicker than autobiographical recall; this finding could explain why recall of a semanticised ABM is significantly faster. Of course, this finding is based on a single case, and it must be emphasised that Santangelo et al. ( 2018 ) did find activity in the hippocampus during access, when non-date cues were used. This review revealed that no group studies have published fMRI data using a date task or have differentiated neural activations among additional features of retrieval, such as direct vs generative retrieval (Harris & Berntsen, 2019 ). Though an ABM network has been previously identified (Svoboda et al., 2006 ), meta-analyses of ABM studies have demonstrated that brain activations differ between studies and this variance indicates that ABM processes fluctuate across individuals. If the theory that HSAM is simply an enhancement of normal ABM is accurate, it would also be reasonable to assume then that not everyone achieves HSAM in the same way either. Furthermore, the notion of neural reserve (e.g., Stern, 2009 ) explains that there is a significant interindividual variability in the neural mechanisms that are engaged when different people perform the same task. These differences might increase the difficulty in correctly interpreting group data and when inferring the specific mechanisms that are responsible for HSAM. With this rationale, single-case studies may be the most effective way to explore HSAM and should be prioritised in the future.

At present, very little is known about the qualitative aspects of this retrieval, such as whether there is any neural specialisation linked to the amount of detail provided during retrieval, or the degree of semantic complexity that characterise the memories. It could be suggested that any differences in functional activations found between HSAM and controls during retrieval are a result of the increased amount of information retrieved in HSAM, rather than the brain functioning in a unique way. Activation levels do not always indicate expertise (Bernardi et al., 2013 ; Jeon & Friederici, 2016 ), however, and our review also found similar patterns of high neural activity during autobiographical retrieval in single-case reports that made within-subject comparisons. In the future, it could be informative to compare HSAM and controls on more recent memories, where the amount retrieved is closer matched. A fMRI study which measures what happens when the number of details between exceptional memory and normal memory are equated could be very beneficial to the field. We hypothesise that differences would still be observed, and the results could provide strong evidence that individuals with HSAM truly are superior. One may argue that qualitative differences are also the reason for resting-state distinctions between HSAM individuals and controls and thus should be considerations taken by researchers when interpreting results (see Heit, 2015 for a discussion on the topic of forward inference). Whilst this could be an explanation, our review did not find any empirical evidence to support that, from a hierarchical point of review, remembering takes priority (i.e., occurs more frequently or for longer) over any other process during resting state (e.g., planning and inner language). Collectively our review revealed that a lot is still unknown about the neural functioning of those with HSAM and that future research is needed to draw clearer conclusions about how the ability is supported.

Collated behavioural evidence supports that each HSAM participant in this review underwent extensive ABM assessment to support their categorisation as exceptional. The enhanced performance and test–retest reliability when providing personal memories leads us to define HSAM as a rare ability involving very rapid, accurate, and extremely detailed retrieval of autobiographical memories, that is effortless, intrinsically tied to dates and that contrasts normal age-related decline. Our review found HSAM manifests itself “spontaneously” that people with HSAM have a heightened trust in memory accuracy (Patihis, 2015 ), a more liberal response bias criterion (Frithsen et al., 2018 ), higher absorption, and fantasy proneness (Patihis, 2015 ), and possibly have a stronger associative memory for faces (LePort et al., 2017 ). Our synthesised results demonstrate that performance in tasks that measure other aspects of memory or cognition is entirely within normal age limits. Considering these findings, Roediger and McDermott ( 2013 ) present an interesting explanation of why HSAM individuals do not excel at other laboratory-based memory tasks. In line with meta-analytical findings (McDermott et al., 2009 ), the authors argue that the specificity of performance enhancement observed in HSAM reflects the retrograde versus anterograde distinction (i.e., ABM for life events and learning of new episodic information in the laboratory, respectively) that characterises episodic memory, with HSAM individuals showing superior levels of retrograde retrieval only. These two distinct forms of retrieval are tested with instruments that prompt different sets of sills, i.e., anterograde memory tasks require participants to engage in retrieval as well as encoding (as the material is new), while retrograde memory requires participants to engage in retrieval only (as it is assumed that encoding occurred in their autobiographical past). Clinical data also offer support to this explanation. It is well established that the systems underlying retrograde and anterograde memory are dissociable (Smith et al., 2013 ); patients with damage to certain brain areas cannot learn and retrieve new memories, but their ability to recall older autobiographical information remains intact. This separation in memory types could also explain why memory champions that possess a form of highly superior memory (Dresler et al., 2017 ; Foer, 2011 ; Maguire et al., 2002 ) only excel at laboratory-like tasks of remembering (Roediger and McDermot, 2013 ). Moreover, whilst both forms of memory retrieval (HSAM and memory champions) can be defined as superior, it should be emphasised that the latter have a “normal” memory that is extremely well-trained and that involves specific learning strategies (for a critical review on the use of strategies in the context of learning and cognitive plasticity, please see Lövdén et al. ( 2010 )). Another possibility is that the self-referential component of ABM is what separates personal memory from the purely episodic memory system. Literature has shown that when the self is involved during encoding, people are better at remembering both past events (Stendardi et al., 2021 ) and imagined future events (Jeunehomme & D'Argembeau, 2021 ). How the role of the self could be related to HSAM has been considered in greater detail earlier in the discussion.

Overall, we argue that the only defining behavioural characteristics substantially supported in the literature are those we have described, including speed of retrieval, number of details remembered, and public event knowledge, that have been objectively measured. As this area remains largely under researched, mainly due to the low frequency of HSAM, future research could lead to further development of this description. This review has highlighted that in HSAM retrieval is vastly heightened, while memory consolidation is possibly enriched. Less is known about how encoding occurs, due to lack of neural data, but we hypothesise that it is comparable to general ABM and in this way is likely susceptible to false memories. The ultimate goal of understanding exceptional memory is to design therapeutic targets that could combat memory impairment. Functional neuroimaging (e.g., fMRI or functional near-infrared spectroscopy, fNIRS), neurophysiological (electroencephalography, EEG) and neuromodulation (transcranial magnetic stimulation, TMS) studies on HSAM could guide researchers to discover target areas, the stimulation of which could enhance ABM (Santangelo et al., 2022 ; for the first HSAM short report using TMS, see Talbot et al., 2022 ).

Availability of Data and Materials

Not applicable.

Readers should note that when occupations are not described within the articles, this does not necessarily mean a HSAM participant is unemployed. News articles on the topic of hyperthymesia have reported that HSAM individuals work in an array of professions, including a professional violinist, radio news anchor and an actress (Stahl, 2014 ).

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Listen to Correa & Nath ( 2022 ) for Podcast anecdotal evidence from a HSAM participant about exercising their own memory.

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Long-term memory

The subjective experience of autobiographical memory

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Nature Reviews Psychology volume  2 ,  page 330 ( 2023 ) Cite this article

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In my favourite episode of the science fiction TV show, Black Mirror , humans are equipped with an internal recording device that allows them to store and replay experiences from their lives with excruciating accuracy. However, decades of research in cognitive neuroscience have demonstrated that memory processing in human brains operates quite differently from this fictional depiction. It is now understood that people store discrete pieces of information (such as the spatial layout, emotional context or sensory details) and that these are recombined at retrieval to form a coherent representation of the past. Thus, episodic memory is not a facsimile of the original event, but rather a dynamic reconstruction that is influenced by external factors such as acute mood states or task demands. For example, when recalling falling off a ski lift, one might describe the experience with a high degree of precision when speaking to a paramedic but embellish the story when later recounting it to a friend at a party.

Ageing is another factor that alters the ability to recall personal past experiences. Using ‘simple’ stimulus sets such as word lists or pictures, memory researchers have established that episodic memory naturally declines with age. Although these studies offered a high degree of experimental rigour, they did not adequately test memory for the personally significant and multisensory experiences that make up everyday life. Thus, the effect of ageing on episodic memory for rich, complex events remained unknown.

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Original article

Levine, B. et al. Aging and autobiographical memory: dissociating episodic from semantic retrieval. Psychol. Aging 17 , 677–689 (2002)

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In This Article Expand or collapse the "in this article" section Autobiographical Memories

Introduction, general overviews.

• Methodology
• Brain Mechanisms
• Autobiographical Memory in Early Childhood
• Childhood Amnesia
• Life Narratives in Adolescence and Adulthood
• Autobiographical Memory and Aging
• Reminiscence Bump
• Flashbulb Memory
• Autobiographical Memory and the Self
• Emotional Memory and Memory for Trauma
• Culture and Autobiographical Memory
• Collective Memory
• Functions of Autobiographical Memory
• False Memory and Suggestibility
• Involuntary Memory
• Autobiographical Memory in Psychopathology
• Episodic Future Thinking

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Autobiographical Memories by Qi Wang , Çağla Aydın , Jessica Zoe Klemfuss LAST REVIEWED: 29 April 2015 LAST MODIFIED: 29 April 2015 DOI: 10.1093/obo/9780199828340-0009

Autobiography as a literary genre has existed for centuries—with Augustine’s (354–430) Confessions being commonly regarded as the first Western autobiography—and has gained increasing popularity in the modern and postmodern eras. The scientific study of autobiographical memory, however, is relatively recent. Autobiographical memories, as the name itself reveals, can be literally taken as the memories that we would write about in our autobiography, if we ever decided to write one, so that we might tell people who we are and how we have become what we are. Autobiographical memories are the memories of significant personal events and experiences from an individual’s life. Research on autobiographical memory has grown with continuous momentum since the mid-1980s. This is in response to the call made by leading cognitive psychologists such as Ulric Neisser to study human memory in natural contexts. It also reflects the increasing interests in pop culture and the research community in life histories and narrative self-making. The rapid development in autobiographical memory research further signals the practical importance of such memory in clinical, legal, and everyday settings. The study of autobiographical memory is now a dynamic, interdisciplinary research field that encompasses exciting discoveries, theoretical debates, controversial issues, intriguing phenomena, and emerging interests. It attracts researchers from all sorts of psychological subdisciplines—cognitive, developmental, social and personality, cultural, clinical, neuroscience—as well as other social sciences and humanities. The first section of this bibliography introduces general overviews about autobiographical memory, focusing on the theoretical discussion concerning its definition, organization, and functioning. The following section on textbooks provides selected resources to help the reader gain initial access to the diverse theoretical and empirical approaches to autobiographical memory and related phenomena. The next section is devoted to methodology, introducing the commonly used methods in the study of autobiographical memory. The bibliography’s remaining sections examine particular issues, questions, and areas that are of current interest to researchers in this field.

Autobiographical memory is generally considered a subset of episodic memory. Episodic memory refers to the conscious recollection of specific events that took place at a particular point in time in the past, involving such information as what, where, and when. It supports the mental time travel of the self to relive previous experiences. Endel Tulving calls episodic memory “a true marvel of nature” ( Tulving 2002 , p. 3). Tulving views episodic memory as a major neurocognitive memory system distinct from semantic memory, which deals with context-free, general knowledge of the world. Not all episodic memories (e.g., where and what did you eat last Thursday) become part of one’s autobiographical history, however. Only those that are highly significant to the individual constitute autobiographical memories. Conway and Rubin 1993 highlights the personal relevance in their definition of autobiographical memory. Nelson 1993 discusses the functional importance of autobiographical memory from an evolutionary standpoint, emphasizing the unique role of such memory in defining the self and facilitating social integration. These three seminal articles are a good place to start in order to understand what autobiographical memory is.

Conway, Martin A., and David C. Rubin. 1993. The structure of autobiographical memory. In Theories of memory . Edited by Alan F. Collins, Susan E. Gathercole, Martin A. Conway, and Peter E. Morris, 103–137. Hillsdale, NJ: Lawrence Erlbaum.

A theoretical discussion of the role of the self and personal relevance in autobiographical memory formation and retrieval. Relates the proposal to empirical work.

Nelson, Katherine. 1993. The psychological and social origins of autobiographical memory. Psychological Science 4:7–14.

DOI: 10.1111/j.1467-9280.1993.tb00548.x

With a particular focus on the development of autobiographical memory, this review piece situates the origins of memory in a sociocultural context. Provides a clear theoretical formulation of how language and narrative are integral in autobiographical memory development.

Tulving, Endel. 2002. Episodic memory: From mind to brain. Annual Review of Psychology 53:1–25.

DOI: 10.1146/annurev.psych.53.100901.135114

With a particular focus on mental time travel (autonoetic consciousness) as the core defining feature of episodic memory, this seminal article highlights differences between episodic and semantic memory and the development of the study of episodic memory (e.g., functional magnetic resonance imaging [fMRI] studies).

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Peer-reviewed

Research Article

Understanding the reminiscence bump: A systematic review

Contributed equally to this work with: Khadeeja Munawar, Sara K. Kuhn

Roles Conceptualization, Data curation, Formal analysis, Project administration, Writing – original draft

Affiliations Department of Psychology, Jeffrey Cheah School of Medicine and Health Sciences, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, Subang Jaya, Selangor, Malaysia, Department of Psychology, University of Wah, Wah Cantt, Pakistan

Roles Writing – review & editing

Affiliation Department of Teaching and Learning, College of Education and Human Development, University of North Dakota, Grand Forks, North Dakota, United States of America

Roles Conceptualization, Funding acquisition, Project administration, Supervision, Writing – review & editing

* E-mail: [email protected]

Affiliation Department of Psychology, Jeffrey Cheah School of Medicine and Health Sciences, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, Subang Jaya, Selangor, Malaysia

• Khadeeja Munawar, 
• Sara K. Kuhn, 
• Shamsul Haque

• Published: December 11, 2018
• https://doi.org/10.1371/journal.pone.0208595
• Reader Comments

One of the most consistently observed phenomena in autobiographical memory research is the reminiscence bump: a tendency for middle-aged and elderly people to access more personal memories from approximately 10–30 years of age. This systematic review (PROSPERO 2017:CRD42017076695) aimed to synthesize peer-reviewed literature pertaining to the reminiscence bump. The researchers conducted searches in nine databases for studies published between the date of inception of each database and the year 2017. Keywords used included: reminiscence, bump, peak, surge, blip, reminiscence effect, and reminiscence component. Sixty-eight quantitative studies, out of 523, met the inclusion criteria. The researchers implemented a thematic analytic technique for data extraction. Four main themes were generated: methods of memory activation/instruction for life scripts, types of memory/life scripts recalled, location of the reminiscence bump, and theoretical accounts for the bump. The two prevailing methods of memory activation implemented were the cuing method and important memories method. Three types of memories/life scripts were recalled: personal/autobiographical memory, memories for public events, and life script events. The findings illustrate differing temporal periods for the bump: approximately 10–30 years for memories for important events, approximately 5–30 years for memories that were induced by word cues, and 6–39 years for studies using life scripts. In explaining the bump, the narrative/identity account and cultural life script account received the most support.

Citation: Munawar K, Kuhn SK, Haque S (2018) Understanding the reminiscence bump: A systematic review. PLoS ONE 13(12): e0208595. https://doi.org/10.1371/journal.pone.0208595

Editor: Maria Semkovska, University of Limerick, IRELAND

Received: March 14, 2018; Accepted: November 20, 2018; Published: December 11, 2018

Copyright: © 2018 Munawar et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper and its Supporting Information files.

Funding: Khadeeja Munawar was supported by a Monash University Malaysia Higher Degree by Research Scholarship. The authors acknowledge Global Asia in the 21st Century (GA21) Platform at Monash University Malaysia for the open access publication funding support. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

When examining the life span distribution of autobiographical memories (AMs), three phenomena are revealed. The first is childhood amnesia, or the limited recollection of AMs from a very young age, which is present in the life span retrieval curve as a steadily rising function between 0–8 years of age [ 1 ]. The second, the recency effect, dictates that memories recalled by most individuals are of recent events, and the frequency of these memories decline gradually [ 2 ]. Lastly, the reminiscence bump—also known as “the bump”—enhances memory recall from approximately 10–30 years of age by people over the age of 30 [ 3 ], and is considered one of the most robust findings in AM research [ 4 ]. As the reminiscence bump features deviation from the standard forgetting curve and forgetting functions [ 2 , 3 ], it is recognized as a peculiar phenomenon and defining feature of AM [ 2 , 3 ]. The reminiscence bump concept is included in most introductory psychology textbooks due to its significance to AM and the field of cognitive psychology [ 5 – 7 ].

The distribution of AMs across the adult life span is often studied through two major types of cueing techniques: the word cuing method and the important memories method [ 8 ]. There are various other methods of activating memories, some of them are; writing home diaries, free-recall of public and private items of news, and answering factual, semantic, general-knowledge, multiple-choice questions about the Academy Awards, the World Series, and current events [ 9 – 11 ]. The word cuing method, originally developed by Francis Galton, was later modified by Crovitz and Schiffman [ 12 , 13 ]. In this technique, participants retrieve and report memories in response to word cues commonly used in everyday conversation [ 14 – 17 ]. The word cuing method was used rigorously in investigating personal memories during the 1970s and 1980s [ 12 , 18 – 20 ]. In the important memories method, participants are instructed to retrieve and report the most important memories from their life [ 8 , 15 , 21 , 22 ], especially the vivid ones [ 23 – 25 ].

A significant amount of research emerging in the last two decades, claims that the previously found reminiscence bump in AM also extends to public events [ 26 ]. Research shows that public events which occurred during adolescence or early adulthood, approximately from the age of 12 to 29 years, are preferentially recalled [ 27 ]. This phenomenon is assessed through two major methodologies: the first asks participants to name significant events from recent history [ 28 ], and the second assesses participants’ level of factual knowledge of specific events [ 11 ].

Researchers propose various theoretical accounts to explain the reminiscence bump [ 8 ], including the: cognitive account, cognitive abilities account, cultural life script account, and narrative/identity account. The cognitive account postulates that it is simply the novelty of many events occurring in the second and third decades of life that is the major factor leading to enhanced memory recall from this period [ 29 ]. According to the cognitive abilities account, people become better equipped to learn, process, and retain information as they move into adolescence and early adulthood due to the maturation of the brain, which leads to maximal cognitive and neurological functioning [ 30 ]. According to the cultural life script account, individuals recall more events from the second and third decades of life because of cultural prescriptions and expectations present in the life script [ 31 – 33 ].

The narrative/identity account states that events occurring during adolescence and early adulthood are vital to the development of the individual’s adult identity. It is this time when an individual engages in activities and relationships that define who the person will finally become, and how they narrate the stories of their lives [ 11 , 24 , 34 , 35 ]. By the time an individual reaches this period of life, the effect of novel experiences on long-term memory, recognition, self-identity, and the development and consolidation of goals, have typically been demonstrated [ 36 , 37 ]. Experiences acquired during this period are integrated into an individual’s lifelong narratives, thus they are more easily recalled later in life. The critical role of early adulthood AMs in identity formation is illustrated in neuropsychological and developmental research [ 38 , 39 ]. The working self is viewed as playing a major role in organizing AMs; and events during this critical period are used as identity markers for the remainder of life each time AMs are reconstructed [ 40 ]. Broadly, the self (or identity) is conceptualized as a multidimensional and complicated set of self-related processes and schema [ 36 , 41 , 42 ].

The research identifies two components of the reminiscence bump: one relating to social identity (i.e. AMs corresponding chiefly to public events individuals experienced when ages 10 to 19 years old), and the other relating to personal identity (i.e. AMs corresponding to personal events that happened between the ages of 20–29) [ 10 ]. While social identity develops, individuals associate themselves with specific cultural, social, political, and/or religious groups with whom they have similar goals and desires [ 43 ]. Alternatively, during the development of personal identity, desires and goals towards establishing interaction with significant others and forming intimate relationships are developed [ 36 ]. The enhanced recollection of social events results in the formation of a reminiscence bump for the ages of 10–19 years, while developing close personal relationships results in a bump for the ages of 20–29 years [ 10 ].

Cultural life scripts are stereotypical episodes comprising multiple events in a specific order, with every event allowing succeeding events to occur [ 44 ]. They are a series of events which occur in a particular sequence and characterize a prototypical life span in a certain culture [ 31 – 33 ]. The scripts have slots with specific conditions for what is allowed to fill them [ 45 ]. The slots in cultural life scripts are culturally significant transitional events likely to occur in a prototypical life span in a certain culture [ 46 ]. According to one study, an important life script characteristic is that it represents a culturally shared part of our semantic knowledge; not the outcome of a few personal experiences [ 32 ]. Another research study opposes this finding, reporting that cultural life scripts are not part of our shared semantic knowledge [ 47 ].

The cultural life script account is based on observation; as reported by Neugarten, Moore and Lowe (1965): Certain age norms are present in every society which organize the expectations, and structure the behaviour, of individuals [ 46 ]. There are prescriptive timetables in every culture for the arrangement of significant life events (e.g., finish school, get a job, get married, and have the first child) and the individuals of that particular culture are aware of these age norms [ 48 ]. Individuals also manage their own timing for the events on these timetables, and assess if they are achieving significant events earlier, or later, than anticipated [ 49 ]. The mechanisms underlying the bump for public events may be different than the mechanisms underlying the bump in autobiographical memory.

The earlier review papers summarized the studies on retention function, reanalyzed the previous findings, reviewed the temporal location of the bump according to different cueing methods, and assessed the current theoretical accounts of the bump in light of the temporal locations of the bump [ 2 , 4 , 8 , 26 ]. These studies presented different bump periods for different methods of memory activation. The past reviews summarized the empirical evidence on the bump from non-clinical sample and excluded findings from the clinical samples as well as immigrants as they were interest in "the location of the bump in the general population" (p.67, Koppel & Berntsen, 2015). The differences in location of the bump with respect to cuing methods has already been shown in the past research studies [ 15 , 16 ]. However, the mechanisms underlying the bump for autobiographical memory activated by different cuing methods (e.g., word cuing method and important memory method) might be different as well as the mechanisms underlying the bump for different types of responses (autobiographical memory vs. Life script events) could also be different.

As the bump is one of the most robust findings in autobiographical memory research, the present review paper added to the empirical body of literature on the bump and attempted to take a step a little further by including and reporting past studies’ findings on: general population, clinical samples, immigrants, or any other samples. Furthermore, no restriction was applied on methods of memory activation and research studies assessing the reminiscence bump through methods other than word cuing methods and important methods were also screened for eligibility. The last review paper on bump was published in 2015, therefore, this review was conducted with the belief that more recent and latest findings from the studies could be assessed and summarized.

The authors attempted to present a thorough summary of all the existing primary research studies on the bump, tried to establish the state of existing knowledge and reviewed the bump for various types of memories apart from autobiographical memories, for instance, flashbulb memories and memories public/private events. The authors developed a clearly defined, predetermined eligibility and relevance criteria for including research studies; the methodology was reproducible, transparent and systematic; carried out a meticulous search to identify all suitable studies; assessed quality of included studies, and systematically synthesized all the evidence in the form of figures and tables. The authors tried to limit selection bias and random error which have been found to mislead the reviews [ 50 , 51 ], and attempted to present a reliable summary of the existing knowledge.

In the existing literature, there is currently no systematic review on the reminiscence bump which implements the guidelines proposed in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [ 52 ]. However, a general review of the location of the reminiscence bump across different methods of memory activation provides additional references and descriptions [ 8 ]. Although there are several theoretical accounts for the bump for various kinds of memories, and all receive some support in the literature, it is necessary to investigate the relative plausibility of each account. The temporal location of the bump is not presented consistently in studies using varied methods of memory activation. The mechanisms underlying the bump may be different across different memory domains and types of memory assessments.

Discovering the most likely temporal location of the reminiscence bump is one of the authors’ aims in conducting the systematic literature review presented in this paper. Another goal of the researchers is to ascertain which theoretical account for the reminiscence bump received the most overarching support, by examining the literature for reported temporal locations of the bump in relation to the methods used to activate different types of memories in participants from various countries.

The authors implemented the PRISMA statement guidelines in designing this systematic review [ 52 ]. After the researchers developed the review protocol, they registered the protocol in PROSPERO (International prospective register of systematic reviews; please see S1 File ) prior to the commencement of the review (registration number CRD42017076695) [ 53 ].

Eligibility criteria

Eligible articles were required to present original research on the bump from qualitative, experimental, quasi-experimental, non-experimental, observational, or mixed-method studies. Neither language of published article, nor sample age group was limited. Only articles published in peer-reviewed or refereed journals were selected. Grey literature and articles that did not mention the reminiscence bump or its synonyms in their titles or abstracts, were excluded. No restriction was imposed on date of publication to allow for a comprehensive background on—and theoretical progression for—the reminiscence bump over time, as presented in the research.

Systematic search strategy

The researchers conducted a systematic search to locate primary source articles. Synonymic keywords searched in each database, using the Boolean OR operator [ 54 ] and wildcard features (e.g. placing an asterisk at the end of a root word to account for a variety of word endings), included: reminiscence*, bump, peak, surge, blip, reminiscence effect*, and reminiscence component (please see S1 Table ). The search strategy combined these synonymic keywords (with OR) to search the following 9 databases for articles published, or added, to the databases between the date of inception of each database and 2017: Ovid MEDLINE, Ovid Embase, Ovid Emcare, CINAHL Plus (EBSCOhost), Proquest Central, PsycInfo, Scopus, Pubmed, and ScienceDirect (please see S2 Table ).

The researchers retrieved a total of 523 records through this search strategy. They then “hand searched” the 523 articles’ reference lists to obtain further relevant studies, yielding 47 additional citations ( Fig 1 ). After the researchers removed duplicates, 261 articles remained.

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https://doi.org/10.1371/journal.pone.0208595.g001

The 261 article records were imported into the EndNote reference/citation manager and screened for eligibility/inclusion in the review. The researchers screened titles and abstracts for inclusion criteria, removing 181 studies that did not meet the criteria and retaining 80 full-text articles to assess for eligibility. After assessing the 80 full-text articles, 12 articles were excluded for not meeting inclusion criteria (e.g. not published in peer reviewed journals, or format was a brief report and not a research article). The researchers hand searched the remaining 68 articles’ references for relevant articles. No additional articles were included from this search. The final group of 68 studies was assessed for methodological quality, after which the researchers performed data extraction and synthesis.

Methodological assessment

The 68 quantitative studies were assessed for quality using the 14 criteria developed by Kmet, Lee, and Cook [ 55 ]. No qualitative or mixed-method studies were present. An overall rating (from 0 to 1) was assigned to every study; higher numerical ratings indicated higher quality. Previous systematic reviews employing the QualSyst quality assessment protocol required a minimum threshold score of 0.55 for study inclusion [ 56 ]. Other reviews included studies falling within the range of 0.74 to 0.91[ 57 ]. The lowest quality rating of studies included in this review was determined to be 0.54, therefore all studies were considered eligible for inclusion. To minimize the risk of bias, two reviewers worked independently to screen studies and extract and synthesize data. Disagreements were settled by applying the 14 criteria [ 55 ] (please see S3 Table ).

Data collection and extraction

The researchers used forms to extracted the data for retrieving relevant information to assess the aims of the review [ 58 ]. They placed extracted information under appropriate sections corresponding to: author, year, country; study objective; sample size (N), and findings ( Table 1 )

https://doi.org/10.1371/journal.pone.0208595.t001

Data synthesis

The researchers employed a narrative synthesis approach to abridge extracted study data. Narrative synthesis was deemed appropriate as it allows for both the synthesis of findings from several studies that use different research designs with varying characteristics of samples, and the application of an overall meaning to the data [ 115 ]. Through extraction, formulation (via a data extraction chart), and translation of the data into narrative summaries, the researchers utilized all accessible data. The initial stage of data familiarization occurred through the repeated systematic review of each research article. Formatting initial codes resulted in data refinement and the generation of themes from the data. Before allocating descriptive terms, themes were further refined so that the crux of all themes could be captured. These themes assisted in generating an analytic narrative during the final stage of report writing. Driven by the narrative synthesis approach, the data synthesis stage of this systematic review achieved the researchers’ goals of effectively drawing out—and giving meaning to—pertinent data from the research articles (please see S4 Table ).

Temporal and geographical representation of studies

The locations of the 68 studies varied: 19 in the U.S.; 9 in Denmark; 8 in the Netherlands; 7 in the United Kingdom; 4 in both Turkey and Japan; 3 in Germany, Canada and France, respectively; and 2 in Australia. One study took place in Bangladesh, Austria, Malaysia, and Trinidad and Tobago, respectively. Two studies included samples from more than one country: the first from Japan, Bangladesh, England, and China; and the second from the Netherlands and the United States (please see S1 Fig ). Dates of published studies ranged from 1988 to 2017: 13 studies from 1988 to 1999, 29 studies from 2000 to 2010, and 26 studies from 2011 to 2017 (please see S2 Fig ).

Research study themes

The articles selected for review comprised 68 quantitative studies (N = 68; see Fig 2 ).

https://doi.org/10.1371/journal.pone.0208595.g002

Four main themes emerged after reviewing these studies on the reminiscence bump: (a) methods of memory activation/instruction for life scripts, (b) types of memory /life scripts recalled, (c) location of the reminiscence bump, and (c) theoretical accounts for the bump. These four themes evolved from 13 sub-themes.

Method of memory activation/Instruction for life scripts.

The researchers used a variety of methods to recall memories or life events. Most studies implemented more than one recall method across a number of different populations.

Important memories method. Previous research consists of asking participants to report their important memories. Several studies ask participants about their most positive and negative, or most important and traumatic experiences, or important and surprising memories, used emotional cues (i.e. positive and negative), and asked participants to recall important positive and negative, or surprising positive and negative, events [ 22 , 33 , 35 , 59 , 72 , 73 , 100 , 104 , 107 ]. Furthermore, the past research studies focused on: (a) memories that are a vital component of life story, (b) life history timeline and significant life events narrative for the description of three events, (c) free narrative of life history about important life events and word cues, (d) descriptions of three self-defining memories, (e) Reminiscence Functions Scale and vivid memories that are important landmarks or turning points in life, (f) the Life Story Questionnaire for listing 15 personally important events or experiences, and (g) free narratives of life history; important life events [ 68 – 70 , 108 , 111 , 113 ]. Two questionnaire-based studies instructed participants to report self-defining “I am” statements which were later used in recalling memories as well as a life event list for gathering distributions of positive and negative events [ 91 , 93 ].

Word cuing method. A number of research studies use cuing methods for collecting memories, for instance odor cues, emotional cues and words cues. However, most of the studies used word cues for activating memories. The type of cues used, and their number, varies from study to study, and some studies use more than one cue recall method: (a) using word cues (i.e. emotional, emotion-provoking, and neutral); (b) instructing children to write future life stories using 10 word cues; (c) using both odor cues and word label cues; (d) reading novels and then implementing a cued recall task; (e) reporting important word-cue memories; (f) using 50 word cues for AMs; (g) employing 15 word cues pertaining to common locations, objects, positive emotions, negative emotions, and significant others; (h) Associative Memory Questionnaire with 18 word cues, (i) modified Autobiographical Memory Test having a list of 16 word cues; and (j) questionnaires implementing both the word cues and important memories method [ 17 , 60 – 62 , 64 , 88 , 89 , 96 , 114 , 116 ].

Several studies assess life events through: (a) the use of various emotional cues to elicit and record events in the course of a typical and hypothetical person, and (b) verbal reporting of events from personal lives and diaries. Cued recall methods include: 10 word cues, 40 word cues for obtaining specific memories, and 20 ambiguous and 20 unambiguous single names [ 24 , 75 , 109 ]. A few studies use the Galton-Crovitz cueing method and Robinson word cuing technique [ 30 , 76 – 78 , 81 , 82 , 86 , 105 , 106 ]. The number of word cues used to elicit memories range from 15–124 [ 15 , 16 , 74 ].

Life Scripts. The studies on life scripts used various instructions to explore the reminiscence bump. For instance, these studies assessed the reminiscence bump through: (a) asking participants to report important events likely to occur in the life of a newborn or an elderly person; (b) the life script questionnaire; and (c) the expected timing of the public event [ 32 , 71 , 84 , 87 , 101 ].

Other methods. Other memory recall methods that were not associated with the three methods already discussed, were found in some studies (e.g., dream diaries, the evaluation of participants’ reactions to nostalgic advertising, a modified version of timeline methods for recording specific memories, issues surrounding the Academy Awards, World Series, and current events, life regrets’ content and chronology, and free recall events having public or private nature[ 9 – 11 , 63 , 65 – 67 , 85 , 110 ]. Adults were interviewed to discover their: recounted and oral life stories; specific autobiographical lifetime events; three favorite books, movies, and records, or the five best football players; and memories using the Yearly News Memory Test (YNMT) comprising 30 open-ended and multiple-choice questions [ 79 , 80 , 83 , 97 , 99 , 102 , 103 , 112 ]. A few studies used music clips and instructed participants to imagine music as a means to access associated memories [ 90 , 92 , 94 , 98 ].

Types of memory/life scripts recalled.

The 68 research studies elicited a variety of responses in types of memories activated.

Personal /autobiographical memories. One study elicited personal future life stories of children, and events from those stories [ 60 ]. In studies where participants reported autobiographical events for various types of cues, retrieved memories were from personal pasts [ 15 , 16 , 30 , 33 , 59 , 61 – 63 , 69 , 74 , 75 , 76 – 78 , 81 , 86 , 89 , 90 , 92 , 93 , 96 , 98 , 104 – 106 , 109 , 114 ]. The following were elicited in various studies: personal memories of adults spanning their life course, meaningful life events, life stories from across participants' life spans, lists of life events for AMs and collective memories, life-lines for both past and future events, and self-defining AMs [ 17 , 22 , 35 , 66 , 70 , 72 , 73 , 91 , 95 , 97 , 99 , 102 , 103 , 107 , 108 , 110 – 113 ]. One study collected participants’ personal specific memories and specific memories related to 70- and 80-year-old hypothetical cases [ 68 ].

Life scripts. These studies asked participants about important events likely to occur in the life of a newborn and important events a newborn or an elderly person would experience during his/her lifetime [ 32 , 71 ]. A few research studies investigated original and modified versions of the life script questionnaire, and probed cultural expectations for the expected timing of the public event [ 84 , 87 , 101 ].

Public events. A number of research studies focused on memories for important public and private news items, as well as public events. [ 10 , 80 , 81 ]. In another study, authors elicited both autobiographical and public events [ 88 ].

Other memory types. Other research studies focused on curves of forgetting after memorizing a 10-chapter autobiographical novel, flashbulb memories, AM recall, and memories from the lives of participants’ parents [ 24 , 64 ]. Some studies investigated dreams of older adults’ life regrets; flashbulb memories; reactions to nostalgic advertisements; and responses to the World Series, academy awards, and current events [ 9 , 11 , 65 , 67 , 85 ]. And in other studies, researchers asked participants to report three favorite books, movies, and records as well as the five best sports players of all time [ 79 , 83 , 94 , 100 ].

Location of the bump.

The temporal range of the bump varied along with the method of memory activation and presence or absence of a mental health issue (see Table 2 ).

https://doi.org/10.1371/journal.pone.0208595.t002

Important memories method. Research studies using this method revealed the location of the bump to be from a minimum of 10 years to a maximum of 40 years of age, and some studies also showed a more localized peak from ages 16 to 20 years of age [ 33 , 35 , 59 , 68 , 93 , 100 , 104 , 107 , 113 ]. Findings of another study did not show the bump for AM distribution, however, a bump appeared for the life script distribution as a result of suppressing typical life events [ 69 ]. Three age intervals corresponded to the bump period (i.e. 16–20, 21–25, and 26–30 years) [ 69 ]. The responses to a newborn questionnaire demonstrated a considerably large bump for positive events in the third decade of life, a very small bump for negative events in the second decade of life, and a clear bump for positive events between 10 to 30 years of age [ 71 ]. The range of the bumps for positive, negative, expected, unexpected events, and significant life events narrative were between 16 to 30 years and 20–29 years of ages, respectively [ 22 , 70 , 72 , 73 , 91 ]. A few studies compared the bumps of control groups and patients with schizophrenia and found different results for both groups: 15–19 and 16–25 years for patients with schizophrenia; 20–24 and 21–25 years for control groups [ 108 , 111 ].

Word cuing method. Using these methods, the distribution of the future life stories of Danish children illustrated a bump in young adulthood, and this bump consisted of life-script events [ 60 ]. Studies discovered bumps from a minimum of 6 years of age to a maximum of 35 years of age [ 11 , 15 , 16 , 24 , 30 , 61 , 62 , 74 – 76 , 78 , 81 , 82 , 88 , 89 , 95 , 106 , 109 , 114 ]. One study, using a novel cuing method, revealed two bumps: The first bump appeared when the protagonist of the story was 22 years old, and the second occurred when she was in her 50s and undergoing significant life changes [ 64 ]. Studies revealed the timing of the bump to be between the ages of 10 to 30, with peaks occurring between 15–18 for men, 13–14 for women, 5–13 for Japanese adults, and from 5–30 for personal and collective memories; a peak in recall occurred between 10–19 years of age for public items and 20–29 years of age for private items, and a bump between 10 to 30 years of age [ 17 , 77 , 78 , 86 , 105 ].

Life scripts. The studies on life scripts revealed varying locations of the bump. These studies revealed the bumps of positive and negative events from a minimum of 6 to a maximum of 39 years of age [ 32 , 71 , 84 , 87 , 101 ].

Other methods. A study using a heterogeneous sample from five countries (i.e. Japan, Bangladesh, U.K., China and the U.S.) revealed that more than 50% of memories were recalled from the ages of 10–30 [ 63 ]. Studies also showed a bump in late adolescence to early adulthood as well as a bump between 15 to 24 years of age [ 9 , 11 , 65 , 67 , 79 , 85 , 92 , 94 , 99 ]. Two studies, using a life history timeline method and a “life-line” interview method, discovered the bump between 10–30 and 10–40 years of age, respectively [ 66 , 110 ]. A study using cues explored memories of younger and older Bangladeshi individuals: Younger adults showed a bump between 10–30 years of age, whereas an older group revealed a second bump between 35–55 years of age [ 62 ]. The bump corresponding to ages of immigrants at the time of immigration, occurred between 6–30 and 10–25 years of age [ 80 , 97 , 98 , 102 , 103 ]. A study using free recall flashbulb memories from personal lives reported a bump between 10 to 30 years of age [ 67 ]. A study conducted on patients with schizophrenia revealed bump between 16–25 years for patients and 21–25 for controls [ 112 ].

Theoretical accounts for the bump.

There are a variety of theoretical accounts for the bump, and each account has received varied levels of support in the research. The narrative/identity account is fully supported by the findings of seven studies [ 9 , 10 , 35 , 36 , 62 , 70 , 93 ], and partially supported by the findings of six studies [ 24 , 72 , 76 , 108 , 111 , 112 ]. Two studies illustrated complete support for the cultural life script account [ 33 , 71 ], while ten studies showed partial support [ 32 , 60 , 75 , 87 , 89 , 100 – 103 , 114 ], and two studies demonstrated no support at all [ 59 , 84 ].

Four studies supported the “life story” account [ 22 , 66 , 73 , 105 ], three studies supported differential encoding and differential sampling [ 78 , 79 , 92 ], one study supported the “life-span perspective” [ 110 ], one study supported the cognitive account [ 97 ], and two studies showed some support for the “biological-maturational” account [ 30 , 74 ]. Several research studies supported more than one theoretical account for the bump: the cultural life script and “novelty” accounts [ 64 ]; the narrative/identity and cultural life script accounts [ 65 , 68 , 107 ]; the narrative/identity, cognitive and “maturational” accounts [ 67 , 96 , 109 ]; the cognitive, narrative/identity, cultural life script, and life story accounts [ 69 , 94 ]; the cognitive, narrative/identity, and cultural life script accounts [ 80 ]; the biological, narrative/identity, and cultural life script accounts [ 83 ]; and the cognitive and narrative/identity accounts [ 11 ].

Despite the wealth of evidence existing on aspects of the reminiscence bump that present when using different methods for activating memories, there is a limited understanding of which theoretical account—or accounts—offers the best explanation for the bump, and the reasons for variation in the exact location of the bump. The aim of this systematic review is to establish a current evidence base concerning the understanding and formation of the bump, and to add to the existing body of literature on AMs, other kinds of memories and the reminiscence bump.

Summary of study findings

The results of this systematic review on the reminiscence bump are based on the analysis of 68 selected studies retrieved from 9 scientific databases and screened by the researchers. The results reveal that methods for activating memories/instruction for life scripts include the important memories method [ 33 , 69 , 100 ], word cuing method [ 17 , 60 , 61 , 64 , 68 , 114 ], life scripts [ 32 , 71 , 84 , 87 , 101 ], and other heterogeneous methods [ 72 , 73 , 79 , 80 , 83 , 97 , 99 , 102 , 103 ]. A variety of responses were elicited from the participants including: AMs [ 15 , 16 , 30 , 33 , 59 , 61 – 63 , 74 , 76 – 78 , 81 , 86 , 90 , 92 , 93 , 96 , 98 , 104 – 106 , 114 ], memories for public events [ 10 , 80 , 81 ], life scripts [ 32 , 71 , 84 , 87 , 101 ], and other heterogeneous responses [ 79 , 83 , 94 , 100 ].

The exact location of the bump varied with each method for activating memories. For instance, with the important memories method, studies showed the bump between 10–30 years of age [ 11 , 22 , 32 , 33 , 35 , 65 , 67 , 69 , 71 , 84 , 87 , 93 , 95 , 100 ]. For word cuing method, the bump began as early as 5 years, and lasted until as late as 30 years of age [ 10 , 17 , 77 , 78 , 86 , 114 ]. Likewise, for the studies using life scripts, the location of the bump was from 6 to 39 years. Also, there are a variety of theoretical accounts offering an explanation for the bump for different kinds of memories activated, such as the narrative/identity account which received significant support from eight studies [ 9 , 10 , 24 , 35 , 63 , 70 , 93 , 94 ], the life story account garnering sound support in four studies [ 22 , 66 , 73 , 105 ], and the cultural life script account finding substantial support in two studies [ 33 , 71 ]. The narrative/identity account received partial support from seven studies [ 10 , 24 , 72 , 76 , 108 , 111 , 112 ], and the cultural life script account received a degree of support from ten studies [ 32 , 60 , 75 , 87 , 89 , 100 – 103 , 114 ].

Interpretation of study findings

Past research indicates that the cues used to induce the memories influence both the proportion of memories recalled and the location of the bump [ 8 , 15 ]. A variety of cuing methods exist, therefore different retrieval processes help to explain the differences in reported location of the bump and the distribution of AMs [ 117 ]. The research demonstrates that word cues initiate an associative, “bottom-up” process in memory, whereas the important memories method prompts a strategic, “top-down” process in memory that is organized around important memories [ 8 , 12 ]. The Attention-to-Memory hypothesis [ 118 , 119 ], proposes that the two major brain regions playing different roles in attention are the dorsal parietal cortex and the ventral parietal cortex. The dorsal parietal cortex is associated with top-down attention (i.e. selecting stimuli on the basis of the internal goals of the individual); the ventral parietal cortex is concerned with bottom-up attention (i.e. permitting the detection of related stimuli) [ 120 , 121 ].

The Attention-to-Memory hypothesis states that in addition to playing a significant role in attention, the two cortexes play similar roles in memory retrieval [ 122 ]. The dorsal parietal cortex initiates the assignment of attentional resources towards retrieval of a specific memory (i.e. top-down Attention-to-Memory) [ 122 ]. The important memories method supports this, as the dorsal parietal cortex initiates top-down attention when retrieval relies on memory search [ 8 , 12 ]. Alternatively, in word cuing methods, the ventral parietal cortex initiates a bottom-up attention focus on the basis of retrieved content. Recent reviews on recognition memory studies support these theories and the localization of the top-down and bottom-up attention [ 123 ]. The instructions for the important memories method initiate a search for relevant memories—a role of the dorsal parietal cortex. The instructions for memories related to word cues initiate rapid detection of memory content—a role of the ventral parietal cortex.

Memory activation methods play a significant role in the nature of the memories activated. Word cuing methods yield unbiased sampling of memories across the entire life span [ 12 ], whereas the important memories method focuses on eliciting the most important memories of a person’s life, and tends to produce a narrative-based search [ 8 , 12 ]. Important and self-defining memories are closely linked to the meaning-making processes of individuals [ 124 , 125 ]. The different types of memory activation methods and theoretical accounts of the bump have common underlying mechanisms influencing bump location. The range and location of the bump vary according to memory activation method: word cuing methods yield a disproportionately large number of recent memories and an earlier bump location (see Table 2 ), as well as with respect to the type of memories activated. Differing locations of the bump have significant implications for theoretical accounts explaining the bump [ 8 ]. In this review, the researchers present a general range of the bump approximated through the analysis of all studies: 16–30 years of age for the important memories method; 5–30 years of age for word cuing methods. A past review paper calculated the mean range and midpoint of the bump formed through different cuing methods. The mean range of bump for word cued and important memories was calculated between 8.7 to 22.5 and 15.1 to 27.9 years of age, respectively [ 8 ]. The differences in location of the bump could be due to different methods of activating memories or differences in memory types.

Disparate bump ranges are indicative of the processes occurring at retrieval, favoring a retrieval-based account of the bump. This contradicts the accounts focusing on characteristics of the memories themselves (i.e. narrative/identity account and cognitive account) [ 11 , 24 , 25 , 35 , 40 ], or the effectiveness of encoding during the bump period (i.e. cognitive abilities account) [ 11 , 126 ]. These findings suggest a schema-based explanation of the bump (i.e. cultural life script account) rather than an individualistic and memory-based account [ 32 ]. There is considerable supporting evidence for the role of cultural life scripts in organizing the retrieval of AMs for important and emotional events [ 32 , 33 , 68 , 125 , 127 ]. According to Rubin (2015), variations in bump peaks cannot be explained solely in terms of encoding, or by theoretical accounts, especially those that consider adolescence and early adulthood periods to be when the emergence of identity or heightened cognitive ability occurs [ 3 , 11 , 117 ]. Different methods of activating memories give rise to different bumps and the mechanism underlying the bumps for different types of memories are different. The mechanism is different when other kinds of responses or memories are elicited (e.g., life scripts, dreams, etc.)

Life scripts allow encoding and rehearsal of an event by attributing the personal events to some culturally shared importance. A majority of the life script events are anticipated, prepared for, and given a certain meaning before they even happen in a person’s life [ 32 ]. According to established empirical evidence, the recall of important life events is structured by the life scripts, however, there is no such structuring of life events through cues, as cues are not likely to initiate culturally shared schemata for important transitional events [ 46 ]. The cultural life script account is possibly the reason behind the prevalence of important memories in the bump and a greater proportion of life script events in important memories.

This review demonstrates that the cultural life script account received considerable support, yet accepting it as a possible explanation for the bump would be problematic owing to some inherent flaws. The cultural life script account utilizes the concept of life scripts—cultural expectations about the timing and arrangement of significant transitional life events in a prototypical life course—to provide a cultural explanation for the bump [ 31 – 33 ]. The empirical evidence partially supporting the cultural life script account also employed other theoretical accounts to explain these findings [ 32 , 60 , 75 , 87 , 89 , 100 – 103 , 114 ]. Very few studies have compared the life script and real life events [ 68 , 75 ]. Thus, there is a lack of evidence for comparison of the bump patterns for life script events and real life events, and the similarity between the cultural life script events and non-scripted events [ 33 , 60 , 68 , 128 ]. The similar bump patterns for both life script events and real life stories, run contrary to the premise of the cultural life script account, meaning that the cultural life script is not solely responsible for the recall of the events and formation of the bump [ 32 ].

The fact that the bump is not based on the age of the memories, but on the age of the person recalling at the time of encoding, implies that findings of an artifactual, retrieval based account of the bump (i.e. the cultural life script account) can be rejected [ 3 ]. Memories may be easily retrieved due to the originality of the experiences (i.e. high emotional valence), or because they play a role in higher-order structures of the personality (i.e. significance or self-relevance). A bump for vivid memories may occur due to salience rather than mere nostalgia, but are significant because they define who a person is [ 24 ]. The ability of a person to consciously recall memories, to identify them as linked to his/her personal past, and to relate them to his/her goals and desires permits the formation of a coherent personal narrative concerned with the present and the future [ 111 , 129 ].

Researchers propose that there is an emergence of adult identity during late adolescence and early adulthood [ 130 ]. This period may contain many self-defining incidents which link the self of an individual to that particular reality [ 131 ]. Therefore, an account focusing on the role of self in the bump (e.g. narrative/identity account) can be used to explain the occurrence of memories from this period [ 62 ]. The bump reveals an era in an individual’s life that is crucial for the development and maintenance of a stable self, as basic cognitive changes across the life span cannot be solely responsible for shaping retrieval [ 11 , 130 ]. It is likely that the development of a new self initiates preferential encoding due to the importance of the formation of particular personal and cultural identities [ 22 , 31 , 132 ]. As AMs ground the self, there is a possibility that the importance of identity development stimulates the use of cognitive mechanisms [ 11 , 36 ].

The narrative/identity account received significant support: a study analyzing dream content, temporally linked with the bump, revealed themes associated with identity and life goals [ 9 ]. However, the dreams were collected from professional career women mostly at retirement age, who were already experiencing a transition—during which most people are concerned with life orientation and purpose—that could trigger the importance of identity. Another study collected benchmarked memories from the life history timeline, revealing a bump associated with identity formation in early adulthood, however, the bump was seen concerning only family or relationships [ 70 ]. A high correlation between levels of rehearsal and preoccupation with stories from participants’ lives could show potential support for the narrative/identity account, but causal claims cannot yet be made [ 35 ].

Free recall of public and private news items revealed differential bumps: 10 to 19 years for public items and 20 to 29 years for private items [ 42 ]. The earlier bump reflects a period of formation of generation identity, while the later bump mirrors a period of formation of intimate relationships [ 42 ]. The generation of self-images in the form of “I am” statements to test the relationship between memory accessibility and self, lends support to the narrative/identity account due to the clustering of AMs around the time of self-formation [ 93 ]. However, this evidence was only found when the first three memories representing each self were compared at age 20 versus age 40. Another study revealed an absence of the bump for AMs when highly self-relevant life events were supressed [ 69 ].

Similar support is shown in studies by examining the relationship of highly positive and highly negative events with life story and identity [ 107 ]; and the role of generational identity with the development of an integrative self behind the bump [ 62 ]. The latter study demonstrates an accessibility of AMs from a period outside of the reminiscence bump that are suggested to be relevant to the self [ 111 ]. In this study, a group of older Bangladeshi participants presented a second bump for the ages of 35 to 55 years, coinciding with the period of Bangladesh’s war for independence in 1971. It is suggested that the second bump is due to the enhanced retrieval of AMs from a period when Bengalis, as a nation, were struggling to establish their own independent country, and to uphold their collective Bengali identity. However, a current debate exists on whether individuals recall public events from the bump period because of their identification with those events, or because of better encoding of them [ 80 ]. The narrative/identity account states that all the memories for adolescent events are not necessarily self-narrative memories, but rather that more events from this phase, with a better availability for recall, form a pool for self-narrative memories.

Alternatively, the ratings of re-living and vividness showed no differences between bump and non-bump memories, thus rejecting the role of phenomenological features of memories in the bump formation [ 82 ]. One study investigating the personal significance of songs showed a bump for both R (remember) and K (know) ratings [ 94 ]. Although a greater number of personally significant songs were associated with R ratings, K ratings also formed a bump, even though according to the narrative/identity account, they should not do so. The question of circularity raised in the research has not been answered (i.e. whether the selection of songs as personally significant is due to their association with particular memories, or high personal importance of a song leads a person to relate this song to specific memories from the time it was heard) [ 94 ].

The preponderance of memories from the bump period does not mean that these memories are related to identity formation unless there is a direct retrieval of identity related memories and an analysis of the lifespan distribution of these memories is performed. Although the narrative/identity account states that the bump is the result of an identity-relevant process, substantial evidence is still needed. Furthermore, the way identity-related questions are formulated, and the functional demands of answering these questions for the participants, impacts the construction of available memories. Various factors such as emotional salience, specific temporal and geographical context, sociocultural factors and a self-reference effect might influence the preferential retrieval of personally significant, over non-significant, events [ 133 , 134 ]. As a result of the interplay between these factors, information associated with the self tends to be remembered best [ 135 ]; and thus, these more easily remembered memories may not necessarily indicate that bump memories are from important identity-forming events.

The heterogeneous methodologies of the studies in this review have created difficulty for the authors in discerning a link between identity formation and the bump, as the studies are based on memories, or important memories, and ratings. A major issue with ratings lies in the fact that these ratings reflect how study participants currently feel about their experiences rather than what they felt at the time of encoding the memories. Therefore, ratings have a limited role in identity related explanations of the bump (e.g. it is difficult to ascertain how study participants recalled and reported their judgments of rehearsal to reflect true/precise rehearsal rates).

The authors did not find any direct request to recall specific self-defining memories (SDMs) in the research, while important life event narratives were requested. Two studies investigated the role of the self in AMs by examining SDMs [ 136 ]. The exploration of SDMs is an important approach for understanding the association between identity and the bump as they are memories of events that one draws on to inform one’s sense of identity [ 137 , 138 ]. A few studies did investigate the self-relevance of autobiographical events in the bump and their centrality to the life story and identity, SDMs, and self-images [ 93 , 107 , 108 , 111 ]. The identity of individuals depends upon their ability to recall personal history, in the form of self-defining memories [ 139 ]. Therefore, there is a need to look deeper into the encoding and retrieval of event-specific temporal knowledge for understanding the self and identity [ 140 ].

The key to understanding the bump may lie in the memories of self-defining events during adolescence and early adulthood, as the narrative/identity account claims that many memories found in the bump are from this period [ 24 , 93 , 131 , 141 ]. These memories play a vital role in the regulation of mood and direct functions of the self [ 142 , 143 ]. Studies using measures for memory function and the self asked participants to report 20 “I am” self-concepts, thereby collecting concepts/roles significant to their definitions of self [ 39 , 93 , 144 , 145 ].

Theoretical implications

The present systematic review extends the body of knowledge on the reminiscence bump, highlights theoretical accounts giving various explanations for the bump, and supports the use of a variety of methods of identity construction as possible explanations for the bump phenomenon. It shows that varying locations of the bump could be due to different methods of activating memories or differences in memory types. Future research could examine memory functions and the measure of the self, along with the role of SDMs in the association of identity and the bump. New research could also be conducted on the salience of identity in memories, and the significance of goals in SDM formation. The role of SDMs in helping familiarize an individual to age-related changes could be investigated.

The authors highlight a gap in the research for which, if any, theoretical account offers the best explanation for the bump. The reviewed studies do not provide enough evidence to construct a clear understanding of the bump and its location and formation using different types of memory assessments. There is a need to conduct further studies investigating methods of memory activation, different types of memories activated, and theories for the bump, particularly to compare the plausibility of several theoretical accounts simultaneously in a single study. A novel research strategy could be developed for use in a large study to determine if the narrative/identity account, or cultural life script account, better explains the bump.

Strengths/Limitations

This review provides a foundation for a more transparent understanding of the relative plausibility of theoretical accounts explaining the bump as reflected in the research. Since the temporal location of the bump varies according to memory activation method, the authors present the bump’s most widely accepted location. This review includes studies in various languages and geographical locations, and with differing population characteristics and lengths. The researchers conducted an extensive quality assessment exercise for study inclusion.

A limitation of this systematic review is simply the heterogeneity of the pool of studies regarding research design, time period conducted, sample size, sample characteristics, intervention strategies implemented during different time periods, and assessment method. Methods used to examine the bump depend on self-report measures, and cannot be evaluated for accuracy of recall due to the potential for self-report bias, which may influence results. The 14 criteria checklist [ 55 ] used to assess the quality of studies with diverse designs has inherent limitations. Although it permits a comparison among studies for quality, it gives no guidance for what score is considered “good,” or represents a satisfactory level of internal validity. The authors adopted this checklist based on its use in other published systematic reviews [ 146 , 147 ].

This systematic review provides a comprehensive summary of the empirical research on the reminiscence bump published between 1988 and 2017. Findings illustrate that the cuing method and important memories method were widely used to induce memories. Results indicate the overall temporal location of the bump to be between 10–30 years of age for the important memories method, 5–30 years of age for word cuing methods, and 6–39 years of age for studies using life scripts. Both the narrative/identity and cultural life scripts accounts received a fair amount of support in explaining the occurrence of the bump. The authors indicate a need for further research in identifying: (a) the theoretical account offering the most comprehensive explanation for the bump, and (b) the most accurate method(s) of memory activation. The strengths and limitations of both accounts of the bump (i.e., the narrative/identity account and cultural life script account) and suggestions for future studies are discussed. The current, empirical evidence on the bump summarized in this paper could be valuable for researchers and professionals in the fields of cognitive psychology and neuroscience.

Supporting information

S1 table. showing key words and alternative words..

Computer-based searches were conducted to search nine databases. In each search, derivatives of “reminiscence bump” were combined using the Boolean OR operator and wildcards.

https://doi.org/10.1371/journal.pone.0208595.s001

S2 Table. Databases searched for the systematic review.

A search of nine databases gave a total of 523 research articles.

https://doi.org/10.1371/journal.pone.0208595.s002

S3 Table. Showing quality assessment of quantitative studies included in this systematic review (n = 68).

The detailed quality assessment of all included studies was carried out through a 14 criteria given by Kmet, Lee, and Cook.

https://doi.org/10.1371/journal.pone.0208595.s003

S4 Table. PRISMA checklist.

A PRISMA checklist showing various section of this review and page numbers on which these sections are reported.

https://doi.org/10.1371/journal.pone.0208595.s004

S1 Fig. Geographical distribution of all 68 studies examined in this systematic review.

The clustering of studies on the basis of geographical location shows that most of the studies (n = 19) were conducted in USA.

https://doi.org/10.1371/journal.pone.0208595.s005

S2 Fig. Temporal distribution of all 68 studies examined in this systematic review.

The clustering of included studies in three groups; 1988 to 1999; 2000 to 2010, and 2011 to 2017.

https://doi.org/10.1371/journal.pone.0208595.s006

S1 File. PROSPERO protocol.

Review protocol registered in PROSPERO (International prospective register of systematic reviews).

https://doi.org/10.1371/journal.pone.0208595.s007

Acknowledgments

Dr. Shogo Moriya, Senior Lecturer, BRIMS, Jeffrey Cheah School of Medicine & Health Sciences, Monash University Malaysia for his assistance in reviewing and extracting data from the Japanese research article. Monash University Malaysia is acknowledged for offering a Higher Degree by Research Scholarship to Khadeeja Munawar. The authors also acknowledge Global Asia in the 21 st Century (GA21) Platform at Monash University Malaysia for the open access publication funding support.

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Hypothesis and theory article, autobiographical memory: a clinical perspective.

• 1 Section of Geriatric Psychiatry, University of Heidelberg, Heidelberg, Germany
• 2 Institute of Gerontology, University of Heidelberg, Heidelberg, Germany

Autobiographical memory (ABM) comprises memories of one’s own past that are characterized by a sense of subjective time and autonoetic awareness. Although ABM deficits are among the primary symptoms of patients with major psychiatric conditions such as mild cognitive impairment (MCI) and Alzheimer Disease (AD) or chronic schizophrenia large clinical studies are scarce. We therefore summarize and discuss the results of our clinical studies on ABM deficits in the respective conditions. In these studies ABM was assessed by using the same instrument – i.e., the Erweitertes Autobiographisches Gedächtnis Inventar (E-AGI) – thus allowing a direct comparison between diagnostic groups. Episodic ABM, especially the richness of details was impaired already in MCI and in beginning AD. Semantic memories were spared until moderate stages, indicating a dissociation between both memory systems. A recency effect was detectable in cognitively unimpaired subjects and vanished in patients with AD. A similar pattern of deficits was found in patients with chronic schizophrenia but not in patients with major depression. These ABM deficits were not accounted for by gender, or education level and did not apply for the physiological ageing process in otherwise healthy elderly. In conclusion, ABM deficits are frequently found in AD and chronic schizophrenia and primarily involve episodic rather than semantic memories. This dissociation corresponds to the multiple trace theory which hypothesized that these memory functions refer to distinct neuronal systems. The semi-structured interview E-AGI used to discern ABM changes provided a sufficient reliability measures, moreover potential effects of a number of important confounders could be falsified so far. These findings underline the relevance of ABM-assessments in clinical practice.

Introduction

Autobiographical memory (ABM) refers to memories of an individual, which are characterized by a sense of subjective time and autonoetic awareness ( Tulving, 1972 , 2002 ) and entailed by feelings of emotional re-experience ( Tulving, 1983 ; Tulving and Markowitsch, 1998 ; Markowitsch, 2003 ). Because of the interaction of episodic and semantic memory and the uniqueness to humans ABM is considered to be crucial for the continuity of the self and the development of personal identity, i.e., processes which are typically disturbed in patients with major psychiatric conditions such as Alzheimer’s disease (AD) or chronic schizophrenia ( Conway and Pleydell-Pearce, 2000 ; Cuervo-Lombard et al., 2007 ; Berna et al., 2012 ; Seidl et al., 2011 ; Herold et al., 2013 ). As a part of the declarative memory, ABM comprises a semantic plus an episodic domain. While semantic ABM involves general facts from different life time periods, episodic ABM includes biographic events with a richness of details and a feeling of re-experiencing when recalled.

According to Ribot’s law ( Ribot, 1881 ) remote memories are more resistant to brain damage than recent one. Ribot’s law stands in opposition to the recency effect that implies a better consolidation of recent memories than remote ones. Declarative mnestic deficits are among the core symptoms of AD and usually go along with anterograde memory impairment in the initial phases and loss of remote memory following Ribot’s gradient in the more advanced stages ( Sagar et al., 1988 ; Dall’Ora et al., 1989 ; Kopelman, 1989 ; Greene and Hodges, 1996 ; Dorrego et al., 1999 ; Piolino et al., 2003 ; Hou et al., 2005 ; Leyhe et al., 2009 ). Two important theoretical approaches regarding the role of the hippocampus on ABM retrieval are the standard model of consolidation and the multiple trace theory ( Squire and Alvarez, 1995 ; Nadel and Moscovitch, 1997 ). The first approach suggests that the function of the hippocampus in ABM is time-limited; hence, memories become gradually independent of the medial temporal lobe (MTL) in the course of time. In contrast, the multiple trace theory predicts that the recall process of the episodic autobiographical memories requires the hippocampal formation irrespective of how old the relevant memories are. The semantic memories, however, could be recalled independently of this structure and were subject to Ribot’s gradient. The majority of studies support the multiple trace theory ( Conway et al., 1999 ; Piolino et al., 2004 ; Viard et al., 2007 ). There are also reports of spared personal-semantic memory but impaired personal episodic memories without a temporal gradient in patients with MTL lesions ( Viskontas et al., 2000 ; Steinvorth et al., 2005 ; Noulhiane et al., 2008 ).

Autobiographical memory deficits are not specific to AD but were also described in mild cognitive impairment (MCI) and chronic schizophrenia. These changes do not only contribute to our understanding of the respective diseases but have the potential to facilitate clinical examination and diagnosis. However, the potential impact of important confounders, such as education, depressive mood, or the aging process as such needs to be addressed.

In the following we summarize and discuss findings from our studies on ABM deficits in MCI and AD, major depression, and chronic schizophrenia with reference to normal aging.

Clinical Studies

Methodological details of the five studies conducted by our group as well as the description of sample characteristics are summarized in Table 1 .

Table 1 . Studies on ABM in major depression, MCI, AD, and chronic schizophrenia .

Autobiographical memory was investigated by using the Erweitertes Autobiographisches Gedächtnis Inventar (E-AGI) ( Kopelman et al., 1990 ; Fast et al., 2007 ) – a semi-structured autobiographical interview based on the ABM Interview of Kopelman and colleagues. A previous version of the E-AGI was used in one study. Both, personal-semantic facts (SEM) as well as free recalled autobiographical events (EP-F) of five different lifetime periods (preschool, primary school, secondary school, early adulthood, recent 5 years) are considered. One autobiographical event from each lifetime period had to be described in detail. The score of maximal 11 points was given considering the number of details of such an event (EP-D). According to Conway ( Conway, 1996 ; Conway and Pleydell-Pearce, 2000 ) event-specific knowledge plays a central role to autobiographical remembering and is stored and encoded in a completely different way than knowledge about “general events” or “lifetime periods,” which can be assigned to semantic autobiographical knowledge. To reduce the time necessary for the examination and to consider the restrictions due to the psychiatric conditions, the interview was modified and limited to the following three lifetime periods (primary school, early adulthood, recent 5 years – Figure 1 ) in four studies.

Figure 1. Group comparison concerning semantic knowledge (SEM), free recalled episodes (EP-F) and episodic details (EP-D) by groups of patients with depression ( Ahlsdorf, 2009 ), schizophrenia ( Herold et al., 2013 ), MCI, different stages of AD and healthy controls ( Seidl et al., 2011 ) .

Study 1: Psychometric Properties of ABM Assessment and Effects of Depressed Mood ( Ahlsdorf, 2009 )

Group difference and effects of depressed mood.

When compared between the four diagnostic groups, SEM scores showed only minor, non-significant differences. In contrast, EP-F scores were significantly higher in healthy controls, patients with major depression and patients with MCI than in those with manifest AD. Similar results applied for the EP-D which were significantly higher in the healthy controls followed by patients with MCI and major depression than in those with manifest AD. Only marginally, non-significant differences in EP-D scores between healthy controls and patients with major depression could be found. The E-AGI total values diminished non-significantly in patients with major depression in comparison to healthy controls. The study yielded an important result in the comparison of the evaluation of memories. Patients with major depression were occupied with negative thoughts and estimated their memories more negative than patients with AD.

Study 2: ABM in Nursing Home Residents with MCI and Manifest AD ( Seidl et al., 2011 )

Autobiographical memory was examined in patients with different stages of AD and MCI, respectively, as well as in healthy controls (Table 1 ). Subjects were recruited in the framework of a large survey in nursing homes across Germany.

Results (Figure 1 ) demonstrated a progressive loss of ABM sum scores with increasing severity of dementia, which primarily involved episodic rather than semantic memories. When compared between controls, MCI, and mild AD diagnostic groups, SEM scores showed only minor, non-significant differences. Patients with moderate and severe AD displayed a significant reduction in SEM from the recent 5 years. Patients with moderate AD showed also a reduction for EP-F scores from the recent 5 years when compared to the childhood period whereas in healthy controls an inverse relationship was observed. This dissociation indicates that these memory functions are subserved by distinct neuronal systems as emphasized by the multiple trace hypothesis.

Further analyses of the temporal gradients in control subjects and MCI patients displayed a better memory performance from adulthood when compared to the childhood period. Both controls and patients with MCI showed lower EP-D scores for the childhood period.

In contrast, this recency effect was not found in patients with moderate AD suggesting an impact of the disease on the formation of recent memories.

Study 3: ABM in Normal Aging and MCI ( Berna et al., 2012 )

Results confirmed a significant impairment of episodic ABM in MCI, but not in normal aging. Old-aged patients with MCI reached significantly lower scores than both Healthy Middle-Aged ( P < 0.001) and Healthy Old-Aged ( P = 0.02) subjects. Significant lower scores were also reached by Old-Aged patients with MCI compared with healthy Middle-Aged patients in the recent period ( P = 0.004). Participants with MCI showed significantly lower scores than both control groups irrespective of age. These deficits were significantly correlated with verbal memory performances, but not with measures of executive functions.

Study 4: Hippocampal Changes and ABM in MCI and AD ( Thomann et al., 2012 )

Autobiographical memory deficits were investigated with respect to hippocampal changes in patients with MCI ( n = 15), patients with mild AD and cognitively unaffected control subjects ( n = 24) (Table 1 ). Associations between ABM sum scores and hippocampal changes were explored using partial correlations, each of the significant correlations was confirmed by regional shape analyses. Results confirmed a significant ABM loss in the in early stages of AD and in MCI. Episodic, but not semantic ABM losses were associated with hippocampal atrophy mainly involving the left hippocampus. Right-sided hippocampal atrophy corresponded to reduced scores in the EP-F of the “childhood” lifetime period. These associations referred to the regional rather than to the global hippocampal changes which primarily affect the hippocampal head and body.

Study 5: ABM Deficits in Chronic Schizophrenia ( Herold et al., 2013 )

Autobiographical memory BM and hippocampal volume were assessed in 33 patients with chronic schizophrenia ( n = 24) or patients with schizoaffective disorder ( n = 9) and 21 healthy volunteers matched for age, gender, and education. The assessment of ABM was part of a large neuropsychological test battery, which also addressed verbal, short-term, and working memory as well as remote semantic memory. Psychopathological symptoms were rated on appropriate rating scales (Table 1 ).

When compared with the healthy controls, patients showed a significantly poorer recollection of episodic ABM as well as a trend toward a lower performance with respect to semantic ABM. Analysis of MRI data revealed lower volumes of left anterior and posterior hippocampus as well as of the right posterior hippocampus in the patients group.

Both, episodic and semantic ABM-scores were significantly correlated with the left hippocampal volume in the patient group. This association applied for both, the left anterior as well as the left posterior part of the hippocampus. These associations accounted for 16% of the variance of episodic ABM and 24% of the variance of semantic ABM with educational level considered as a covariate.

The present studies yielded the following main findings: (i) a confirmation that episodic rather than semantic ABM is impaired in major psychiatric conditions such as AD and chronic schizophrenia; (ii) evidence that this effect is not accounted for by potential confounding factors such as age, education, or depressed mood; and (iii) an indication that ABM deficits refer to hippocampal changes in both AD and chronic schizophrenia.

That episodic rather than semantic ABM is impaired already in the early stages of AD including MCI is made evident by a wealth of studies. This effect involves the recognition of past events and also includes the remembrance of recent experiences such as a consultation in the doctors’ office and can facilitate clinical examination and diagnosis in early dementia ( Donix et al., 2010 ). While semantic recall followed Ribot’s law in patients with manifest dementia in all stages, episodic ABM recall showed this effect in patients with mild and moderate dementia only, since the respective deficits also included earlier life time periods.

A significant effect of potential confounding variables – in particular age, education, or depressed mood – on these findings was not confirmed. Age is a variable difficult to consider in any study on AD since the disease progresses with it. We therefore investigated potential age effects in a 332 otherwise healthy volunteers from two birth cohorts and demonstrated only minor non-significant episodic ABM differences with age. School education had to be considered as another potential confounder since this variable is a robust marker of cognitive reserve ( Fratiglioni and Wang, 2007 ; Sattler, 2011 ; Schröder and Pantel, 2011 ). However, an effect of school education could not be confirmed ( Berna et al., 2012 ). Depressive mood was primarily considered by Ahlsdorf (2009) who described an effect on the emotional content of the memories reported rather than their recollection per se . Depressed patients showed a significantly higher rate of negative valuations in both, semantic and episodic ABM. Along with this, Seidl et al. (2011) did not find the severity of ABM deficits to be significantly correlated with depressive mood although their sample of 239 nursing home residents provided a sufficient effect size.

Two of the studies summarized here – each one involving patients with MCI and AD or patients with chronic schizophrenia – investigated ABM deficits with respect to MRI derived measures of hippocampal volume and shape. Irrespective of the diagnosis, episodic ABM deficits were associated with left hippocampal changes. An additional association of ABM deficits with right hippocampal changes was restricted to patients with MCI and AD. The respective associations clearly underline the importance of the hippocampus for the recollection of episodic ABM although these associations only accounted for a small proportion of the variance. Beginning in the early 1990s a wealth of neuroimaging studies found the hippocampus to be critically involved in MCI, AD, and chronic schizophrenia ( Pantel et al., 1997 ; Heckers et al., 1998 ; Herold, 2011 ; Schröder and Pantel, 2011 ). Hence, it is plausible that the respective changes may result in similar deficits in both conditions. Differences refer to the extent of hippocampal changes and ABM deficits as well as to additional factors contributing to them. Further studies need to differentiate the association of hippocampal changes and ABM deficits by comparing hippocampal substructures for potential differences between these conditions or by considering additional clinical factors such as lifelong withdrawal, living without partnership, or long term hospitalization in patients with schizophrenia. Taken together, these finding conform with the multiple trace theory. Episodic ABM was already compromised in MCI and mild AD whereas recall of SEM was still preserved. This dissociation is generally referred to the hippocampus role for the recall of episodic but not semantic ABM since the former is already involved in the early and in the preclinical stages of AD ( Pantel et al., 2003 ).

The results of our studies correspond to Conway’s formal differentiation of event-specific knowledge and “general events” or “lifetime periods” ( Conway, 1996 ; Conway and Pleydell-Pearce, 2000 ). From a more phenomenological standpoint, the failure of episodic remembrance in the more advanced stages of AD and schizophrenia causes a breakdown of subjective coherence and identity since life stories ( McAdams, 1985 ) stop to be accessible nor retrievable anymore. This effect may be associated with psychopathological symptoms such as apathy which is another common features in both AD and chronic schizophrenia.

In conclusion the present studies underline the importance of episodic ABM changes in MCI, AD, and chronic schizophrenia, i.e., conditions which share hippocampal changes as a common feature. While deficits of episodic ABM are already present in the early stages of AD, those of semantic ABM are confined to the more severe stages. In both, AD and chronic schizophrenia, ABM deficits were correlated with hippocampal changes. These findings demonstrate that ABM deficits can facilitate the clinical examination of patients with MCI, AD, and chronic schizophrenia.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The studies reported here were supported in part by the Dietmar Hopp Foundation (Walldorf).

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Keywords: autobiographical memory, semantic memory, episodic memory, mild cognitive impairment, Alzheimer’s disease, chronic schizophrenia, hippocampus, multiple trace theory

Citation: Urbanowitsch N, Gorenc L, Herold CJ and Schröder J (2013) Autobiographical memory: a clinical perspective. Front. Behav. Neurosci. 7 :194. doi: 10.3389/fnbeh.2013.00194

Received: 23 May 2013; Accepted: 21 November 2013; Published online: 10 December 2013.

Reviewed by:

Copyright: © 2013 Urbanowitsch, Gorenc, Herold and Schröder. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Nadja Urbanowitsch, Section of Geriatric Psychiatry, University of Heidelberg, Voßstraße 4, 69115 Heidelberg, Germany e-mail: nadja.urbanowitsch@med.uni-heidelberg.de

This article is part of the Research Topic

Progress in Episodic Memory Research

• Health News

My earliest memory is from the womb and I remember 95% of my life – it took me weeks to get fluent in French and Spanish

• Eliza Loukou , Health Reporter
• Published : 7:02 ET, May 31 2024
• Updated : 8:18 ET, May 31 2024
• Published : Invalid Date,

A WOMAN with a rare condition claims she can remember 95 per cent of everything that's ever happened to her - including being in the womb.

Rebecca Sharrock, 34, lives with highly superior autobiographical memory (H-SAM) - a neurological condition which leaves people able to remember most of their life in intricate detail.

It's extraordinarily rare and Rebecca is just one of 62 people in the world who have been diagnosed with the condition.

Research shows that for the average person, the earliest memory they can retrieve will be from when they were about two and a half years old.

As for Rebecca, her earliest dated memory is from when she was just 12 days old - but she claims she can also remember being a foetus in her mum's womb.

Despite needing therapy for the condition, Rebecca says she "puts it to good use" and has been able to become fluent in two languages over the last 10 weeks alone.

Rebecca, an author and public speaker, from Brisbane, Australia , said: "My mind is just unable to let go of rubbish from the past.

"My earliest memory is possibly from when I was a foetus in the womb.

"The interesting thing is I don't have to have any personal significance attached to a memory.

"They don't need to mean much for some reason.

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"At the moment, I'd say I can remember 95 per cent of everything that's happened to me in my life."

INTENSE FLASHBACKS

Rebecca wasn't diagnosed with H-SAM until she was 21 years old, on January 23, 2011.

She says she was originally misdiagnosed with obsessive compulsive disorder (OCD) at the age of 16 because she'd "obsessively" relive past experiences.

Her childhood memories - like having toys taken away from her at school - would cause her genuine upset despite seeming "trivial".

"I always put my flashbacks down to OCD," she said.

"When I'd have a flashback my emotions would remember too.

"I don't just remember the past - I get all the sensory information that I had in that moment.

I have this memory of me being scrunched up, having my head tucked between my legs. Researchers have told me this could've been from when I was in the womb. Rebecca Sharrock

"When I'd relive a moment from my childhood, I'd feel immature emotions.

"Very often this would be from something that happened when I was very small.

"I'd become upset over trivial things - like when someone stole my lollipop in primary school.

"Or the time I built a Lego tower in preschool, and a kid came and knocked it over.

"Now I'd call it trivial - but I'd get such anger from it.

"And of course, when you're really small, it's not trivial."

EARLIEST MEMORIES

Rebecca says her earliest dated memory was from December 23, 1989 - when she was just 12 days old and had her photo taken.

"I can remember everything from about 12 days old," she said.

"The earliest one I can date, I had a photograph taken - and spent many years afterwards telling my mum about this experience.

"I do actually have some memory prior to that, though - but I couldn't date them as I was too young to understand the concept of the calendar."

Rebecca claims her "true" earliest memory was from when she was in the womb - or potentially in the minutes after being born.

All these flashbacks going through my mind are constant and throughout them I do experience insomnia. My mind will just never stay quiet Rebecca Sharrock

She said: "The earliest recollection I have I can't date because I was far too young.

"I have this memory of me being scrunched up, having my head tucked between my legs.

"Researchers have told me this could've been from when I was in the womb.

"Or, it could've been straight after being born."

A BLESSING AND A CURSE

H-SAM can cause significant mental health issues and Rebecca says she struggles with insomnia and needs therapy for trauma and anxiety.

Often plagued by intense flashbacks at night, she'll try to ease her insomnia by listening to classical music at night and has been prescribed valium to help her relax.

She added: "This memory condition is essentially a medical condition which does cause issues in my daily living.

"All these flashbacks going through my mind are constant and throughout them I do experience insomnia.

"My mind will just never stay quiet.

"I have to listen to classical music before I go to sleep - it keeps my mind away from flashbacks.

"But sometimes, if my mind is very chaotic, classical music won't work, so I take valium which has been prescribed by my doctor.

"I have to have therapy, because H-SAM causes a lot of anxiety and depression.

"There are very few therapists who can treat my condition - so they have to borrow things from PTSD therapy. It's quite experimental."

What is highly superior autobiographical memory (H-SAM)?

PEOPLE with highly superior autobiographical memory (H-SAM) a superior ability to recall specific details of autobiographical events.

They tend to spend a large amount of time thinking about their past.

The neurological condition is incredibly rare and only 62 people across the world have been diagnosed with it.

H-SAM was first identified by researchers at the Center for the Neurobiology of Learning and Memory at UC Irvine in 2006.

Professor James McGaugh and colleagues reported the first known case of H-SAM in a research participant known as “AJ”, later identified as Jill Price.

When provided with a date, Jill could specify on which day of the week it fell and what she did that day.

Since then, more people with this bizarre and extraordinary ability have been identified.

So far, the studies conducted at UC Irvine suggest that individuals with H-SAM have superior abilities in autobiographical memories, but are no different from other their on standard laboratory memory tests.

MRI studies of their brain also show that specific regions and networks may be different from the average person, though this work is still in its early stages.

Source: UCI

Rebecca has tried to harness her condition for something positive and started learning French and Spanish 10 weeks ago.

She went from a total beginner to near-fluent in two months.

"My French teacher is from Marseilles and I can have a conversation with her in French now," she said.

"I can watch Spanish and French TV without subtitles.

Read More on The US Sun

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Jenelle Evans wants to 'celebrate teen years she never had,' but is faking it

"Everything I've been taught in class I now know - it's just a case of practicing my pronunciation.

"I'm hoping to learn Italian next ."

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Cognitive neuroscience perspective on memory: overview and summary

Sruthi sridhar.

1 Department of Psychology, Mount Allison University, Sackville, NB, Canada

Abdulrahman Khamaj

2 Department of Industrial Engineering, College of Engineering, Jazan University, Jazan, Saudi Arabia

Manish Kumar Asthana

3 Department of Humanities and Social Sciences, Indian Institute of Technology Roorkee, Roorkee, India

4 Department of Design, Indian Institute of Technology Roorkee, Roorkee, India

Associated Data

The original contributions presented in this study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

This paper explores memory from a cognitive neuroscience perspective and examines associated neural mechanisms. It examines the different types of memory: working, declarative, and non-declarative, and the brain regions involved in each type. The paper highlights the role of different brain regions, such as the prefrontal cortex in working memory and the hippocampus in declarative memory. The paper also examines the mechanisms that underlie the formation and consolidation of memory, including the importance of sleep in the consolidation of memory and the role of the hippocampus in linking new memories to existing cognitive schemata. The paper highlights two types of memory consolidation processes: cellular consolidation and system consolidation. Cellular consolidation is the process of stabilizing information by strengthening synaptic connections. System consolidation models suggest that memories are initially stored in the hippocampus and are gradually consolidated into the neocortex over time. The consolidation process involves a hippocampal-neocortical binding process incorporating newly acquired information into existing cognitive schemata. The paper highlights the role of the medial temporal lobe and its involvement in autobiographical memory. Further, the paper discusses the relationship between episodic and semantic memory and the role of the hippocampus. Finally, the paper underscores the need for further research into the neurobiological mechanisms underlying non-declarative memory, particularly conditioning. Overall, the paper provides a comprehensive overview from a cognitive neuroscience perspective of the different processes involved in memory consolidation of different types of memory.

Introduction

Memory is an essential cognitive function that permits individuals to acquire, retain, and recover data that defines a person’s identity ( Zlotnik and Vansintjan, 2019 ). Memory is a multifaceted cognitive process that involves different stages: encoding, consolidation, recovery, and reconsolidation. Encoding involves acquiring and processing information that is transformed into a neuronal representation suitable for storage ( Liu et al., 2021 ; Panzeri et al., 2023 ). The information can be acquired through various channels, such as visual, auditory, olfactory, or tactile inputs. The acquired sensory stimuli are converted into a format the brain can process and retain. Different factors such as attention, emotional significance, and repetition can influence the encoding process and determine the strength and durability of the resulting memory ( Squire et al., 2004 ; Lee et al., 2016 ; Serences, 2016 ).

Consolidation includes the stabilization and integration of memory into long-term storage to increase resistance to interference and decay ( Goedert and Willingham, 2002 ). This process creates enduring structural modification in the brain and thereby has consequential effects on the function by reorganizing and strengthening neural connections. Diverse sources like sleep and stress and the release of neurotransmitters can influence memory consolidation. Many researchers have noted the importance of sleep due to its critical role in enabling a smooth transition of information from transient repositories into more stable engrams (memory traces) ( McGaugh, 2000 ; Clawson et al., 2021 ; Rakowska et al., 2022 ).

Retrieval involves accessing, selecting, and reactivating or reconstructing the stored memory to allow conscious access to previously encoded information ( Dudai, 2002 ). Retrieving memories depends on activating relevant neural pathways while reconstructing encoded information. Factors like contextual or retrieval cues and familiarity with the material can affect this process. Forgetting becomes a possibility if there are inadequate triggers for associated memory traces to activate upon recall. Luckily, mnemonic strategies and retrieval practice offer effective tools to enhance recovery rates and benefit overall memory performance ( Roediger and Butler, 2011 ).

Previous research implied that once a memory has been consolidated, it becomes permanent ( McGaugh, 2000 ; Robins, 2020 ). However, recent studies have found an additional phase called “reconsolidation,” during which stored memories, when reactivated, enter a fragile or liable state and become susceptible to modification or update ( Schiller et al., 2009 ; Asthana et al., 2015 ). The process highlights the notion that memory is not static but a dynamic system influenced by subsequent encounters. The concept of reconsolidation has much significance in memory modification therapies and interventions, as it offers a promising opportunity to target maladaptive or traumatic memories for modification specifically. However, more thorough investigations are needed to gain insight into the mechanisms and concrete implications of employing memory reconsolidation within therapeutic settings ( Bellfy and Kwapis, 2020 ).

The concept of memory is not reducible to a single unitary phenomenon; instead, evidence suggests that it can be subdivided into several distinct but interrelated constituent processes and systems ( Richter-Levin and Akirav, 2003 ). There are three major types of human memory: working memory, declarative memory (explicit), and non-declarative memory (implicit). All these types of memories involve different neural systems in the brain. Working memory is a unique transient active store capable of manipulating information essential for many complex cognitive operations, including language processing, reasoning, and judgment ( Atkinson and Shiffrin, 1968 ; Baddeley and Logie, 1999 ; Funahashi, 2017 ; Quentin et al., 2019 ). Previous models suggest the existence of three components that make up the working memory ( Baddeley and Hitch, 1974 ; Baddeley, 1986 ). One master component, the central executive, controls the two dependent components, the phonological loop (speech perception and language comprehension) and the visuospatial sketchpad (visual images and spatial impressions processing). Some models mention a third component known as the episodic buffer. It is theorized that the episodic buffer serves as an intermediary between perception, long-term memory, and two components of working memory (the phonological loop and visuospatial sketchpad) by storing integrated episodes or chunks from both sources ( Baddeley, 2000 ). Declarative memory (explicit memory) can be recalled consciously, including facts and events that took place in one’s life or information learned from books. It encompasses memories of both autobiographical experiences and memories associated with general knowledge. It is usually associated with the hippocampus–medial temporal lobe system ( Thompson and Kim, 1996 ; Ober, 2014 ). Non-declarative memory (implicit memory) refers to unconscious forms of learning such as skills, habits, and priming effects; this type of implicit learning does not involve conscious recollection but can include motor skill tasks that often require no thought prior to execution nor later recall upon completion. This type of memory usually involves the amygdala and other systems ( Thompson and Kim, 1996 ; Ober, 2014 ).

Working memory

Working memory is primarily associated with the prefrontal and posterior parietal cortex ( Sarnthein et al., 1998 ; Todd and Marois, 2005 ). Working memory is not localized to a single brain region, and research suggests that it is an emergent property arising from functional interactions between the prefrontal cortex (PFC) and the rest of the brain ( D’Esposito, 2007 ). Neuroimaging studies have explored the neural basis for the three components proposed by Baddeley and Hitch (1974) , the Central executive, the phonological loop, and the visuospatial sketch pad; there is evidence for the existence of a fourth component called the episodic buffer ( Baddeley, 2000 ).

The central executive plays a significant role in working memory by acting as the control center ( Shallice, 2002 ). It facilitates critical functions like attention allocation and coordination between the phonological loop and the visuospatial sketchpad ( Yu et al., 2023 ). Recent findings have illuminated the dual-functional network regulation, the cingulo-opercular network (CON) and the frontoparietal network (FPN), that underpins the central executive system ( Yu et al., 2023 ). The CON comprises the dorsal anterior cingulate cortex (dACC) and anterior insula (AI). In contrast, the FPN encompasses various regions, such as the dorsolateral prefrontal cortex (DLPFC) and frontal eye field (FEF), along with the intraparietal sulcus (IPS) ( Yu et al., 2023 ). Neuroimaging research has found evidence that elucidates the neural underpinnings of the executive attention control system to the dorsolateral prefrontal cortex (DLPFC) and the anterior cingulate cortex (ACC) ( Jung et al., 2022 ). The activation patterns indicate that the CON may have a broader top-down control function across the working memory process. At the same time, the FPN could be more heavily implicated in momentary control or processing at the trial level ( Yu et al., 2023 ). Evidence suggests that the central executive interacts with the phonological loop and visuospatial sketchpad to support working memory processes ( Baddeley, 2003 ; Buchsbaum, 2010 ; Menon and D’Esposito, 2021 ). The function, localization, and neural basis of this interaction are thought to involve the activation of specific brain regions associated with each component of working memory, as discussed in detail below.

The phonological loop is divided into two components: a storage system that maintains information (a few seconds) and a component involving subvocal rehearsal—which maintains and refreshes information in the working memory. Neuroanatomically, the phonological loop is represented in the Brodmann area (BA) 40 in the parietal cortex and the rehearsal components in BA 44 and 6, both situated in the frontal cortex ( Osaka et al., 2007 ). The left inferior frontal gyrus (Broca’s area) and the left posterior superior temporal gyrus (Wernicke’s area) has been proposed to play a critical role in supporting phonological and verbal working memory tasks, specifically the subvocal rehearsal system of the articulatory loop ( Paulesu et al., 1993 ; Buchsbaum et al., 2001 ; Perrachione et al., 2017 ). The phonological store in verbal short-term memory has been localized at the left supramarginal gyrus ( Graves et al., 2008 ; Perrachione et al., 2017 ).

Studies utilizing neuroimaging techniques have consistently yielded results indicating notable activation in these brain regions during phonological activities like recalling non-words and maintaining verbal information in memory ( Awh et al., 1996 ; Graves et al., 2008 ). During tasks that require phonological rehearsal, there was an increase in activation in the left inferior frontal gyrus ( Paulesu et al., 1993 ). Researchers have noted an increase in activity within the superior temporal gyrus-which plays a significant role in auditory processing-in individuals performing tasks that necessitate verbal information maintenance and manipulation ( Smith et al., 1998 ; Chein et al., 2003 ).

Additionally, lesion studies have provided further confirmation regarding the importance of these regions. These investigations have revealed that impairment in performing phonological working memory tasks can transpire following damage inflicted upon the left hemisphere, particularly on perisylvian language areas ( Koenigs et al., 2011 ). It is common for individuals with lesions affecting regions associated with the phonological loop, such as the left inferior frontal gyrus and superior temporal gyrus, to have difficulty performing verbal working memory tasks. Clinical cases involving patients diagnosed with aphasia and specific language impairments have highlighted challenges related to retaining and manipulating auditory information. For example, those who sustain damage specifically within their left inferior frontal gyrus often struggle with tasks involving phonological rehearsal and verbal working memory activities, and therefore, they tend to perform poorly in tasks that require manipulation or repetition of verbal stimuli ( Saffran, 1997 ; Caplan and Waters, 2005 ).

The visuospatial sketchpad is engaged in the temporary retention and manipulation of visuospatial facts, including mental pictures, spatial associations, and object placements ( Miyake et al., 2001 ). The visuospatial sketchpad is localized to the right hemisphere, including the occipital lobe, parietal and frontal areas ( Osaka et al., 2007 ). Ren et al. (2019) identified the localization of the visuospatial sketchpad, and these areas were the right infero-lateral prefrontal cortex, lateral pre-motor cortices, right inferior parietal cortex, and the dorsolateral occipital cortices ( Burbaud et al., 1999 ; Salvato et al., 2021 ). Moreover, the posterior parietal cortex and the intraparietal sulcus have been implicated in spatial working memory ( Xu and Chun, 2006 ). Additionally, some evidence is available for an increase in brain regions associated with the visuospatial sketchpad during tasks involving mental imagery and spatial processing. Neuroimaging studies have revealed increased neural activation in some regions of the parietal cortex, mainly the superior and posterior parietal cortex, while performing mental rotation tasks ( Cohen et al., 1996 ; Kosslyn et al., 1997 ). However, further research is needed to better understand the visuospatial working memory and its integration with other cognitive processes ( Baddeley, 2003 ). Lesions to the regions involving the visuospatial sketchpad can have detrimental effects on visuospatial working memory tasks. Individuals with lesions to the posterior parietal cortex may exhibit deficits in mental rotation tasks and may be unable to mentally manipulate the visuospatial representation ( Buiatti et al., 2011 ). Moreover, studies concerning lesions have shown that damage to the parietal cortex can result in short-term deficits in visuospatial memory ( Shafritz et al., 2002 ). Damage to the occipital cortex can lead to performance impairments in tasks that require the generation and manipulation of mental visual images ( Moro et al., 2008 ).

The fourth component of the working memory, termed episodic buffer, was proposed by Baddeley (2000) . The episodic buffer is a multidimensional but essentially passive store that can hold a limited number of chunks, store bound features, and make them available to conscious awareness ( Baddeley et al., 2010 ; Hitch et al., 2019 ). Although research has suggested that episodic buffer is localized to the hippocampus ( Berlingeri et al., 2008 ) or the inferior lateral parietal cortex, it is thought to be not dependent on a single anatomical structure but instead can be influenced by the subsystems of working memory, long term memory, and even through perception ( Vilberg and Rugg, 2008 ; Baddeley et al., 2010 ). The episodic buffer provides a crucial link between the attentional central executive and the multidimensional information necessary for the operation of working memory ( Baddeley et al., 2011 ; Gelastopoulos et al., 2019 ).

The interdependence of the working memory modules, namely the phonological loop and visuospatial sketchpad, co-relates with other cognitive processes, for instance, spatial cognition and attention allocation ( Repovs and Baddeley, 2006 ). It has been found that the prefrontal cortex (PFC) and posterior parietal cortex (PPC) have a crucial role in several aspects of spatial cognition, such as the maintenance of spatially oriented attention and motor intentions ( Jerde and Curtis, 2013 ). The study by Sellers et al. (2016) and the review by Ikkai and Curtis (2011) posits that other brain areas could use the activity in PFC and PPC as a guide and manifest outputs to guide attention allocation, spatial memory, and motor planning. Moreover, research indicates that verbal information elicits an activation response in the left ventrolateral prefrontal cortex (VLPFC) when retained in the phonological loop, while visuospatial information is represented by a corresponding level of activity within the right homolog region ( Narayanan et al., 2005 ; Wolf et al., 2006 ; Emch et al., 2019 ). Specifically, the study by Yang et al. (2022) investigated the roles of two regions in the brain, the right inferior frontal gyrus (rIFG) and the right supra-marginal gyrus (rSMG), as they relate to spatial congruency in visual working memory tasks. A change detection task with online repetitive transcranial magnetic stimulation applied concurrently at both locations during high visual WM load conditions determined that rIFG is involved in actively repositioning the location of objects. At the same time, rSMG is engaged in passive perception of the stability of the location of objects.

Recent academic studies have found evidence to support the development of a new working memory model known as the state-based model ( D’Esposito and Postle, 2015 ). This theoretical model proposes that the allocation of attention toward internal representations permits short-term retention within working memory ( Ghaleh et al., 2019 ). The state-based model consists of two main categories: activated LTM models and sensorimotor recruitment models; the former largely focuses upon symbolic stimuli categorized under semantic aspects, while the latter has typically been applied to more perceptual tasks in experiments. This framework posits that prioritization through regulating cognitive processes provides insight into various characteristics across different activity types, including capacity limitations, proactive interference, etcetera ( D’Esposito and Postle, 2015 ). For example, the paper by Ghaleh et al. (2019) provides evidence for two separate mechanisms involved in maintenance of auditory information in verbal working memory: an articulatory rehearsal mechanism that relies more heavily on left sensorimotor areas and a non-articulatory maintenance mechanism that critically relies on left superior temporal gyrus (STG). These findings support the state-based model’s proposal that attentional allocation is necessary for short-term retention in working memory.

State-based models were found to be consistent with the suggested storage mechanism as they do not require representation transfer from one dedicated buffer type; research has demonstrated that any population of neurons and synapses may serve as such buffers ( Maass and Markram, 2002 ; Postle, 2006 ; Avraham et al., 2017 ). The review by D’Esposito and Postle (2015) examined the evidence to determine whether a persistent neural activity, synaptic mechanisms, or a combination thereof support representations maintained during working memory. Numerous neural mechanisms have been hypothesized to support the short-term retention of information in working memory and likely operate in parallel ( Sreenivasan et al., 2014 ; Kamiński and Rutishauser, 2019 ).

Persistent neural activity is the neural mechanism by which information is temporarily maintained ( Ikkai and Curtis, 2011 ; Panzeri et al., 2023 ). Recent review by Curtis and Sprague (2021) has focused on the notion that persistent neural activity is a fundamental mechanism for memory storage and have provided two main arcs of explanation. The first arc, mainly underpinned by empirical evidence from prefrontal cortex (PFC) neurophysiology experiments and computational models, posits that PFC neurons exhibit sustained firing during working memory tasks, enabling them to store representations in their active state ( Thuault et al., 2013 ). Intrinsic persistent firing in layer V neurons in the medial PFC has been shown to be regulated by HCN1 channels, which contribute to the executive function of the PFC during working memory episodes ( Thuault et al., 2013 ). Additionally, research has also found that persistent neural firing could possibly interact with theta periodic activity to sustain each other in the medial temporal, prefrontal, and parietal regions ( Düzel et al., 2010 ; Boran et al., 2019 ). The second arc involves advanced neuroimaging approaches which have, more recently, enabled researchers to decode content stored within working memories across distributed regions of the brain, including parts of the early visual cortex–thus extending this framework beyond just isolated cortical areas such as the PFC. There is evidence that suggests simple, stable, persistent activity among neurons in stimulus-selective populations may be a crucial mechanism for sustaining WM representations ( Mackey et al., 2016 ; Kamiński et al., 2017 ; Curtis and Sprague, 2021 ).

Badre (2008) discussed the functional organization of the PFC. The paper hypothesized that the rostro-caudal gradient of a function in PFC supported a control hierarchy, whereas posterior to anterior PFC mediated progressively abstract, higher-order controls ( Badre, 2008 ). However, this outlook proposed by Badre (2008) became outdated; the paper by Badre and Nee (2018) presented an updated look at the literature on hierarchical control. This paper supports neither a unitary model of lateral frontal function nor a unidimensional abstraction gradient. Instead, separate frontal networks interact via local and global hierarchical structures to support diverse task demands. This updated perspective is supported by recent studies on the hierarchical organization of representations within the lateral prefrontal cortex (LPFC) and the progressively rostral areas of the LPFC that process/represent increasingly abstract information, facilitating efficient and flexible cognition ( Thomas Yeo et al., 2011 ; Nee and D’Esposito, 2016 ). This structure allows the brain to access increasingly abstract action representations as required ( Nee and D’Esposito, 2016 ). It is supported by fMRI studies showing an anterior-to-posterior activation movement when tasks become more complex. Anatomical connectivity between areas also supports this theory, such as Area 10, which has projections back down to Area 6 but not vice versa.

Finally, studies confirm that different regions serve different roles along a hierarchy leading toward goal-directed behavior ( Badre and Nee, 2018 ). The paper by Postle (2015) exhibits evidence of activity in the prefrontal cortex that reflects the maintenance of high-level representations, which act as top-down signals, and steer the circulation of neural pathways across brain networks. The PFC is a source of top-down signals that influence processing in the posterior and subcortical regions ( Braver et al., 2008 ; Friedman and Robbins, 2022 ). These signals either enhance task-relevant information or suppress irrelevant stimuli, allowing for efficient yet effective search ( D’Esposito, 2007 ; D’Esposito and Postle, 2015 ; Kerzel and Burra, 2020 ). The study by Ratcliffe et al. (2022) provides evidence of the dynamic interplay between executive control mechanisms in the frontal cortex and stimulus representations held in posterior regions for working memory tasks. Moreover, the review by Herry and Johansen (2014) discusses the neural mechanisms behind actively maintaining task-relevant information in order for a person to carry out tasks and goals effectively. This review of data and research suggests that working memory is a multi-component system allowing for both the storage and processing of temporarily active representations. Neural activity throughout the brain can be differentially enhanced or suppressed based on context through top-down signals emanating from integrative areas such as PFC, parietal cortex, or hippocampus to actively maintain task-relevant information when it is not present in the environment ( Herry and Johansen, 2014 ; Kerzel and Burra, 2020 ).

In addition, Yu et al. (2022) examined how brain regions from the ventral stream pathway to the prefrontal cortex were activated during working memory (WM) gate opening and closing. They defined gate opening as the switch from maintenance to updating and gate closing as the switch from updating to maintenance. The data suggested that cognitive branching increases during the WM gating process, thus correlating the gating process and an information approach to the PFC function. The temporal cortices, lingual gyrus (BA19), superior frontal gyri including frontopolar cortices, and middle and inferior parietal regions are involved in processes of estimating whether a response option available will be helpful for each case. During gate closing, on the other hand, medial and superior frontal regions, which have been associated with conflict monitoring, come into play, as well as orbitofrontal and dorsolateral prefrontal processing at later times when decreasing activity resembling stopping or downregulating cognitive branching has occurred, confirming earlier theories about these areas being essential for estimation of usefulness already stored within long-term memories ( Yu et al., 2022 ).

Declarative and non-declarative memory

The distinctions between declarative and non-declarative memory are often based on the anatomical features of medial temporal lobe regions, specifically those involving the hippocampus ( Squire and Zola, 1996 ; Squire and Wixted, 2011 ). In the investigation of systems implicated in the process of learning and memory formation, it has been posited that the participation of the hippocampus is essential for the acquisition of declarative memories ( Eichenbaum and Cohen, 2014 ). In contrast, a comparatively reduced level of hippocampal involvement may suffice for non-declarative memories ( Squire and Zola, 1996 ; Williams, 2020 ).

Declarative memory (explicit) pertains to knowledge about facts and events. This type of information can be consciously retrieved with effort or spontaneously recollected without conscious intention ( Dew and Cabeza, 2011 ). There are two types of declarative memory: Episodic and Semantic. Episodic memory is associated with the recollection of personal experiences. It involves detailed information about events that happened in one’s life. Semantic memory refers to knowledge stored in the brain as facts, concepts, ideas, and objects; this includes language-related information like meanings of words and mathematical symbol values along with general world knowledge (e.g., capitals of countries) ( Binder and Desai, 2011 ). The difference between episodic and semantic memory is that when one retrieves episodic memory, the experience is known as “remembering”; when one retrieves information from semantic memory, the experience is known as “knowing” ( Tulving, 1985 ; Dew and Cabeza, 2011 ). The hippocampus, medial temporal lobe, and the areas in the diencephalon are implicated in declarative memory ( Richter-Levin and Akirav, 2003 ; Derner et al., 2020 ). The ventral parietal cortex (VPC) is involved in declarative memory processes, specifically episodic memory retrieval ( Henson et al., 1999 ; Davis et al., 2018 ). The evidence suggests that VPC and hippocampus is involved in the retrieval of contextual details, such as the location and timing of the event, and the information is critical for the formation of episodic memory ( Daselaar, 2009 ; Hutchinson et al., 2009 ; Wiltgen et al., 2010 ). The prefrontal cortex (PFC) is involved in the encoding (medial PFC) and retrieval (lateral PFC) of declarative memories, specifically in the integration of information across different sensory modalities ( Blumenfeld and Ranganath, 2007 ; Li et al., 2010 ). Research also suggests that the amygdala may modulate other brain regions involved with memory processing, thus, contributing to an enhanced recall of negative or positive experiences ( Hamann, 2001 ; Ritchey et al., 2008 ; Sendi et al., 2020 ). Maintenance of the integrity of hippocampal circuitry is essential for ensuring that episodic memory, along with spatial and temporal context information, can be retained in short-term or long-term working memory beyond 15 min ( Ito et al., 2003 ; Rasch and Born, 2013 ). Moreover, studies have suggested that the amygdala plays a vital role in encoding and retrieving explicit memories, particularly those related to emotionally charged stimuli which are supported by evidence of correlations between hippocampal activity and amygdala modulation during memory formation ( Richter-Levin and Akirav, 2003 ; Qasim et al., 2023 ).

Current findings in neuroimaging studies assert that a vast array of interconnected brain regions support semantic memory ( Binder and Desai, 2011 ). This network merges information sourced from multiple senses alongside different cognitive faculties necessary for generating abstract supramodal views on various topics stored within our consciousness. Modality-specific sensory, motor, and emotional system within these brain regions serve specialized tasks like language comprehension, while larger areas of the brain, such as the inferior parietal lobe and most of the temporal lobe, participate in more generalized interpretation tasks ( Binder and Desai, 2011 ; Kuhnke et al., 2020 ). These regions lie at convergences of multiple perceptual processing streams, enabling increasingly abstract, supramodal representations of perceptual experience that support a variety of conceptual functions, including object recognition, social cognition, language, and the remarkable human capacity to remember the past and imagine the future ( Binder and Desai, 2011 ; Binney et al., 2016 ). The following section will discuss the processes underlying memory consolidation and storage within declarative memory.

Non-declarative (implicit) memories refer to unconscious learning through experience, such as habits and skills formed from practice rather than memorizing facts; these are typically acquired slowly and automatically in response to sensory input associated with reward structures or prior exposure within our daily lives ( Kesner, 2017 ). Non-declarative memory is a collection of different phenomena with different neural substrates rather than a single coherent system ( Camina and Güell, 2017 ). It operates by similar principles, depending on local changes to a circumscribed brain region, and the representation of these changes is unavailable to awareness ( Reber, 2008 ). Non-declarative memory encompasses a heterogenous collection of abilities, such as associative learning, skills, and habits (procedural memory), priming, and non-associative learning ( Squire and Zola, 1996 ; Camina and Güell, 2017 ). Studies have concluded that procedural memory for motor skills depends upon activity in diverse set areas such as the motor cortex, striatum, limbic system, and cerebellum; similarly, perceptual skill learning is thought to be associated with sensory cortical activation ( Karni et al., 1998 ; Mayes, 2002 ). Research suggests that mutual connections between brain regions that are active together recruit special cells called associative memory cells ( Wang et al., 2016 ; Wang and Cui, 2018 ). These cells help integrate, store, and remember related information. When activated, these cells trigger the recall of memories, leading to behaviors and emotional responses. This suggests that co-activated brain regions with these mutual connections are where associative memories are formed ( Wang et al., 2016 ; Wang and Cui, 2018 ). Additionally, observational data reveals that priming mechanisms within distinct networks, such as the “repetition suppression” effect observed in visual cortical areas associated with sensory processing and in the prefrontal cortex for semantic priming, are believed to be responsible for certain forms of conditioning and implicit knowledge transfer experiences exhibited by individuals throughout their daily lives ( Reber, 2008 ; Wig et al., 2009 ; Camina and Güell, 2017 ). However, further research is needed to better understand the mechanisms of consolidation in non-declarative memory ( Camina and Güell, 2017 ).

The process of transforming memory into stable, long-lasting from a temporary, labile memory is known as memory consolidation ( McGaugh, 2000 ). Memory formation is based on the change in synaptic connections of neurons representing the memory. Encoding causes synaptic Long-Term potentiation (LTP) or Long-Term depression (LTD) and induces two consolidation processes. The first is synaptic or cellular consolidation which involves remodeling synapses to produce enduring changes. Cellular consolidation is a short-term process that involves stabilizing the neural trace shortly after learning via structural brain changes in the hippocampus ( Lynch, 2004 ). The second is system consolidation, which builds on synaptic consolidation where reverberating activity leads to redistribution for long-term storage ( Mednick et al., 2011 ; Squire et al., 2015 ). System consolidation is a long-term process during which memories are gradually transferred to and integrated with cortical neurons, thus promoting their stability over time. In this way, memories are rendered less susceptible to forgetting. Hebb postulated that when two neurons are repeatedly activated simultaneously, they become more likely to exhibit a coordinated firing pattern of activity in the future ( Langille, 2019 ). This proposed enduring change in synchronized neuronal activation was consequently termed cellular consolidation ( Bermudez-Rattoni, 2010 ).

The following sections of this paper incorporate a more comprehensive investigation into various essential procedures connected with memory consolidation- namely: long-term potentiation (LTP), long-term depression (LTD), system consolidation, and cellular consolidation. Although these mechanisms have been presented briefly before this paragraph, the paper aims to offer greater insight into each process’s function within the individual capacity and their collective contribution toward memory consolidation.

Synaptic plasticity mechanisms implicated in memory stabilization

Long-Term Potentiation (LTP) and Long-Term Depression (LTP) are mechanisms that have been implicated in memory stabilization. LTP is an increase in synaptic strength, whereas LTD is a decrease in synaptic strength ( Ivanco, 2015 ; Abraham et al., 2019 ).

Long-Term Potentiation (LTP) is a phenomenon wherein synaptic strength increases persistently due to brief exposures to high-frequency stimulation ( Lynch, 2004 ). Studies of Long-Term Potentiation (LTP) have led to an understanding of the mechanisms behind synaptic strengthening phenomena and have provided a basis for explaining how and why strong connections between neurons form over time in response to stimuli.

The NMDA receptor-dependent LTP is the most commonly described LTP ( Bliss and Collingridge, 1993 ; Luscher and Malenka, 2012 ). In this type of LTP, when there is high-frequency stimulation, the presynaptic neuron releases glutamate, an excitatory neurotransmitter. Glutamate binds to the AMPA receptor on the postsynaptic neuron, which causes the neuron to fire while opening the NMDA receptor channel. The opening of an NMDA channel elicits a calcium ion influx into the postsynaptic neuron, thus initiating a series of phosphorylation events as part of the ensuing molecular cascade. Autonomously phosphorylated CaMKII and PKC, both actively functional through such a process, have been demonstrated to increase the conductance of pre-existing AMPA receptors in synaptic networks. Additionally, this has been shown to stimulate the introduction of additional AMPA receptors into synapses ( Malenka and Nicoll, 1999 ; Lynch, 2004 ; Luscher and Malenka, 2012 ; Bailey et al., 2015 ).

There are two phases of LTP: the early phase and the late phase. It has been established that the early phase LTP (E-LTP) does not require RNA or protein synthesis; therefore, its synaptic strength will dissipate in minutes if late LTP does not stabilize it. On the contrary, late-phase LTP (L-LTP) can sustain itself over a more extended period, from several hours to multiple days, with gene transcription and protein synthesis in the postsynaptic cell ( Frey and Morris, 1998 ; Orsini and Maren, 2012 ). The strength of presynaptic tetanic stimulation has been demonstrated to be a necessary condition for the activation of processes leading to late LTP ( Luscher and Malenka, 2012 ; Bailey et al., 2015 ). This finding is supported by research examining synaptic plasticity, notably Eric Kandel’s discovery that CREB–a transcription factor–among other cytoplasmic and nuclear molecules, are vital components in mediating molecular changes culminating in protein synthesis during this process ( Kaleem et al., 2011 ; Kandel et al., 2014 ). Further studies have shown how these shifts ultimately lead to AMPA receptor stabilization at post-synapses facilitating long-term potentiation within neurons ( Luscher and Malenka, 2012 ; Bailey et al., 2015 ).

The “synaptic tagging and capture hypothesis” explains how a weak event of tetanization at synapse A can transform to late-LTP if followed shortly by the strong tetanization of a different, nearby synapse on the same neuron ( Frey and Morris, 1998 ; Redondo and Morris, 2011 ; Okuda et al., 2020 ; Park et al., 2021 ). During this process, critical plasticity-related proteins (PRPs) are synthesized, which stabilize their own “tag” and that from the weaker synaptic activity ( Moncada et al., 2015 ). Recent evidence suggests that calcium-permeable AMPA receptors (CP-AMPARs) are involved in this form of heterosynaptic metaplasticity ( Park et al., 2018 ). The authors propose that the synaptic activation of CP-AMPARs triggers the synthesis of PRPs, which are then engaged by the weak induction protocol to facilitate LTP on the independent input. The paper also suggests that CP-AMPARs are required during the induction of LTP by the weak input for the full heterosynaptic metaplastic effect to be observed ( Park et al., 2021 ). Additionally, it has been further established that catecholamines such as dopamine plays an integral part in memory persistence by inducing PRP synthesis ( Redondo and Morris, 2011 ; Vishnoi et al., 2018 ). Studies have found that dopamine release in the hippocampus can enhance LTP and improve memory consolidation ( Lisman and Grace, 2005 ; Speranza et al., 2021 ).

Investigations into neuronal plasticity have indicated that synaptic strength alterations associated with certain forms of learning and memory may be analogous to those underlying Long-Term Potentiation (LTP). Research has corroborated this notion, demonstrating a correlation between these two phenomena ( Lynch, 2004 ). The three essential properties of Long-Term Potentiation (LTP) that have been identified are associativity, synapse specificity, and cooperativity ( Kandel and Mack, 2013 ). These characteristics provide empirical evidence for the potential role of LTP in memory formation processes. Specifically, associativity denotes the amplification of connections when weak stimulus input is paired with a powerful one; synapse specificity posits that this potentiating effect only manifests on synaptic locations exhibiting coincidental activity within postsynaptic neurons, while cooperativity suggests stimulated neuron needs to attain an adequate threshold of depolarization before LTP can be induced again ( Orsini and Maren, 2012 ).

There is support for the idea that memories are encoded by modification of synaptic strengths through cellular mechanisms such as LTP and LTD ( Nabavi et al., 2014 ). The paper by Nabavi et al. (2014) shows that fear conditioning, a type of associative memory, can be inactivated and reactivated by LTD and LTP, respectively. The findings of the paper support a causal link between these synaptic processes and memory. Moreover, the paper suggests that LTP is used to form neuronal assemblies that represent a memory, and LTD could be used to disassemble them and thereby inactivate a memory ( Nabavi et al., 2014 ). Hippocampal LTD has been found to play an essential function in regulating synaptic strength and forming memories, such as long-term spatial memory ( Ge et al., 2010 ). However, it is vital to bear in mind that studies carried out on LTP exceed those done on LTD; hence the literature on it needs to be more extensive ( Malenka and Bear, 2004 ; Nabavi et al., 2014 ).

Cellular consolidation and memory

For an event to be remembered, it must form physical connections between neurons in the brain, which creates a “memory trace.” This memory trace can then be stored as long-term memory ( Langille and Brown, 2018 ). The formation of a memory engram is an intricate process requiring neuronal depolarization and the influx of intracellular calcium ( Mank and Griesbeck, 2008 ; Josselyn et al., 2015 ; Xu et al., 2017 ). This initiation leads to a cascade involving protein transcription, structural and functional changes in neural networks, and stabilization during the quiescence period, followed by complete consolidation for its success. Interference from new learning events or disruption caused due to inhibition can abort this cycle leading to incomplete consolidation ( Josselyn et al., 2015 ).

Cyclic-AMP response element binding protein (CREB) has been identified as an essential transcription factor for memory formation ( Orsini and Maren, 2012 ). It regulates the expression of PRPs and enhances neuronal excitability and plasticity, resulting in changes to the structure of cells, including the growth of dendritic spines and new synaptic connections. Blockage or enhancement of CREB in certain areas can affect subsequent consolidation at a systems level–decreasing it prevents this from occurring, while aiding its presence allows even weak learning conditions to produce successful memory formation ( Orsini and Maren, 2012 ; Kandel et al., 2014 ).

Strengthening weakly encoded memories through the synaptic tagging and capture hypothesis may play an essential role in cellular consolidation. Retroactive memory enhancement has also been demonstrated in human studies, mainly when items are initially encoded with low strength but later paired with shock after consolidation ( Dunsmoor et al., 2015 ). The synaptic tagging and capture theory (STC) and its extension, the behavioral tagging hypothesis (BT), have both been used to explain synaptic specificity and the persistence of plasticity ( Moncada et al., 2015 ). STC proposed that electrophysiological activity can induce long-term changes in synapses, while BT postulates similar effects of behaviorally relevant neuronal events on learning and memory models. This hypothesis proposes that memory consolidation relies on combining two distinct processes: setting a “learning tag” and synthesizing plasticity-related proteins ( De novo protein synthesis, increased CREB levels, and substantial inputs to nearby synapses) at those tagged sites. BT explains how it is possible for event episodes with low-strength inputs or engagements can be converted into lasting memories ( Lynch, 2004 ; Moncada et al., 2015 ). Similarly, the emotional tagging hypothesis posits that the activation of the amygdala in emotionally arousing events helps to mark experiences as necessary, thus enhancing synaptic plasticity and facilitating transformation from transient into more permanent forms for encoding long-term memories ( Richter-Levin and Akirav, 2003 ; Zhu et al., 2022 ).

Cellular consolidation, the protein synthesis-dependent processes observed in rodents that may underlie memory formation and stabilization, has been challenging to characterize in humans due to the limited ability to study it directly ( Bermudez-Rattoni, 2010 ). Additionally, multi-trial learning protocols commonly used within human tests as opposed to single-trial experiments conducted with non-human subjects suggest there could be interference from subsequent information that impedes individual memories from being consolidated reliably. This raises important questions regarding how individuals can still form strong and long-lasting memories when exposed to frequent stimuli outside controlled laboratory conditions. Although this phenomenon remains undiscovered by science, it is of utmost significance for gaining a deeper understanding of our neural capacities ( Genzel and Wixted, 2017 ).

The establishment of distributed memory traces requires a narrow temporal window following the initial encoding process, during which cellular consolidation occurs ( Nader and Hardt, 2009 ). Once this period ends and consolidation has been completed, further protein synthesis inhibition or pharmacological disruption will be less effective at altering pre-existing memories and interfering with new learning due to the stabilization of the trace in its new neuronal network connections ( Nader and Hardt, 2009 ). Thus, systems consolidation appears critical for the long-term maintenance of memory within broader brain networks over extended periods after their formation ( Bermudez-Rattoni, 2010 ).

System consolidation and memory

Information is initially stored in both the hippocampus and neocortex ( Dudai et al., 2015 ). The hippocampus subsequently guides a gradual process of reorganization and stabilization whereby information present within the neocortex becomes autonomous from that in the hippocampal store. Scholars have termed this phenomenon “standard memory consolidation model” or “system consolidation” ( Squire et al., 2015 ).

The Standard Model suggests that information acquired during learning is simultaneously stored in both the hippocampus and multiple cortical modules. Subsequently, it posits that over a period of time which may range from weeks to months or longer, the hippocampal formation directs an integration process by which these various elements become enclosed into single unified structures within the cortex ( Gilboa and Moscovitch, 2021 ; Howard et al., 2022 ). These newly learned memories are then assimilated into existing networks without interference or compression when necessary ( Frankland and Bontempi, 2005 ). It is important to note that memory engrams already exist within cortical networks during encoding. They only need strengthening through links enabled by hippocampal assistance-overtime allowing remote memory storage without reliance on the latter structure. Data appears consistent across studies indicating that both AMPA-and NMDA receptor-dependent “tagging” processes occurring within the cortex are essential components of progressive rewiring, thus enabling longer-term retention ( Takeuchi et al., 2014 ; Takehara-Nishiuchi, 2020 ).

Recent studies have additionally demonstrated that the rate of system consolidation depends on an individual’s ability to relate new information to existing networks made up of connected neurons, popularly known as “schemas” ( Robin and Moscovitch, 2017 ). In situations where prior knowledge is present and cortical modules are already connected at the outset of learning, it has been observed that a hippocampal-neocortical binding process occurs similarly to when forming new memories ( Schlichting and Preston, 2015 ). The proposed framework involves the medial temporal lobe (MTL), which is involved in acquiring new information and binds different aspects of an experience into a single memory trace. In contrast, the medial prefrontal cortex (mPFC) integrates this information with the existing knowledge ( Zeithamova and Preston, 2010 ; van Kesteren et al., 2012 ). During consolidation and retrieval, MTL is involved in replaying memories to the neocortex, where they are gradually integrated with existing knowledge and schemas and help retrieve memory traces. During retrieval, the mPFC is thought to use existing knowledge and schemas to guide retrieval and interpretation of memory. This may involve the assimilation of newly acquired information into existing cognitive schemata as opposed to the comparatively slow progression of creating intercortical connections ( Zeithamova and Preston, 2010 ; van Kesteren et al., 2012 , 2016 ).

Medial temporal lobe structures are essential for acquiring new information and necessary for autobiographical (episodic) memory ( Brown et al., 2018 ). The consolidation of autobiographical memories depends on a distributed network of cortical regions. Brain areas such as entorhinal, perirhinal, and parahippocampal cortices are essential for learning new information; however, they have little impact on the recollection of the past ( Squire et al., 2015 ). The hippocampus is a region of the brain that forms episodic memories by linking multiple events to create meaningful experiences ( Cooper and Ritchey, 2019 ). It receives information from all areas of the association cortex and cingulate cortex, subcortical regions via the fornix, as well as signals originating within its entorhinal cortex (EC) and amygdala regarding emotionally laden or potentially hazardous stimuli ( Sorensen, 2009 ). Such widespread connectivity facilitates the construction of an accurate narrative underpinning each remembered episode, transforming short-term into long-term recollections ( Richter-Levin and Akirav, 2003 ).

Researchers have yet to establish a consensus regarding where semantic memory information is localized within the brain ( Roldan-Valadez et al., 2012 ). Some proponents contend that such knowledge is lodged within perceptual and motor systems, triggered when we initially associate with a given object. This point of view is supported by studies highlighting how neural activity occurs initially in the occipital cortex, followed by left temporal lobe involvement during processing and pertinent contributions to word selection/retrieval via activation of left inferior frontal cortices ( Patterson et al., 2007 ). Moreover, research indicates elevated levels of fusiform gyrus engagement (a ventral surface region encompassing both temporal lobes) occurring concomitantly with verbal comprehension initiatives, including reading and naming tasks ( Patterson et al., 2007 ).

Research suggests that the hippocampus is needed for a few years after learning to support semantic memory (factual information), yet, it is not needed for the long term ( Squire et al., 2015 ). However, some forms of memory remain dependent on the hippocampus, such as the retrieval of spatial memory ( Wiltgen et al., 2010 ). Similarly, the Multiple-trace theory ( Moscovitch et al., 2006 ), also known as the transformation hypothesis ( Winocur and Moscovitch, 2011 ), posits that hippocampal engagement is necessary for memories that retain contextual detail such as episodic memories. Consolidation of memories into the neocortex is theorized to involve a loss of specific finer details, such as temporal and spatial information, in addition to contextual elements. This transition ultimately results in an evolution from episodic memory toward semantic memory, which consists mainly of gist-based facts ( Moscovitch et al., 2006 ).

Sleep and memory consolidation

Sleep is an essential physiological process crucial to memory consolidation ( Siegel, 2001 ). Sleep is divided into two stages: Non-rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep. NREM sleep is divided into three stages: N1, N2, and N3 (AKA Slow Wave Sleep or SWS) ( Rasch and Born, 2013 ). Each stage displays unique oscillatory patterns and phenomena responsible for consolidating memories in distinct ways. The first stage, or N1 sleep, is when an individual transitions between wakefulness and sleep. This type of sleep is characterized by low-amplitude, mixed-frequency brain activity. N1 sleep is responsible for the initial encoding of memories ( Rasch and Born, 2013 ). The second stage, or N2 sleep, is characterized by the occurrence of distinct sleep spindles and K-complexes in EEG. N2 is responsible for the consolidation of declarative memories ( Marshall and Born, 2007 ). The third stage of sleep N3, also known as slow wave sleep (SWS), is characterized by low-frequency brain activity, slow oscillations, and high amplitude. The slow oscillations which define the deepest stage of sleep are trademark rhythms of NREM sleep. These slow oscillations are delta waves combined to indicate slow wave activity (SWA), which is implicated in memory consolidation ( Tononi and Cirelli, 2003 ; Stickgold, 2005 ; Kim et al., 2019 ). Sleep spindles are another trademark defining NREM sleep ( Stickgold, 2005 ). Ripples are high-frequency bursts, and when combined with irregularly occurring sharp waves (high amplitude), they form the sharp-wave ripple (SWR). These spindles and the SWRs coordinate the reactivation and redistribution of hippocampus-dependent memories to neocortical sites ( Ngo et al., 2020 ; Girardeau and Lopes-dos-Santos, 2021 ). The third stage is also responsible for the consolidation of procedural memories, such as habits and motor skills ( Diekelmann and Born, 2010 ). During SWS, there is minimal cholinergic activity and intermediate noradrenergic activity ( Datta and MacLean, 2007 ).

Finally, the fourth stage of sleep is REM sleep, characterized by phasic REMs and muscle atonia ( Reyes-Resina et al., 2021 ). During REM sleep, there is high cholinergic activity, serotonergic and noradrenergic activity are at a minimum, and high theta activity ( Datta and MacLean, 2007 ). REM sleep is also characterized by local increases in plasticity-related immediate-early gene activity, which might favor the subsequent synaptic consolidation of memories in the cortex ( Ribeiro, 2007 ; Diekelmann and Born, 2010 ; Reyes-Resina et al., 2021 ). The fourth stage of sleep is responsible for the consolidation of emotional memories and the integration of newly acquired memories into existing knowledge structures ( Rasch and Born, 2013 ). Studies indicate that the cholinergic system plays an imperative role in modifying these processes by toggling the entire thalamo-cortico-hippocampal network between distinct modes, namely high Ach encoding mode during active wakefulness and REM sleep and low Ach consolidation mode during quiet wakefulness and NREM sleep ( Bergmann and Staresina, 2017 ; Li et al., 2020 ). Consequently, improving neocortical hippocampal communication results in efficient memory encoding/synaptic plasticity, whereas hippocampo-neocortical interactions favor better systemic memory consolidation ( Diekelmann and Born, 2010 ).

The dual process hypothesis of memory consolidation posits that SWS facilitates declarative, hippocampus-dependent memory, whereas REM sleep facilitates non-declarative hippocampus-independent memory ( Maquet, 2001 ; Diekelmann and Born, 2010 ). On the other hand, the sequential hypothesis states that different sleep stages play a sequential role in memory consolidation. Memories are encoded during wakefulness, consolidated during NREM sleep, and further processed and integrated during REM sleep ( Rasch and Born, 2013 ). However, there is evidence present that contradicts the sequential hypothesis. A study by Goerke et al. (2013) found that declarative memories can be consolidated during REM sleep, suggesting that the relationship between sleep stages and memory consolidation is much more complex than a sequential model. Moreover, other studies indicate the importance of coordinating specific sleep phases with learning moments for optimal memory retention. This indicates that the timing of sleep has more influence than the specific sleep stages ( Gais et al., 2006 ). The active system consolidation theory suggests that an active consolidation process results from the selective reactivation of memories during sleep; the brain selectively reactivates newly encoded memories during sleep, which enhances and integrates them into the network of pre-existing long-term memories ( Born et al., 2006 ; Howard et al., 2022 ). Research has suggested that slow-wave sleep (SWS) and rapid eye movement (REM) sleep have complementary roles in memory consolidation. Declarative and non-declarative memories benefiting differently depending on which sleep stage they rely on ( Bergmann and Staresina, 2017 ). Specifically, during SWS, the brain actively reactivates and reorganizes hippocampo-neocortical memory traces as part of system consolidation. Following this, REM sleep is crucial for stabilizing these reactivated memory traces through synaptic consolidation. While SWS may initiate early plastic processes in hippocampo-neocortical memory traces by “tagging” relevant neocortico-neocortical synapses for later consolidation ( Frey and Morris, 1998 ), long-term plasticity requires subsequent REM sleep ( Rasch and Born, 2007 , 2013 ).

The active system consolidation hypothesis is not the only mechanism proposed for memory consolidation during sleep. The synaptic homeostasis hypothesis proposes that sleep is necessary for restoring synaptic homeostasis, which is challenged by synaptic strengthening triggered by learning during wake and synaptogenesis during development ( Tononi and Cirelli, 2014 ). The synaptic homeostasis hypothesis assumes consolidation is a by-product of the global synaptic downscaling during sleep ( Puentes-Mestril and Aton, 2017 ). The two models are not mutually exclusive, and the hypothesized processes probably act in concert to optimize the memory function of sleep ( Diekelmann and Born, 2010 ).

Non-rapid eye movement sleep plays an essential role in the systems consolidation of memories, with evidence showing that different oscillations are involved in this process ( Düzel et al., 2010 ). With an oscillatory sequence initiated by a slow frontal cortex oscillation (0.5–1 Hz) traveling to the medial temporal lobe and followed by a sharp-wave ripple (SWR) in the hippocampus (100–200 Hz). Replay activity of memories can be measured during this oscillatory sequence across various regions, including the motor cortex and visual cortex ( Ji and Wilson, 2006 ; Eichenlaub et al., 2020 ). Replay activity of memory refers to the phenomenon where the hippocampus replays previously experienced events during sharp wave ripples (SWRs) and theta oscillations ( Zielinski et al., 2018 ). During SWRs, short, transient bursts of high-frequency oscillations occur in the hippocampus. During theta oscillations, hippocampal spikes are ordered according to the locations of their place fields during behavior. These sequential activities are thought to play a role in memory consolidation and retrieval ( Zielinski et al., 2018 ). The paper by Zielinski et al. (2018) suggests that coordinated hippocampal-prefrontal representations during replay and theta sequences play complementary and overlapping roles at different stages in learning, supporting memory encoding and retrieval, deliberative decision-making, planning, and guiding future actions.

Additionally, the high-frequency oscillations of SWR reactivate groups of neurons attributed to spatial information encoding to align synchronized activity across an array of neural structures, which results in distributed memory creation ( Swanson et al., 2020 ; Girardeau and Lopes-dos-Santos, 2021 ). Parallel to this process is slow oscillation or slow-wave activity within cortical regions, which reflects synced neural firing and allows regulation of synaptic weights, which is in accordance with the synaptic homeostasis hypothesis (SHY). The SHY posits that downscaling synaptic strengths help incorporate new memories by avoiding saturation of resources during extended periods–features validated by discoveries where prolonged wakefulness boosts amplitude while it diminishes during stretches of enhanced sleep ( Girardeau and Lopes-dos-Santos, 2021 ).

During REM sleep, the brain experiences “paradoxical” sleep due to the similarity in activity to wakefulness. This stage plays a significant role in memory processing. Theta oscillations which are dominant during REM sleep, are primarily observed in the hippocampus, and these are involved in memory consolidation ( Landmann et al., 2014 ). There has been evidence of coherence between theta oscillations in the hippocampus, medial frontal cortex, and amygdala, which support their involvement in memory consolidation ( Popa et al., 2010 ). During REM sleep, phasic events such as ponto-geniculo-occipital waves originating from the brainstem coordinate activity across various brain structures and may contribute to memory consolidation processes ( Rasch and Born, 2013 ). Research has suggested that sleep-associated consolidation may be mediated by the degree of overlap between new and already known material whereby, if the acquired information is similar to the information one has learned, it is more easily consolidated during sleep ( Tamminen et al., 2010 ; Sobczak, 2017 ).

In conclusion, understanding more about how the brains cycle through different stages of sleep, including specific wave patterns, offers valuable insight into the ability to store memories effectively. While NREM sleep is associated with SWRs and slow oscillations, facilitating memory consolidation and synaptic downscaling, REM sleep, characterized by theta oscillations and phasic events, contributes to memory reconsolidation and the coordination of activity across brain regions. By exploring the interactions between sleep stages, oscillations, and memory processes, one may learn more about how sleep impacts brain function and cognition in greater detail.

Century has passed since we addressed memory, and several notable findings have moved from bench-to-bedside research. Several cross-talks between multidiscipline have been encouraged. Nevertheless, further research is needed into neurobiological mechanisms of non-declarative memory, such as conditioning ( Gallistel and Balsam, 2014 ). Modern research indicates that structural change that encodes information is likely at the level of the synapse, and the computational mechanisms are implemented at the level of neural circuitry. However, it also suggests that intracellular mechanisms realized at the molecular level, such as micro RNAs, should not be discounted as potential mechanisms. However, further research is needed to study the molecular and structural changes brought on by implicit memory ( Gallistel and Balsam, 2014 ).

The contribution of non-human animal studies toward our understanding of memory processes cannot be understated; hence recognizing their value is vital for moving forward. While this paper predominantly focused on cognitive neuroscience perspectives, some articles cited within this paper were sourced from non-human animal studies providing fundamental groundwork and identification of critical mechanisms relevant to human memories. A need persists for further investigation—primarily with humans—which can validate existing findings from non-human animals. Moving forward, it is prudent for researchers to bridge the gap between animal and human investigations done while exploring parallels and exploring unique aspects of human memory processes. By integrating findings from both domains, one can gain a more comprehensive understanding of the complexities of memory and its underlying neural mechanisms. Such investigations will broaden the horizon of our memory process and answer the complex nature of memory storage.

This paper attempted to provide an overview and summarize memory and its processes. The paper focused on bringing the cognitive neuroscience perspective on memory and its processes. This may provide the readers with the understanding, limitations, and research perspectives of memory mechanisms.

Data availability statement

Author contributions.

SS and MKA: conceptualization, framework, and manuscript writing. AK: review and editing of the manuscript. All authors contributed to the article and approved the submitted version.

Acknowledgments

We gratefully thank students and Indian Institute of Technology Roorkee (IITR) office staff for their conditional and unconditional support. We also thank the Memory and Anxiety Research Group (MARG), IIT Roorkee for its constant support.

Funding Statement

MKA was supported by the F.I.G. grant (IITR/SRIC/2741). The funding agency had no role in the preparation of the manuscript.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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