presentation on types of memory

Types of Memory

Reviewed by Psychology Today Staff

A person’s memory is a sea of images and other sensory impressions, facts and meanings, echoes of past feelings, and ingrained codes for how to behave—a diverse well of information. Naturally, there are many ways (some experts suggest there are hundreds) to describe the varieties of what people remember and how. While the different brands of memory are not always described in exactly the same way by memory researchers, some key concepts have emerged.

These forms of memory, which can overlap in daily life, have also been arranged into broad categories. Memory that lingers for a moment (or even less than a second) could be described as short-term memory , while any kind of information that is preserved for remembering at a later point can be called long-term memory . Memory experts have also distinguished explicit memory , in which information is consciously recalled, from implicit memory , the use of saved information without conscious awareness that it’s being recalled.

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• Episodic Memory
• Semantic Memory
• Procedural Memory
• Short-Term Memory and Working Memory
• Sensory Memory
• Prospective Memory

When a person recalls a particular event (or “episode”) experienced in the past, that is episodic memory . This kind of long-term memory brings to attention details about anything from what one ate for breakfast to the emotions that were stirred up during a serious conversation with a romantic partner. The experiences conjured by episodic memory can be very recent or decades-old.

A related concept is autobiographical memory , which is the memory of information that forms part of a person’s life story. However, while autobiographical memory includes memories of events in one’s life (such as one’s sixteenth birthday party), it can also encompass facts (such as one’s birth date) and other non-episodic forms of information.

• The details of a phone call you had 20 minutes ago

• How you felt during your last argument

• What it was like receiving your high-school diploma

Semantic memory is someone’s long-term store of knowledge: It’s composed of pieces of information such as facts learned in school, what concepts mean and how they are related, or the definition of a particular word. The details that make up semantic memory can correspond to other forms of memory. One may remember factual details about a party, for instance—what time it started, at whose house it took place, how many people were there, all part of semantic memory—in addition to recalling the sounds heard and excitement felt. But semantic memory can also include facts and meanings related to people, places, or things one has no direct relation to.

• What year it currently is

• The capital of a foreign country

• The meaning of a slang term

Sitting on a bike after not riding one for years and recalling just what to do is a quintessential example of procedural memory . The term describes long-term memory for how to do things, both physical and mental, and is involved in the process of learning skills—from the basic ones people take for granted to those that require considerable practice. A related term is kinesthetic memory , which refers specifically to memory for physical behaviors.

• How to tie your shoes

• How to send an email

• How to shoot a basketball

The terms short-term memory and working memory are sometimes used interchangeably, and both refer to storage of information for a brief amount of time. Working memory can be distinguished from general short-term memory, however, in that working memory specifically involves the temporary storage of information that is being mentally manipulated.

Short-term memory is used when, for instance, the name of a new acquaintance, a statistic, or some other detail is consciously processed and retained for at least a short period of time. It may then be saved in long-term memory, or it may be forgotten within minutes. With working memory , information—the preceding words in a sentence one is reading, for example—is held in mind so that it can be used in the moment.

• The appearance of someone you met a minute ago

• The current temperature, immediately after looking it up

• What happened moments ago in a movie

• A number you have calculated as part of a mental math problem

• The person named at the beginning of a sentence

• Holding a concept in mind (such as ball ) and combining it with another ( orange )

Sensory memories are what psychologists call the short-term memories of just-experienced sensory stimuli such as sights and sounds. The brief memory of something just seen has been called iconic memory, while the sound-based equivalent is called echoic memory. Additional forms of short-term sensory memory are thought to exist for the other senses as well.

Sense-related memories, of course, can also be preserved long-term. Visual-spatial memory refers to memory of how objects are organized in space—tapped when a person remembers which way to walk to get to the grocery store. Auditory memory , olfactory memory , and haptic memory are terms for stored sensory impressions of sounds, smells, and skin sensations, respectively.

• The sound of a piano note that was just played

• The appearance of a car that drove by

• The smell of a restaurant you passed

Prospective memory is forward-thinking memory: It means recalling an intention from the past in order to do something in the future. It is essential for daily functioning, in that memories of previous intentions, including very recent ones, ensure that people execute their plans and meet their obligations when the intended behaviors can’t be carried out right away, or have to be carried out routinely.

• To call someone back

• To stop at the drugstore on the way home

• To pay the rent every month

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8.1 Memories as Types and Stages

Learning objectives.

• Compare and contrast explicit and implicit memory, identifying the features that define each.
• Explain the function and duration of eidetic and echoic memories.
• Summarize the capacities of short-term memory and explain how working memory is used to process information in it.

As you can see in Table 8.1 “Memory Conceptualized in Terms of Types, Stages, and Processes” , psychologists conceptualize memory in terms of types , in terms of stages , and in terms of processes . In this section we will consider the two types of memory, explicit memory and implicit memory , and then the three major memory stages: sensory , short-term , and long-term (Atkinson & Shiffrin, 1968). Then, in the next section, we will consider the nature of long-term memory, with a particular emphasis on the cognitive techniques we can use to improve our memories. Our discussion will focus on the three processes that are central to long-term memory: encoding , storage , and retrieval .

Table 8.1 Memory Conceptualized in Terms of Types, Stages, and Processes

Explicit Memory

When we assess memory by asking a person to consciously remember things, we are measuring explicit memory . Explicit memory refers to knowledge or experiences that can be consciously remembered . As you can see in Figure 8.2 “Types of Memory” , there are two types of explicit memory: episodic and semantic . Episodic memory refers to the firsthand experiences that we have had (e.g., recollections of our high school graduation day or of the fantastic dinner we had in New York last year). Semantic memory refers to our knowledge of facts and concepts about the world (e.g., that the absolute value of −90 is greater than the absolute value of 9 and that one definition of the word “affect” is “the experience of feeling or emotion”).

Figure 8.2 Types of Memory

Explicit memory is assessed using measures in which the individual being tested must consciously attempt to remember the information. A recall memory test is a measure of explicit memory that involves bringing from memory information that has previously been remembered . We rely on our recall memory when we take an essay test, because the test requires us to generate previously remembered information. A multiple-choice test is an example of a recognition memory test , a measure of explicit memory that involves determining whether information has been seen or learned before .

Your own experiences taking tests will probably lead you to agree with the scientific research finding that recall is more difficult than recognition. Recall, such as required on essay tests, involves two steps: first generating an answer and then determining whether it seems to be the correct one. Recognition, as on multiple-choice test, only involves determining which item from a list seems most correct (Haist, Shimamura, & Squire, 1992). Although they involve different processes, recall and recognition memory measures tend to be correlated. Students who do better on a multiple-choice exam will also, by and large, do better on an essay exam (Bridgeman & Morgan, 1996).

A third way of measuring memory is known as relearning (Nelson, 1985). Measures of relearning (or savings) assess how much more quickly information is processed or learned when it is studied again after it has already been learned but then forgotten . If you have taken some French courses in the past, for instance, you might have forgotten most of the vocabulary you learned. But if you were to work on your French again, you’d learn the vocabulary much faster the second time around. Relearning can be a more sensitive measure of memory than either recall or recognition because it allows assessing memory in terms of “how much” or “how fast” rather than simply “correct” versus “incorrect” responses. Relearning also allows us to measure memory for procedures like driving a car or playing a piano piece, as well as memory for facts and figures.

Implicit Memory

While explicit memory consists of the things that we can consciously report that we know, implicit memory refers to knowledge that we cannot consciously access. However, implicit memory is nevertheless exceedingly important to us because it has a direct effect on our behavior. Implicit memory refers to the influence of experience on behavior, even if the individual is not aware of those influences . As you can see in Figure 8.2 “Types of Memory” , there are three general types of implicit memory: procedural memory, classical conditioning effects, and priming.

Procedural memory refers to our often unexplainable knowledge of how to do things . When we walk from one place to another, speak to another person in English, dial a cell phone, or play a video game, we are using procedural memory. Procedural memory allows us to perform complex tasks, even though we may not be able to explain to others how we do them. There is no way to tell someone how to ride a bicycle; a person has to learn by doing it. The idea of implicit memory helps explain how infants are able to learn. The ability to crawl, walk, and talk are procedures, and these skills are easily and efficiently developed while we are children despite the fact that as adults we have no conscious memory of having learned them.

A second type of implicit memory is classical conditioning effects, in which we learn, often without effort or awareness, to associate neutral stimuli (such as a sound or a light) with another stimulus (such as food), which creates a naturally occurring response, such as enjoyment or salivation. The memory for the association is demonstrated when the conditioned stimulus (the sound) begins to create the same response as the unconditioned stimulus (the food) did before the learning.

The final type of implicit memory is known as priming , or changes in behavior as a result of experiences that have happened frequently or recently . Priming refers both to the activation of knowledge (e.g., we can prime the concept of “kindness” by presenting people with words related to kindness) and to the influence of that activation on behavior (people who are primed with the concept of kindness may act more kindly).

One measure of the influence of priming on implicit memory is the word fragment test , in which a person is asked to fill in missing letters to make words. You can try this yourself: First, try to complete the following word fragments, but work on each one for only three or four seconds. Do any words pop into mind quickly?

_ i b _ a _ y

_ h _ s _ _ i _ n

_ h _ i s _

Now read the following sentence carefully:

Then try again to make words out of the word fragments.

I think you might find that it is easier to complete fragments 1 and 3 as “library” and “book,” respectively, after you read the sentence than it was before you read it. However, reading the sentence didn’t really help you to complete fragments 2 and 4 as “physician” and “chaise.” This difference in implicit memory probably occurred because as you read the sentence, the concept of “library” (and perhaps “book”) was primed, even though they were never mentioned explicitly. Once a concept is primed it influences our behaviors, for instance, on word fragment tests.

Our everyday behaviors are influenced by priming in a wide variety of situations. Seeing an advertisement for cigarettes may make us start smoking, seeing the flag of our home country may arouse our patriotism, and seeing a student from a rival school may arouse our competitive spirit. And these influences on our behaviors may occur without our being aware of them.

Research Focus: Priming Outside Awareness Influences Behavior

One of the most important characteristics of implicit memories is that they are frequently formed and used automatically , without much effort or awareness on our part. In one demonstration of the automaticity and influence of priming effects, John Bargh and his colleagues (Bargh, Chen, & Burrows, 1996) conducted a study in which they showed college students lists of five scrambled words, each of which they were to make into a sentence. Furthermore, for half of the research participants, the words were related to stereotypes of the elderly. These participants saw words such as the following:

The other half of the research participants also made sentences, but from words that had nothing to do with elderly stereotypes. The purpose of this task was to prime stereotypes of elderly people in memory for some of the participants but not for others.

The experimenters then assessed whether the priming of elderly stereotypes would have any effect on the students’ behavior—and indeed it did. When the research participant had gathered all of his or her belongings, thinking that the experiment was over, the experimenter thanked him or her for participating and gave directions to the closest elevator. Then, without the participants knowing it, the experimenters recorded the amount of time that the participant spent walking from the doorway of the experimental room toward the elevator. As you can see in Figure 8.3 “Results From Bargh, Chen, and Burrows, 1996” , participants who had made sentences using words related to elderly stereotypes took on the behaviors of the elderly—they walked significantly more slowly as they left the experimental room.

Figure 8.3 Results From Bargh, Chen, and Burrows, 1996

Bargh, Chen, and Burrows (1996) found that priming words associated with the elderly made people walk more slowly.

Adapted from Bargh, J. A., Chen, M., & Burrows, L. (1996). Automaticity of social behavior: Direct effects of trait construct and stereotype activation on action. Journal of Personality & Social Psychology, 71 , 230–244.

To determine if these priming effects occurred out of the awareness of the participants, Bargh and his colleagues asked still another group of students to complete the priming task and then to indicate whether they thought the words they had used to make the sentences had any relationship to each other, or could possibly have influenced their behavior in any way. These students had no awareness of the possibility that the words might have been related to the elderly or could have influenced their behavior.

Stages of Memory: Sensory, Short-Term, and Long-Term Memory

Another way of understanding memory is to think about it in terms of stages that describe the length of time that information remains available to us. According to this approach (see Figure 8.4 “Memory Duration” ), information begins in sensory memory , moves to short-term memory , and eventually moves to long-term memory . But not all information makes it through all three stages; most of it is forgotten. Whether the information moves from shorter-duration memory into longer-duration memory or whether it is lost from memory entirely depends on how the information is attended to and processed.

Figure 8.4 Memory Duration

Memory can characterized in terms of stages—the length of time that information remains available to us.

Adapted from Atkinson, R. C., & Shiffrin, R. M. (1968). Human memory: A proposed system and its control processes. In K. Spence (Ed.), The psychology of learning and motivation (Vol. 2). Oxford, England: Academic Press.

Sensory Memory

Sensory memory refers to the brief storage of sensory information . Sensory memory is a memory buffer that lasts only very briefly and then, unless it is attended to and passed on for more processing, is forgotten. The purpose of sensory memory is to give the brain some time to process the incoming sensations, and to allow us to see the world as an unbroken stream of events rather than as individual pieces.

Visual sensory memory is known as iconic memory . Iconic memory was first studied by the psychologist George Sperling (1960). In his research, Sperling showed participants a display of letters in rows, similar to that shown in Figure 8.5 “Measuring Iconic Memory” . However, the display lasted only about 50 milliseconds (1/20 of a second). Then, Sperling gave his participants a recall test in which they were asked to name all the letters that they could remember. On average, the participants could remember only about one-quarter of the letters that they had seen.

Figure 8.5 Measuring Iconic Memory

Sperling (1960) showed his participants displays such as this one for only 1/20th of a second. He found that when he cued the participants to report one of the three rows of letters, they could do it, even if the cue was given shortly after the display had been removed. The research demonstrated the existence of iconic memory.

Adapted from Sperling, G. (1960). The information available in brief visual presentation. Psychological Monographs, 74 (11), 1–29.

Sperling reasoned that the participants had seen all the letters but could remember them only very briefly, making it impossible for them to report them all. To test this idea, in his next experiment he first showed the same letters, but then after the display had been removed , he signaled to the participants to report the letters from either the first, second, or third row. In this condition, the participants now reported almost all the letters in that row. This finding confirmed Sperling’s hunch: Participants had access to all of the letters in their iconic memories, and if the task was short enough, they were able to report on the part of the display he asked them to. The “short enough” is the length of iconic memory, which turns out to be about 250 milliseconds (¼ of a second).

Auditory sensory memory is known as echoic memory . In contrast to iconic memories, which decay very rapidly, echoic memories can last as long as 4 seconds (Cowan, Lichty, & Grove, 1990). This is convenient as it allows you—among other things—to remember the words that you said at the beginning of a long sentence when you get to the end of it, and to take notes on your psychology professor’s most recent statement even after he or she has finished saying it.

In some people iconic memory seems to last longer, a phenomenon known as eidetic imagery (or “photographic memory”) in which people can report details of an image over long periods of time. These people, who often suffer from psychological disorders such as autism, claim that they can “see” an image long after it has been presented, and can often report accurately on that image. There is also some evidence for eidetic memories in hearing; some people report that their echoic memories persist for unusually long periods of time. The composer Wolfgang Amadeus Mozart may have possessed eidetic memory for music, because even when he was very young and had not yet had a great deal of musical training, he could listen to long compositions and then play them back almost perfectly (Solomon, 1995).

Short-Term Memory

Most of the information that gets into sensory memory is forgotten, but information that we turn our attention to, with the goal of remembering it, may pass into short-term memory . Short-term memory (STM) is the place where small amounts of information can be temporarily kept for more than a few seconds but usually for less than one minute (Baddeley, Vallar, & Shallice, 1990). Information in short-term memory is not stored permanently but rather becomes available for us to process, and the processes that we use to make sense of, modify, interpret, and store information in STM are known as working memory .

Although it is called “memory,” working memory is not a store of memory like STM but rather a set of memory procedures or operations. Imagine, for instance, that you are asked to participate in a task such as this one, which is a measure of working memory (Unsworth & Engle, 2007). Each of the following questions appears individually on a computer screen and then disappears after you answer the question:

Is 10 × 2 − 5 = 15? (Answer YES OR NO) Then remember “S”

Is 12 ÷ 6 − 2 = 1? (Answer YES OR NO) Then remember “R”

Is 10 × 2 = 5? (Answer YES OR NO) Then remember “P”

Is 8 ÷ 2 − 1 = 1? (Answer YES OR NO) Then remember “T”

Is 6 × 2 − 1 = 8? (Answer YES OR NO) Then remember “U”

Is 2 × 3 − 3 = 0? (Answer YES OR NO) Then remember “Q”

To successfully accomplish the task, you have to answer each of the math problems correctly and at the same time remember the letter that follows the task. Then, after the six questions, you must list the letters that appeared in each of the trials in the correct order (in this case S, R, P, T, U, Q).

To accomplish this difficult task you need to use a variety of skills. You clearly need to use STM, as you must keep the letters in storage until you are asked to list them. But you also need a way to make the best use of your available attention and processing. For instance, you might decide to use a strategy of “repeat the letters twice, then quickly solve the next problem, and then repeat the letters twice again including the new one.” Keeping this strategy (or others like it) going is the role of working memory’s central executive —the part of working memory that directs attention and processing. The central executive will make use of whatever strategies seem to be best for the given task. For instance, the central executive will direct the rehearsal process, and at the same time direct the visual cortex to form an image of the list of letters in memory. You can see that although STM is involved, the processes that we use to operate on the material in memory are also critical.

Short-term memory is limited in both the length and the amount of information it can hold. Peterson and Peterson (1959) found that when people were asked to remember a list of three-letter strings and then were immediately asked to perform a distracting task (counting backward by threes), the material was quickly forgotten (see Figure 8.6 “STM Decay” ), such that by 18 seconds it was virtually gone.

Figure 8.6 STM Decay

Peterson and Peterson (1959) found that information that was not rehearsed decayed quickly from memory.

Adapted from Peterson, L., & Peterson, M. J. (1959). Short-term retention of individual verbal items. Journal of Experimental Psychology, 58 (3), 193–198.

One way to prevent the decay of information from short-term memory is to use working memory to rehearse it. Maintenance rehearsal is the process of repeating information mentally or out loud with the goal of keeping it in memory . We engage in maintenance rehearsal to keep a something that we want to remember (e.g., a person’s name, e-mail address, or phone number) in mind long enough to write it down, use it, or potentially transfer it to long-term memory.

If we continue to rehearse information it will stay in STM until we stop rehearsing it, but there is also a capacity limit to STM. Try reading each of the following rows of numbers, one row at a time, at a rate of about one number each second. Then when you have finished each row, close your eyes and write down as many of the numbers as you can remember.

If you are like the average person, you will have found that on this test of working memory, known as a digit span test , you did pretty well up to about the fourth line, and then you started having trouble. I bet you missed some of the numbers in the last three rows, and did pretty poorly on the last one.

The digit span of most adults is between five and nine digits, with an average of about seven. The cognitive psychologist George Miller (1956) referred to “seven plus or minus two” pieces of information as the “magic number” in short-term memory. But if we can only hold a maximum of about nine digits in short-term memory, then how can we remember larger amounts of information than this? For instance, how can we ever remember a 10-digit phone number long enough to dial it?

One way we are able to expand our ability to remember things in STM is by using a memory technique called chunking . Chunking is the process of organizing information into smaller groupings (chunks), thereby increasing the number of items that can be held in STM . For instance, try to remember this string of 12 letters:

You probably won’t do that well because the number of letters is more than the magic number of seven.

Now try again with this one:

Would it help you if I pointed out that the material in this string could be chunked into four sets of three letters each? I think it would, because then rather than remembering 12 letters, you would only have to remember the names of four television stations. In this case, chunking changes the number of items you have to remember from 12 to only four.

Experts rely on chunking to help them process complex information. Herbert Simon and William Chase (1973) showed chess masters and chess novices various positions of pieces on a chessboard for a few seconds each. The experts did a lot better than the novices in remembering the positions because they were able to see the “big picture.” They didn’t have to remember the position of each of the pieces individually, but chunked the pieces into several larger layouts. But when the researchers showed both groups random chess positions—positions that would be very unlikely to occur in real games—both groups did equally poorly, because in this situation the experts lost their ability to organize the layouts (see Figure 8.7 “Possible and Impossible Chess Positions” ). The same occurs for basketball. Basketball players recall actual basketball positions much better than do nonplayers, but only when the positions make sense in terms of what is happening on the court, or what is likely to happen in the near future, and thus can be chunked into bigger units (Didierjean & Marmèche, 2005).

Figure 8.7 Possible and Impossible Chess Positions

Experience matters: Experienced chess players are able to recall the positions of the game on the right much better than are those who are chess novices. But the experts do no better than the novices in remembering the positions on the left, which cannot occur in a real game.

If information makes it past short term-memory it may enter long-term memory (LTM) , memory storage that can hold information for days, months, and years . The capacity of long-term memory is large, and there is no known limit to what we can remember (Wang, Liu, & Wang, 2003). Although we may forget at least some information after we learn it, other things will stay with us forever. In the next section we will discuss the principles of long-term memory.

Key Takeaways

• Memory refers to the ability to store and retrieve information over time.
• For some things our memory is very good, but our active cognitive processing of information assures that memory is never an exact replica of what we have experienced.
• Explicit memory refers to experiences that can be intentionally and consciously remembered, and it is measured using recall, recognition, and relearning. Explicit memory includes episodic and semantic memories.
• Measures of relearning (also known as savings) assess how much more quickly information is learned when it is studied again after it has already been learned but then forgotten.
• Implicit memory refers to the influence of experience on behavior, even if the individual is not aware of those influences. The three types of implicit memory are procedural memory, classical conditioning, and priming.
• Information processing begins in sensory memory, moves to short-term memory, and eventually moves to long-term memory.
• Maintenance rehearsal and chunking are used to keep information in short-term memory.
• The capacity of long-term memory is large, and there is no known limit to what we can remember.

Exercises and Critical Thinking

• List some situations in which sensory memory is useful for you. What do you think your experience of the stimuli would be like if you had no sensory memory?
• Describe a situation in which you need to use working memory to perform a task or solve a problem. How do your working memory skills help you?

Atkinson, R. C., & Shiffrin, R. M. (1968). Human memory: A proposed system and its control processes. In K. Spence (Ed.), The psychology of learning and motivation (Vol. 2). Oxford, England: Academic Press.

Baddeley, A. D., Vallar, G., & Shallice, T. (1990). The development of the concept of working memory: Implications and contributions of neuropsychology. In G. Vallar & T. Shallice (Eds.), Neuropsychological impairments of short-term memory (pp. 54–73). New York, NY: Cambridge University Press.

Bargh, J. A., Chen, M., & Burrows, L. (1996). Automaticity of social behavior: Direct effects of trait construct and stereotype activation on action. Journal of Personality & Social Psychology, 71 , 230–244.

Bridgeman, B., & Morgan, R. (1996). Success in college for students with discrepancies between performance on multiple-choice and essay tests. Journal of Educational Psychology, 88 (2), 333–340.

Cowan, N., Lichty, W., & Grove, T. R. (1990). Properties of memory for unattended spoken syllables. Journal of Experimental Psychology: Learning, Memory, and Cognition, 16 (2), 258–268.

Didierjean, A., & Marmèche, E. (2005). Anticipatory representation of visual basketball scenes by novice and expert players. Visual Cognition, 12 (2), 265–283.

Haist, F., Shimamura, A. P., & Squire, L. R. (1992). On the relationship between recall and recognition memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 18 (4), 691–702.

Miller, G. A. (1956). The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychological Review, 63 (2), 81–97.

Nelson, T. O. (1985). Ebbinghaus’s contribution to the measurement of retention: Savings during relearning. Journal of Experimental Psychology: Learning, Memory, and Cognition, 11 (3), 472–478.

Peterson, L., & Peterson, M. J. (1959). Short-term retention of individual verbal items. Journal of Experimental Psychology, 58 (3), 193–198.

Simon, H. A., & Chase, W. G. (1973). Skill in chess. American Scientist, 61 (4), 394–403.

Solomon, M. (1995). Mozart: A life . New York, NY: Harper Perennial.

Sperling, G. (1960). The information available in brief visual presentation. Psychological Monographs, 74 (11), 1–29.

Unsworth, N., & Engle, R. W. (2007). On the division of short-term and working memory: An examination of simple and complex span and their relation to higher order abilities. Psychological Bulletin, 133 (6), 1038–1066.

Wang, Y., Liu, D., & Wang, Y. (2003). Discovering the capacity of human memory. Brain & Mind, 4 (2), 189–198.

Introduction to Psychology Copyright © 2015 by University of Minnesota is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

Human memory: How we make, remember, and forget memories

Human memory happens in many parts of the brain at once, and some types of memories stick around longer than others.

From the moment we are born, our brains are bombarded by an immense amount of information about ourselves and the world around us. So, how do we hold on to everything we've learned and experienced? Memories.

Humans retain different types of memories for different lengths of time . Short-term memories last seconds to hours, while long-term memories last for years. We also have a working memory, which lets us keep something in our minds for a limited time by repeating it. Whenever you say a phone number to yourself over and over to remember it, you're using your working memory.

Another way to categorize memories is by the subject of the memory itself, and whether you are consciously aware of it. Declarative memory, also called explicit memory, consists of the sorts of memories you experience consciously. Some of these memories are facts or “common knowledge”: things like the capital of Portugal (Lisbon), or the number of cards in a standard deck of playing cards (52). Others consist of past events you've experienced, such as a childhood birthday.

Nondeclarative memory, also called implicit memory, unconsciously builds up. These include procedural memories, which your body uses to remember the skills you've learned. Do you play an instrument or ride a bicycle? Those are your procedural memories at work. Nondeclarative memories also can shape your body's unthinking responses, like salivating at the sight of your favorite food or tensing up when you see something you fear.

In general, declarative memories are easier to form than nondeclarative memories. It takes less time to memorize a country's capital than it does to learn how to play the violin. But nondeclarative memories stick around more easily. Once you've learned to ride a bicycle, you're not likely to forget.

The types of amnesia

To understand how we remember things, it's incredibly helpful to study how we forget— which is why neuroscientists study amnesia, the loss of memories or the ability to learn . Amnesia is usually the result of some kind of trauma to the brain, such as a head injury, a stroke, a brain tumor, or chronic alcoholism.

For Hungry Minds

There are two main types of amnesia. The first, retrograde amnesia, occurs where you forget things you knew before the brain trauma. Anterograde amnesia is when brain trauma curtails or stops someone's ability to form new memories.

The most famous case study of anterograde amnesia is Henry Molaison , who in 1953 had parts of his brain removed as a last-ditch treatment for severe seizures. While Molaison—known when he was alive as H.M.—remembered much of his childhood, he was unable to form new declarative memories. People who worked with him for decades had to re-introduce themselves with every visit.

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By studying people such as H.M., as well as animals with different types of brain damage, scientists can trace where and how different kinds of memories form in the brain. It seems that short-term and long-term memories don't form in exactly the same way, nor do declarative and procedural memories.

There's no one place within the brain that holds all of your memories; different areas of the brain form and store different kinds of memories, and different processes may be at play for each. For instance, emotional responses such as fear reside in a brain region called the amygdala. Memories of the skills you've learned are associated with a different region called the striatum. A region called the hippocampus is crucial for forming, retaining, and recalling declarative memories. The temporal lobes, the brain regions that H.M. was partially missing, play a crucial role in forming and recalling memories.

How memories are formed, stored, and recalled

Since the 1940s scientists have surmised that memories are held within groups of neurons, or nerve cells, called cell assemblies. Those interconnected cells fire as a group in response to a specific stimulus, whether it's your friend's face or the smell of freshly baked bread. The more the neurons fire together, the more the cells' interconnections strengthen . That way, when a future stimulus triggers the cells, it's more likely that the whole assembly fires. The nerves' collective activity transcribes what we experience as a memory. Scientists are still working through the details of how it works.

For a short-term memory to become a long-term memory, it must be strengthened for long-term storage, a process called memory consolidation. Consolidation is thought to take place by several processes. One, called long-term potentiation, consists of individual nerves modifying themselves to grow and talk to their neighboring nerves differently. That remodeling alters the nerves' connections in the long term, which stabilizes the memory. All animals that have long-term memories use this same basic cellular machinery; scientists worked out the details of long-term potentiation by studying California sea slugs . However, not all long-term memories necessarily have to start as short-term memories.

As we recall a memory, many parts of our brain rapidly talk to each other, including regions in the brain's cortex that do high-level information processing, regions that handle our senses' raw inputs, and a region called the medial temporal lobe that seems to help coordinate the process. One recent study found that at the moment when patients recalled newly formed memories, ripples of nerve activity in the medial temporal lobe synced up with ripples in the brain's cortex.

Many mysteries of memory remain. How precisely are memories encoded within groups of neurons? How widely distributed in the brain are the cells that encode a given memory? How does our brain activity correspond to how we experience memories? These active areas of research may one day provide new insight into brain function and how to treat memory-related conditions .

For instance, recent work has demonstrated that some memories must be “reconsolidated” each time they're recalled. If so, the act of remembering something makes that memory temporarily malleable—letting it be strengthened, weakened, or otherwise altered. Memories may be more easily targeted by medications during reconsolidation, which could help treat conditions such as post-traumatic stress disorder, or PTSD .

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What Is Memory?

How memories help us

Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Daniel B. Block, MD, is an award-winning, board-certified psychiatrist who operates a private practice in Pennsylvania.

Fabio / Getty Images

• Organization

Why We Forget

How to improve memory.

• How to Protect Memory

Memory refers to the psychological processes of acquiring, storing, retaining, and later retrieving information. There are three major processes involved in memory: encoding, storage, and retrieval.

Human memory involves the ability to both preserve and recover information. However, this is not a flawless process. Sometimes people forget or misremember things. Other times, information is not properly encoded in memory in the first place.

Memory problems are often relatively minor annoyances, like forgetting birthdays. However, they can also be a sign of serious conditions such as Alzheimer's disease  and other kinds of dementia . These conditions affect quality of life and ability to function.

This article discusses how memories are formed and why they are sometimes forgotten. It also covers the different types of memory and steps you can take to both improve and protect your memory.

How Memories Are Formed

In order to create a new memory, information must be changed into a usable form, which occurs through a process known as encoding . Once the information has been successfully encoded, it must be stored in memory for later use.

Researchers have long believed that memories form due to changes in brain neurons (nerve cells). Our understanding today is that memories are created through the connections that exist between these neurons—either by strengthening these connections or through the growth of new connections.

Changes in the connections between nerve cells (known as synapses ) are associated with the learning and retention of new information. Strengthening these connections helps commit information to memory.

This is why reviewing and rehearsing information improves the ability to remember it. Practice strengthens the connections between the synapses that store that memory.

Much of our stored memory lies outside of our awareness most of the time, except when we actually need to use it. The memory retrieval process allows us to bring stored memories into conscious awareness.

How Long Do Memories Last?

You can't discuss what memory is without also talking about how long memories last. Some memories are very brief, just seconds long, and allow people to take in sensory information about the world.

Short-term memories are a bit longer and last about 20 to 30 seconds. These memories mostly consist of the information people are currently focusing on and thinking about.

Some memories are capable of enduring much longer—lasting days, weeks, months, or even decades. Most of these long-term memories lie outside of immediate awareness but can be drawn into consciousness when needed.

Why Do We Remember Painful Memories?

Have you ever noticed that many times, painful memories tend to hang on for long periods of time? Research suggests that this is because of increased biological arousal during the negative experience, which increases the longevity of that memory.

Using Memory

To use the information that has been encoded into memory, it first has to be retrieved. There are many factors that can influence this process, including the type of information being used and the retrieval cues that are present.

Of course, this process is not always perfect. Have you ever felt like you had the answer to a question just out of your reach, for instance? This is an example of a perplexing memory retrieval issue known as lethologica or the tip-of-the-tongue phenomenon.

Organizing Memory

The ability to access and retrieve information from long-term memory allows us to actually use these memories to make decisions, interact with others, and solve problems . But in order to be retrievable, memories have to be organized in some way.

One way of thinking about memory organization is the semantic network model. This model suggests that certain triggers activate associated memories. Seeing or remembering a specific place might activate memories that have occurred in that location.

Thinking about a particular campus building, for example, might trigger memories of attending classes, studying, and socializing with peers.

Certain stimuli can also sometimes act as powerful triggers that draw memories into conscious awareness. Scent is one example. Smelling a particular smell, such as a perfume or fresh-baked cookies, can bring forth a rush of vivid memories connected to people and events from a person's past. 

In order to identify a scent, a person must remember when they have smelled it before, then connect it to visual information that occurred at the same time. So, when areas of the brain connected to memory are damaged, the ability to identify smells is actually impaired.

At the same time, researchers have found that scent can help trigger autobiographical memories in people who have Alzheimer's disease. This underscores just how powerful memories can be.

Types of Memory

While several different models of memory have been proposed, the stage model of memory is often used to explain the basic structure and function of memory. Initially proposed in 1968 by Richard Atkinson and Richard Shiffrin, this theory outlines three separate stages or types of memory : sensory memory, short-term memory, and long-term memory.

Sensory Memory

Sensory memory is the earliest stage of memory. During this stage, sensory information from the environment is stored for a very brief period of time, generally for no longer than a half-second for visual information and three or four seconds for auditory information.

People only pay attention to certain aspects of this sensory memory. Attending to sensory memory allows some of this information to pass into the next stage: short-term memory.

Short-Term Memory

Short-term memory, also known as active memory, is the information we are currently aware of or thinking about. In Freudian psychology, this memory would be referred to as the conscious mind . Paying attention to sensory memories generates information in short-term memory.

While many of our short-term memories are quickly forgotten, attending to this information allows it to continue to the next stage: long-term memory. Most of the information stored in active memory will be kept for approximately 20 to 30 seconds.

This capacity can be stretched somewhat by using memory strategies such as chunking , which involves grouping related information into smaller chunks.

The term "short-term memory" is often used interchangeably with "working memory," which refers to the processes that are used to temporarily store, organize, and manipulate information.

In a famous paper published in 1956, psychologist George Miller suggested that the capacity of short-term memory for storing a list of items was somewhere between five and nine. Some memory researchers now believe that the true capacity of short-term memory is probably closer to four.

Long-Term Memory

Long-term memory refers to the continuing storage of information. In Freudian psychology , long-term memory would be called the preconscious and unconscious .

This information is largely outside of our awareness but can be called into working memory to be used when needed. Some memories are fairly easy to recall, while others are much more difficult to access.

One model suggests that there are three main types of memory: sensory memory, short-term memory, and long-term memory. Sensory memory is very brief, short-term memory is slightly longer, and long-term memory can last a lifetime.

Forgetting is a surprisingly common event. Just consider how easy it is to forget someone’s name or overlook an important appointment. Why do people so often forget information they have learned in the past?

There are four basic explanations for why forgetting occurs :

• Failure to store a memory
• Interference
• Motivated forgetting
• Retrieval failure

Research has shown that one of the critical factors that influence memory failure is time. Information is often quickly forgotten, particularly if people do not actively review and rehearse the information.

Sometimes information is simply lost from memory and, in other cases, it was never stored correctly in the first place. Some memories compete with one another, making it difficult to remember certain information. In other instances, people actively try to forget things that they simply don’t want to remember.

No matter how great your memory is, there are probably a few things you can do to make it even better. Useful strategies to deal with mild memory loss include:

• Write it down : The act of writing with a pen and paper helps implant the memory into your brain—and can also serve as a reminder or reference later on.
• Attach meaning to it : You can remember something more easily if you attach meaning to it. For instance, if you associate a person you just meet with someone you already know, you may be able to remember their name better.
• Repeat it : Repetition helps the memory become encoded beyond your short-term memory.
• Group it : Information that is categorized becomes easier to remember and recall.
• Test yourself : While it may seem like studying and rehearsing information is the best way to ensure that you will remember it, researchers have found that being tested on information is actually one of the best ways to improve recall .
• Take a mental picture : Systematically trying to make a mental note of things you often forget (such as where you left your car keys) can help you remember things better.
• Get enough rest : Research has also found that sleep plays a critical role in learning and the formation of new memories.
• Use memorization techniques : Rehearsing information, employing mnemonics, and other memorization strategies can help combat minor memory problems.

Using strategies to boost memory can be helpful for recall and retention. By learning how to use these strategies effectively, you can sidestep the faulty areas of your memory and train your brain to function in new ways.

How to Protect Your Memory

While Alzheimer's disease and other age-related memory problems affect many older adults, the loss of memory during later adulthood might not inevitable. Certain abilities do tend to decline with age, but researchers have found that individuals in their 70s often perform just as well on many cognitive tests as those in their 20s.

By the time people reach their 80s, it is common to experience some decline in cognitive function. But some types of memory even increase with age.

To help protect your brain as you age, try some of these lifestyle strategies:

• Avoid stress : Research has found that stress can have detrimental effects on areas of the brain associated with memory, including the hippocampus.
• Avoid drugs, alcohol, and other neurotoxins : Drug use and excessive alcohol consumption have been linked to the deterioration of synapses (the connections between neurons). Exposure to dangerous chemicals such as heavy metals and pesticides can also have detrimental effects on the brain.
• Get enough exercise : Regular physical activity helps improve oxygenation of the brain, which is vital for synaptic formation and growth.
• Stimulate your brain : When it comes to memory, there is a lot of truth to the old adage of "use it or lose it." Researchers have found that people who have more mentally stimulating jobs are less likely to develop dementia.
• Maintain a sense of self-efficacy : Having a strong sense of self-efficacy has been associated with maintaining good memory abilities during old age. Self-efficacy refers to the sense of control that people have over their own lives and destiny. A strong sense of self-efficacy has also been linked to lowered stress levels.

While there is no quick fix for ensuring that your memory stays intact as you age, researchers believe that avoiding stress, leading an active lifestyle, and remaining mentally engaged are important ways to decrease your risk of memory loss.

A Word From Verywell

Human memory is a complex process that researchers are still trying to better understand. Our memories make us who we are, yet the process is not perfect. While we are capable of remembering an astonishing amount of information, we are also susceptible to memory-related mistakes and errors.

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Zemla JC, Austerweil JL. Estimating semantic networks of groups and individuals from fluency data .  Comput Brain Behav . 2018;1(1):36-58. doi:10.1007/s42113-018-0003-7

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Camina E, Güell F. The neuroanatomical, neurophysiological and psychological basis of memory: Current models and their origins .  Front Pharmacol . 2017;8:438. doi:10.3389/fphar.2017.00438

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Darby K, Sloutsky V. The cost of learning: Interference effects in memory development .  J Exp Psychol Gen . 2015;144(2):410-431. doi:10.1037/xge0000051

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McEwen BS, Nasca C, Gray JD. Stress effects on neuronal structure: Hippocampus, amygdala, and prefrontal cortex .  Neuropsychopharmacology . 2016;41(1):3-23. doi:10.1038/npp.2015.171

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By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

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The Neuroanatomical, Neurophysiological and Psychological Basis of Memory: Current Models and Their Origins

Eduardo camina.

1 Mind-Brain Group: Biology and Subjectivity in Philosophy and Contemporary Neuroscience, Institute for Culture and Society, University of Navarra, Pamplona, Spain

2 Department of Learning and Curriculum, Faculty of Education and Psychology, University of Navarra, Pamplona, Spain

Francisco Güell

This review aims to classify and clarify, from a neuroanatomical, neurophysiological, and psychological perspective, different memory models that are currently widespread in the literature as well as to describe their origins. We believe it is important to consider previous developments without which one cannot adequately understand the kinds of models that are now current in the scientific literature. This article intends to provide a comprehensive and rigorous overview for understanding and ordering the latest scientific advances related to this subject. The main forms of memory presented include sensory memory, short-term memory, and long-term memory. Information from the world around us is first stored by sensory memory, thus enabling the storage and future use of such information. Short-term memory (or memory) refers to information processed in a short period of time. Long-term memory allows us to store information for long periods of time, including information that can be retrieved consciously (explicit memory) or unconsciously (implicit memory).

Introduction

A life full of unconnected events, of errors that do not lead to any lessons and of emotions without the ability to remember them is no life at all. Memory is precisely the capacity that allows us to connect experiences, learn and make sense of our lives. In short, it allows us to build our story. The full range of this complex capacity’s neuroanatomical, neurobiological, neurophysiological, and psychological mechanism remain unknown and it presents a challenge for psychologists and neuroscientists who try to explain it. This review attempts to provide a rigorous overview that permits anyone who wants to approach the latest scientific findings on memory to do so, as well as to understand them and properly order them. We will focus on neuroanatomical, neurophysiological, and psychological mechanisms of the different types of memory.

Although knowledge of molecular mechanisms is important for constructing a complete vision of memory models, in this article we can only point out general traits as summarized in this introduction [for more information see ( Kandel et al., 2014 )]. In addition, knowledge gained from neuroimaging studies ( Binder and Desai, 2011 ), as well as knowledge of the neural markers associated with memory ( Meneses, 2015 ), will likely play a key role in future models of memory mechanisms, but in this review, as stated above, we focus mainly on neuroanatomical, neurophysiological, and psychological mechanisms.

We believe it is important to consider previous developments without which one cannot adequately understand the classifications of memories and the kinds of memory models that are now current in the scientific literature.

The three major classifications of memory that the scientific community deals with today are as follows: sensory memory, short-term memory, and long-term memory. Information from the world around us begins to be stored by sensory memory, making it possible for this information to be accessible in the future. Short-term memory refers to the information processed by the individual in a short period of time. Working memory performs this processing. Long-term memory allows us to store information for long periods of time. This information may be retrieved consciously (explicit memory) or unconsciously (implicit memory).

As Squire (2004) points out, the first theoretical approaches relevant to current neuroscience come from the 19th century. These include Maine de Biran (1804/1929) ( Maine de Biran, 1929 ) who, at the beginning of the century, wrote of mechanical memory, sensitive memory, and representative memory. The philosopher James, and his book The Principles of Psychology ( James, 1890 ), is also especially worth highlighting. Therein, James distinguishes between primary and secondary memory, thereby referring to short- and long-term memory, respectively.

The importance of Pavlov (1927) and Fitts and Posner (1967) are especially noteworthy during the first two thirds of the 20th century. Pavlov’s studies are related to a type of memory that later would be called associative memory. Meanwhile, Fitts and Posner’s studies are considered the first model to explain procedural memory.

Prior to the 60’s, most systematizations of memory distinguished a more mechanical type of memory related to the acquisition of skills, which is, in turn, related to activity of the intellect. Unlike what followed, debates in this period were mainly philosophical or based on psychological intuition ( Ribot, 1881 ; Korsafoff, 1890 ).

Beginning in the 1960s, a series of experimental studies on how the brain stores information emerged, using animals and amnesic patients. Within this decade, Milner, Atkinson, and Shiffrin were especially important researchers.

The experimental modern era arguably began when Milner (1962) demonstrated, with HM experiments, that a seriously ill patient could acquire a new skill (hand-eye coordination) without any memory of having encountered the task before. “While this finding showed that memory is not unitary, discussions at the time tended to set aside motor skills as a special case representing a less cognitive form of memory. The suggestion was that the rest of memory is of one piece and is dependent on medial temporal lobe structures” ( Squire and Wixted, 2016 ).

A few years later, Atkinson and Shiffrin (1968) proposed a modal model of memory that constitutes one of the most influential explanations for the existence of different components in the memory system. The importance of this model is such that it must be explained in the next section, but for now it should simply be mentioned that the modal model establishes the existence of short-term storage (ACP), which receives sensory information that is processed by sensory and data storehouses within long-term memory. This storage system can generate reasoning and new deductions from existing ones.

In the seventies, Tulving, Baddeley, and Hitch and Kandel’s investigations are especially noteworthy. Tulving (1972) first proposed the distinction between episodic memory and semantic memory. Baddeley and Hitch (1974) conducted research on the components of working memory. Both authors considered working memory as a limited capacity system that allows temporary storage and manipulation of information necessary to perform complex tasks such as understanding, learning, and reasoning. As explained later on, at first (1974), they proposed the existence of three subsystems within the multi-storehouse model of short-term memory: the central executive, a phonological or articulatory loop and a visuospatial sketchpad. Later, Baddeley (2000) included a fourth subsystem, the episodic buffer, which combines information from the subsystems in a form of temporal representation. Kandel (1976) proposed a model to explain the mechanism of operation in habituation and sensitization. To do this, he used the notion of non-associative memory, which, as we shall see, is one of the four types of non-declarative or implicit memory, like that which refers to new behaviors learned through repeated exposure to a single stimulus. According to Kandel, new behaviors can be classified into two processes: sensitization and habituation. On the one hand, for habituation, acetylcholine is progressively consumed, decreasing the effectiveness of the stimulus and thereby the motor response. On the other hand, the presence of serotonin in sensitization, secreted by another sensory nerve terminal, causes an excess of acetylcholine. An enhanced motor response thus emerges.

In the 1980s, the differences between declarative and non-declarative memories were consolidated and disseminated. This, together with contributions from Tulving and others, such as Di Lollo or Graf and Schacter, enabled a more precise classification of different types of memory. To date, Di Lollo’s model of iconic memory ( Di Lollo, 1980 ) has been the most widely accepted and studied of the three existing types of sensory memory. As discussed in the next section, Di Lollo considered iconic memory a storage unit consisting of two components: the persistence of vision and information. Graf and Schacter (1985) proposed a general difference between declarative memory (explicit) and non-declarative memory (implicit/procedural). This stems from the distinction that Tulving (1972) proposed between the aforementioned episodic memory and semantic memory (both, as we will see, are currently included in declarative memory).

In the 90’s, a classification of the types of memory emerged, but the way they act and their interrelation was still unclear. In order to clarify its operation, Packard and McGaugh (1996) proposed that memory systems operate independently and in parallel. For example, an adverse event in childhood (e.g., seeing your grandfather being run over by a combine) can, on the one hand, consolidate as a stable declarative memory for the event itself (the sound of a combine always makes you remember that moment-episodic memory) and, on the other hand, can crystallize in non-declarative memory and result in a phobia experienced as a personality trait rather than as a mere memory (being near a combine will always produce panic and induces a desire to escape that situation-associative memory). Several authors ( Tulving et al., 1982 ) had already mentioned the idea of priming as a separate type of memory, but it was not until the 90’s that experiments were conducted to show it ( Hamann and Squire, 1997 ; Stark and Squire, 2000 ; Levy et al., 2004 ). These studies show that severely amnesic patients can exhibit completely intact priming while performing memory tests that include conventional recognition of the same test items ( Squire, 2004 ).

Thanks to the development of new 21st century technologies, researchers have been able to more accurately locate brain areas that are associated with different types of memory. Although this pertains to topics to be addressed in detail in the next section, there are two examples that we consider significant to the application of these new techniques and the significant progress made in understanding memory storage. On the one hand, Ergorul and Eichenbaum’s experiment ( Ergorul and Eichenbaum, 2004 ) shows that animals are able to remember the “context in which they experienced specific stimuli, and that this capacity also depends on the hippocampus” ( Dickerson and Eichenbaum, 2010 ). This process is closely related to the formation of episodic memory. On the other hand, neuroimaging studies that Binder and Desai (2011) conducted show “two striking results: the participation of modality-specific sensory, motor, and emotion systems in language comprehension, and the existence of large brain regions that participate in comprehension tasks but are not modality-specific.” With this in mind, Binder and Desai (2011) claims that semantic memory consists of two representations, including a specific mode and a supramodal mode. Again, this will be explained in more detail in what follows.

The research of the cellular and molecular substrates of memory has received much attention since Lomo (1966) described in the 60’s “a cellular model of experience-dependent plasticity—long-term potentiation (LTP)” ( Kandel et al., 2014 ). According to Lisman et al. (2012) : “LTP is a process whereby brief periods of synaptic activity can produce a long-lasting increase in the strength of a synapse, as shown by an increase in the size of the excitatory postsynaptic current.” NMDA receptors are double-gated, as their activation requires both postsynaptic membrane depolarization as well as presynaptic release of glutamate. Once activated by these conditions, NMDA receptors trigger a strong postsynaptic influx of Ca2+ that induce LTP through a variety of pathways including CaMKII, PKC, PKA, and MAPK ( Kandel et al., 2014 ).

With this brief historical and conceptual introduction laid out, we intend to delve into different types of memory in order to present the models that the scientific community has most accepted thus far. In the last section, and before the glossary, we identify the likely directions for future research. Now we turn on to our main task, presenting an overview of the latest scientific findings on memory, classified according to different types and mechanisms.

Sensory Memory: Iconic Memory

“Sensory memory is the capacity for briefly retaining the large amounts of information that people encounter daily” ( Siegler and Alibali, 2005 ). There are three types of sensory memory: echoic memory, iconic memory, and haptic memory. Iconic memory retains information that is gathered through sight, echoic memory retains information gathered through auditory stimuli and haptic memory retains data acquired through touch.

Scientific research has focused mainly on iconic memory; information on echoic and haptic memory is comparatively scarce. Thus, taking into account the goals of this article and that it is aimed at a higher education audience, presenting iconic memory as a paradigm of sensory memory is sufficient for an introductory overview.

Iconic memory retains information from the sense of sight with an approximate duration of 1 s. This reservoir of information then passes to short-term vision memory (which is analogous, as we shall see shortly, to the visuospatial sketchpad with which working memory operates).

Di Lollo’s model ( Di Lollo, 1980 ) is the most widely accepted model of iconic memory. Therein, he considered iconic memory a storehouse constituted by two components: the persistence of vision and information.

(a) Persistence of vision. Iconic memory corresponds to the pre-categorical representation image/visual that remains between 100 and 300 ms. It is sensitive to physical parameters, such that it depends on retinal photoreceptors (rods and cones). It also depends on various cells in the visual system and on retinal ganglion cells M (transition cells) and P (sustained cells). It concludes its representation in the primary visual cortex (V1) of the occipital lobes. “The occipital lobe is responsible for processing visual information” ( Kamel and Malik, 2014 ).

(b) Persistence of information. Iconic memory is a storehouse of information that lasts 800 ms and that represents a codified and already categorized version of the visual image. It plays the role of storehouse for post-categorical memory, which provides visual short-term memory with information to be consolidated. For this, it travels through the ventral route (V) (V1 → V2 → V5 → inferior temporal cortex).

Subsequent research on visual persistence from Coltheart ( Coltheart, 1983 ) and Sperling’s studies ( Sperling, 1960 ) on the persistence of information led to the definition of three characteristics pertaining to iconic memory: a large capacity, a short duration, and a pre-categorical nature.

Sperling (1960) demonstrated this large capacity after presenting the results of his total and partial reports. The full report consisted in presenting a 3 × 3 or 3 × 4 matrix of alphanumeric characters for a short period of time to subjects and later asking them which characters they remembered. On the other hand, in the partial report, subjects were directed to remember the characters in a row specifically assigned to them in the instructions. The total report’s results showed that subjects were only able to recall between 3 or 4 letters of the total number. However, in the partial report, subjects remembered around 75% of those that were asked. In extrapolating the partial report’s data to the total, it follows that individuals could report 9 of the 12 letters contained in the instructions (80% of the total), thus demonstrating a large capacity.

Regarding short-term, Sperling interpreted the results of the partial report as due to the rapid decline of the visual sign and reaffirmed this short duration by obtaining a decrease in the number of letters reported by the subject in delaying the audio signal for choosing a row to remember in the presentation. Averbach and Coriell’s experiments ( Averbach and Coriell, 1961 ) corroborated Sperling’s conclusion; they presented a variety of letters for a certain period of time to the subject. After each letter, and in the same position, they showed a particular visual sign. The participant’s task was to name the letter that occupied the position of the visual sign. When the visual sign appeared immediately after the letters, participants could correctly name the letter that occupied the position of the sign, however, as the presentation of the sign became more delayed, participant performance worsened. These results also show the rapid decline of visual information.

Finally, regarding its pre-categorical nature, Sperling considered the information contained in this storehouse as physical information that maintains the raw data that is not related to the meaning of stimuli. Subsequently, evidence has been obtained that this system is not entirely pre-categorical ( Loftus et al., 1992 ) since the task improves when the stimuli to remember are letters or numbers instead of meaningless forms.

Short-Term Memory

Short-term memory is the ability to keep a small amount of information available for a short period of time. Atkinson and Shiffrin’s modal model ( Atkinson and Shiffrin, 1968 ) is one of the most influential explanations for the existence of different components in the memory system ( Figure ​ Figure1 1 ). This model has some similarities with Broadbent’s previous model ( Broadbent, 1958 ). The modal model establishes the existence of a short-term storehouse with limited capacity. The short-term storehouse receives sensory information processed by sensory storehouses and data in long-term memory. In addition, the short-term storehouse can also send information to the structures involved in long-term memory. This storehouse can generate reasoning and new deductions from existing ones. This model implies that the short-term storehouse functions as a kind of working memory, a system to retain and manipulate information temporarily as part of a wide range of essential cognitive tasks such as learning, reasoning, and understanding. They, in turn, give short-term storage central importance in the overall processing of information by attributing to it the role of controlling the executive system, responsible for the coordination and control of many complex subroutines in charge of acquiring new material and recovering old material in long-term storage.

Atkinson and Shiffrin’s modal model.

Despite the explanatory power of Atkinson and Shiffrin’s model, there were a number of issues that this model could not resolve, causing criticism of it. For example, this model implies that the longer an item remains in memory, the more likely it is to be transferred to long-term storage. But studies like those of Tulving and Pearlstone (1966) and Craik and Watkins (1973) show that said relationship does not exist.

Given these criticisms, new models began to appear to explain memory, such as those from Cowan (1988 , 1995 , 1999 ) and Goldman-Rakic (1995) . Among them, Craik and Lockhart’s process model ( Craik and Lockhart, 1972 ) and Baddeley and Hitch’s structural model ( Baddeley and Hitch, 1974 ) were the most prominent; the latter is the most commonly accepted one today and thus we will focus on it in this article.

As an introduction, it can be argued that Craik and Lockhart (1972) understood memory not as a process through which information is deepened at higher levels until it becomes part of long-term memory, but rather as a system of storehouses. Despite an emphasis on information processing (instead of structure), they continued to accept the existence of short-term memory as independent from long-term memory. For their part, Baddeley and Hitch’s proposal ( Baddeley and Hitch, 1974 ) contemplated a multi-component working memory instead of a storage unit in the short term.

Working Memory

“The term working memory refers to a brain system that provides temporary storage and manipulation of the information necessary for such complex cognitive tasks as language comprehension, learning and reasoning” ( Baddeley, 1992 ). At first (1974), they proposed the existence of three subsystems within the multi-storehouse model of short-term memory: the central executive, a phonological or articulatory loop and a visuospatial sketchpad.

In general, we can say that the central executive controls attention, “the phonological loop ensures retention of verbal information and the visuospatial sketchpad is responsible for storage visual and spatial information” ( Grigorenko et al., 2012 ). The latter two sub-memory systems are equivalent to verbal and visual short-term memory systems, respectively.

Wang and Bellugi (1994) presented a genetically based test that supports the functional and anatomical separation of Baddeley’s model with phonological and visuospatial storehouses. They compared two genetic syndromes (Williams and Down) with different brain morphology. Williams syndrome patients, despite having widespread mental handicaps, preserve their language skills, while Down syndrome patients preserve more partial capacities, but have very limited language skills. It was therefore assumed that the former would be better at verbal tasks related to operative memory, and that the latter would be better at visuospatial tasks related to operative memory. As expected, subjects with Williams syndrome performed better at phonological tasks, while subjects with Down syndrome, in turn, performed better at spatial tasks.

Later, Baddeley (2000) included a fourth subsystem, the episodic buffer ( Figure ​ Figure2 2 ), which combines information from the different subsystems in a kind of temporal representation.

Baddeley’s model.

Here we will focus on the different subsystems that make up Baddeley’s multi-storehouse model (2000), i.e., the central executive, the phonological or articulatory loop, the visuospatial sketchpad and the episodic buffer.

The Central Executive

The central executive is a system of attention control with limited processing capacity. Baddeley (1986) adopted a model originally proposed by Norman and Shallice (1986) , in which actions are controlled in two ways. “Behavior that is routine and habitual is controlled automatically by a range of schemas, well-learned processes that allow us to respond appropriately to the environment” ( Baddeley and Hitch, 2010 ). Processes that are not recognized as habitual are controlled by a second system, the supervisory attention system. This system uses long-term knowledge to propose novel behavioral solutions and to weigh options before deciding on a response.

In its original version, the central executive was considered an overall system capable of processing and storing. However, Baddeley and Logie (1999) proposed that it only has attention capacity.

Subsequent studies have proposed to complement the executive system with the episodic buffer as other separate storage system: “the episodic buffer clearly does represent a change within the working memory framework, whether conceived as a new component, or as a fractionation of the older version of the central executive”( Baddeley, 2000 ).

Baddeley and Logie understand the central executive as the result of the integration of several processes: the ability to focus attention, the ability to divide attention between two or more tasks, and the ability to control long-term memory access ( Baddeley et al., 1991 ; Logie et al., 2004 ; Baddeley, 2007 ). The way to accomplish this may be with one or more types of inhibition ( Engle et al., 1999 ; Miyake et al., 2000 ). This approach accepts that the frontal lobes play an important role in executive control, although there are differing opinions on the functions’ precise location ( Duncan and Owen, 2000 ; Shallice, 2002 ).

Visuospatial Sketchpad

It has been suggested that the sketchpad’s main function is to create and maintain a visuospatial representation that persists through the irregular form found in eye movement and that characterizes our exploration of the visual world ( Luck, 2007 ).

It has been shown that spatial tasks such as driving a car can interfere with spatial skills, while exclusively visual tasks, such as watching a series of images or colored shapes, can interfere with the recall of objects or shapes ( Logie, 1986 ; Klauer and Zhao, 2004 ). These patterns of interference, together with cases of brain-damaged patients that show a deficit in one kind of task but not the other ( Della Sala and Logie, 2002 ), suggest that spatial information and visual characteristics can be stored separately.

The visuospatial sketchpad seems to involve a number of areas, predominantly in the brain’s right hemisphere. On the one hand, it contains a visual component that reflects the processing and storage of objects and their visual features. On the other hand, it contains a second parietal area, presumably involved in spatial aspects.

Phonological Buffer

It can be argued that the phonological buffer supports language acquisition by providing the ability to store new words, while they are consolidated into long-term memory ( Baddeley et al., 1998 ). Within this phonological loop, two basic sub-processes emerge: a short-term acoustic storehouse and a subvocal articulatory rehearsal process. The existence of the former is indicated by the effect of phonological similarity, where speech is less accurate when repeating “similar-sounding words such as MAN CAP CAT MAT CAN, than dissimilar words such as PIT DAY COW PEN TOP. Similarity of meaning (HUGE LARGE BIG WIDE TALL) has little effect on immediate recall. On the other hand if several trials are given to learn a longer list of say 10 words, meaning becomes all-important and sound loses it power, consistent with different systems for short-term and long-term storage ( Baddeley, 1966a , b ). Evidence for the importance of rehearsal comes from the word length effect, whereby immediate recall of long words (e.g., REFRIGERATOR UNIVERSITY TUBERCULOSIS OPPORTUNITY HIPPOPOTAMUS) is much more error-prone than for short words ( Baddeley et al., 1975 )” ( Baddeley and Hitch, 2010 ). Baddeley and Hitch (1974) proposed that retention of items in the short-term storehouse quickly fade, but can be maintained by repeating them.

With respect to cerebral location, the phonological loop is found in the brain’s left hemisphere “The loop is assumed to hold verbal and acoustic information using a temporary store and an articulatory rehearsal system, which clinical lesion studies, and subsequently neuroradiological studies, suggested are principally associated with Brodmann areas, 40 and 44, respectively” ( Baddeley, 2000 ).

Episodic Buffer

The verbal and visual systems within the conventional model of working memory may explain many aspects, but Baddeley (2000) points out that evidence from patients with short-term memory deficits— who resist memorizing prose (with a verbal span much higher than that of isolated words) and resist serial memory of articulatory suppression— leads to supposing that a storehouse of additional support exists. This is seen in the existence of a new mechanism that combines information from multiple subsystems into a form of temporal representation. Baddeley (2000) proposed the term episodic buffer for this new kind of representation.

The episodic buffer is thus a temporary storage system capable of integrating information from different sources, likely controlled by the central executive. “The buffer is episodic in the sense that it holds episodes whereby information is integrated across space and potentially extended across time” ( Baddeley, 2000 ). It can be preserved in patients with advanced amnesia and severe impairment of long-term episodic memory.

With that said, it is possible to consider the episodic buffer as conceptual short-term memory. Studies to date do not specify activity in a specific area. As Potter (1999) said: “The conceptual short-term memory hypothesis proposes that when a stimulus is identified, its meaning is rapidly activated and maintained briefly in conceptual short-term memory.”

Long-Term Memory

Long-term memory refers to unlimited storage information to be maintained for long periods, even for life. There are two types of long-term memory: declarative or explicit memory and non-declarative or implicit memory.

Explicit memory refers to information that can be consciously evoked. There are two types of declarative memory: episodic memory and semantic memory. For its part, implicit memory encompasses all unconscious memories, such as certain abilities or skills. There are four types of implicit memory, including procedural, associative, non-associative, and priming.

Declarative/Explicit Memory

Explicit memory refers to information that can be evoked consciously. There are two types of declarative memory: episodic memory and semantic memory. As shown below, episodic memory stores personal experiences and semantic memory stores information about facts.

Episodic Memory

“Episodic memory involves the ability to learn, store, and retrieve information about unique personal experiences that occur in daily life. These memories typically include information about the time and place of an event, as well as detailed information about the event itself.” ( Dickerson and Eichenbaum, 2010 ).

There are a number of neural components that are closely related to the proper functioning of episodic memory, which include the following: the cortex near the hippocampus [as discussed below, the perirhinal cortex (PRC), the entorhinal cortex, and the parahippocampal cortex (PHC)], cortical and subcortical structures, and the circuits within the medial temporal lobe and hippocampus.

The cortices near the hippocampus extensively interact with a number of cortical and subcortical structures; cortical components have key roles in various aspects of perception and cognition, while the medial temporal lobe structures mediate the organization and the persistence of the memory network, whose data is stored in these cortical areas ( Dickerson and Eichenbaum, 2010 ).

The structures directly related to the hippocampus include the entorhinal, the parahippocampal, and the perirhinal cortices. Each one is discussed in detail below.

The entorhinal cortex is the main interface between the hippocampus and neocortex, thus it is associated with the distribution of information to and from the hippocampus. The surface layers (II and III) of the entorhinal cortex project out toward the dentate gyrus and hippocampus. While layer II mainly projects out toward the dentate gyrus and the CA3 region of the hippocampus, layer III mainly projects out toward the hippocampal CA1 region and the subiculum. These layers receive input signals from other cortical areas, particularly the association cortices, the PRC and the parahippocampal gyrus, as well as the prefrontal cortex. Layers II and III receive highly processed inputs from each sensory modality, and inputs related to ongoing cognitive processes. Deep layers, particularly layer V, receive one of the three output signals from the hippocampus and, in turn, exchange connections with other cortical areas that project out toward the superficial entorhinal cortex.

The PRC has a role in visual object recognition, while the PHC is involved in the perception of the local environment and processing information related to that place. Thus, fMRI studies indicate that the PHC becomes very active when human subjects receive topographical stimuli such as landscapes or rooms. Epstein and Kanwisher (1998) first described the PHC and Aguirre et al. (1996 , 1998 ) and Ishai et al. (1999) later backed up that description.

Finally the hippocampus is responsible for the formation and retrieval of memories. That is, the information that the three cortices described above process reach the hippocampus where new memories are generated and from which they can later be retrieved. Episodic memory recall involves a spatial and temporal context of specific experiences. For further review of the mechanisms of memory formation see Craver (2003) .

As Dickerson and Eichenbaum (2010) point out in their review, “several investigators have argued that animals are indeed capable of remembering the context in which they experienced specific stimuli, and that this capacity also depends on the hippocampus.” Ergorul and Eichenbaum (2004) published a significant study to this effect in which they developed a series of tasks for rats to assess their memory of events, which combined an odor (what), the place of the experience (where), and the relation to other experiences (when). The rats were presented with a sample of an odor in one specific place along the edge of a large open field. Subsequently, as a way of testing their memory, they were presented with a choice between two arbitrarily selected odors in their original locations. The results of the test showed that normal rats use a combination of where and what information to judge the timing of the events, while rats with a damaged hippocampus cannot manage to effectively combine what, when, and where information in order to form a recovered memory.

Three years later Eichenbaum et al. (2007) proposed a functional organization of memory’s medial temporal lobe system: “Neocortical input regarding the object features (“what”) converges in the PRC and lateral entorhinal area (LEA), whereas details about the location (“where”) of objects converge in the PHC and medial entorhinal area (MEA). These streams converge in the hippocampus, which represents items in the context in which they were experienced. Reverse projections follow the same pathways back to the parahippocampal and neocortical regions”( Eichenbaum et al., 2007 ).

It should be noted that memory of faces is typically associated with activity in the perirhinal and hippocampus rostral regions, while memory of objects is typically associated with wider-ranging activity ( Preston et al., 2010 ).

Both results concerning functional an anatomic and characterizations in animal models are consistent with the hypothesis that is guided by anatomic criteria about the functional organization of the hippocampal system ( Dickerson and Eichenbaum, 2010 ).

“The ventral temporal cortex, including fusiform gyrus, is commonly engaged when pictures of visual objects are presented, and the lateral temporal cortex including superior temporal gyrus is typically engaged during the encoding of auditory information” ( Dickerson and Eichenbaum, 2010 ).

Semantic Memory

As noted, in the context of long-term memory, there were two types of memory, corresponding to declarative and non-declarative memory. Within declarative memory, we find both episodic memory, as discussed above, and semantic memory, as discussed below.

Human beings have the ability to represent concepts in language. This ability allows us not only to disseminate conceptual knowledge to others, but also to manipulate, associate, and combine these concepts. Therefore, as Binder and Desai shows, “humans use conceptual knowledge for much more than merely interacting with objects. All of human culture, including science, literature, social institutions, religion, and art, is constructed from conceptual knowledge” ( Binder and Desai, 2011 ). Activities such as reasoning, planning for the future or reminiscing about the past depend on the activation of concepts stored in semantic memory ( Mahon and Caramazza, 2008 ).

Binder and Desai showed two striking results related to neuroimaging research: on the one hand, the participation of the specific sensory, motor and emotional modality in understanding language and, on the other hand, the existence of large regions of the brain (the inferior parietal lobe and a large part of the temporal lobe) involved in tasks related to understanding. These latter regions converge on the many currents involved in perception processing, and these convergences allow supramodal representations of perceptual experience that support a variety of conceptual functions, including language, social cognition, object recognition, and the extraordinary human ability to remember the past and imagine the future ( Binder and Desai, 2011 ). Therefore, accepting their argument, semantic memory consists of two representations: a specific modality and supramodal modality.

In this regard, Binder and Desai found several objections. A not inconsiderable one is that activations observed in imaging experiments could be an epiphenomenon rather than causally related to understanding. Therefore, the involvement of the motor system for processing a text would contribute to understanding and is not a mere product. Another critical point is the possibility of interpreting that collected activations represent images after understanding takes place. However, and as they showed in their review ( Binder and Desai, 2011 ), in studies of neuroimaging with high temporal resolution, the activation of motor regions during the processing of a text appears to be rapid, about 150–200 ms after each word ( Pulvermüller et al., 2005 ; Boulenger et al., 2006 ; Hoenig et al., 2008 ; Revill et al., 2008 ).

“These converging results provide compelling evidence that sensory-motor cortices play an essential role in conceptual representation. Although it is often overlooked in reviews of embodied cognition, emotion is as much a modality of experience as sensory and motor processing ( Vigliocco et al., 2009 ). Words and concepts vary in the magnitude and specific type of emotional response they evoke, and these emotional responses are a large part of the meaning of many concepts” ( Binder and Desai, 2011 ).

Following Binder and Desai, brain appears to use supramodal abstract representations for conceptual tasks. In this regard, it can be convincingly argued that the human brain has large areas of cortex that are between the sensory systems and motor modalities and, therefore, Damasio’s idea convergence zones seems plausible ( Damasio, 1989 ). “These heteromodal areas include the inferior parietal cortex (angular and supramarginal gyri), large parts of the middle and inferior temporal gyri, and anterior portions of the fusiform gyrus ( Mesulam, 1985 )” ( Binder and Desai, 2011 ).

A second argument supporting the hypothesis that the brain appears to use supramodal abstract representations during conceptual work comes from patients with damage to the lower and lateral temporal lobe. The clinical profile of semantic dementia is marked by progressive atrophy in the temporal lobe and loss of multimodal semantic memory ( Hodges et al., 1992 ; Mummery et al., 2000 ). Patients with semantic dementia is characterized by a loss of conceptual knowledge, and this loss may reflect the disruption of a central semantic hub or the degeneration of a temporosylvian language network for verbal concepts ( Irish et al., 2014 ). These patients manifesting in striking alterations in naming and comprehension ( Irish et al., 2016 ). These patients are “characterized by a clear dissociation between marked single-word comprehension” ( Agosta et al., 2010 ), unable to retrieve the names of objects, irregular word reading deficits, identify the color the correct objects, and sparing of fluency, phonology, syntax and working memory ( Binder and Desai, 2011 ).

Basically, these deficits do not seem to be categorical, constituting further evidence that semantic impairment does not imply strongly modal representations and, therefore, the modular and supramodal systems are presented as an interactive continuum of hierarchically ordered neuronal combinations, supporting representations that are progressively more idealized and combined ( Binder and Desai, 2011 ). These systems correspond to Damasio’s idea of areas of local convergence and with Barsalou’s idea of systems of unimodal perceptual symbols ( Lambon Ralph et al., 2007 ). In addition to bottom-up input within their associated modality, each system receives top-down input from other modal and attention systems. These systems are modal in the sense that their output is a analogic or isomorphic representation of the information that they receive bottom-up within their associated modality ( Barsalou, 1999a ).

As observed by Binder and Desai: “These modal convergence zones then converge with each other in higher-level cortices located in the inferior parietal lobe and much of the ventral and lateral temporal lobe (…). One function of these high-level convergences is to bind representations from two or more modalities, such as the sound and visual appearance of an animal, or the visual representation and action knowledge associated with a hand tool ( Wernicke, 1974 ; Damasio, 1989 ; Barsalou, 1999b ; Patterson et al., 2007 ). Such supramodal representations capture similarity structures that define categories, such as the collection of attributes that place ‘pear’ and ‘light bulb’ in different categories despite a superficial similarity of appearance, and ‘pear’ and ‘pine-apple’ in the same category despite very different appearances ( Rogers and McClelland, 2004 ). More generally, supramodal representations allow the efficient manipulation of abstract, schematic conceptual knowledge that characterizes natural language, social cognition, and other forms of highly creative thinking ( Dove, 2011 ; Diefenbach et al., 2013 )” ( Binder and Desai, 2011 ).

Non-declarative/Implied Memory

As noted, long-term memory refers to unlimited information storage that can be maintained for long periods, even for life. There are two types of long-term memory: declarative or explicit memory and non-declarative or implied memory.

Implicit memory encompasses all unconscious memories, as well as certain abilities or skills. There are four types of implicit memory: procedural, associative, non-associative, and priming. Each one is detailed below.

Procedural Memory: Habits and Skill

Procedural memory is the part of memory that participates in recalling motor and executive skills that are necessary to perform a task. It is an executive system that guides activity and usually works at an unconscious level. When necessary, procedural memories are retrieved automatically for use in the implementation of complex procedures related to motor and intellectual skills.

Development of these rote capacities occurs through procedural learning, that is, by systematically repeating a complex activity until acquiring and automatizing the capacity of all neural systems involved in performing the task to work together.

The acquisition of skills requires practice. However, the simple repetition of a task does not ensure skill acquisition. A skill is thought to be acquired when behavior changes as a result of experience or practice. This is known as learning and it is not a directly observable phenomenon. Here we will discuss two models for acquiring skills.

The first model comes from Fitts’s team ( Fitts, 1954 ; Fitts and Posner, 1967 ). These scientists propose an explanatory model of skill acquisition, based on the idea of learning as a process in three phases:

• simple (a) Cognitive phase: The process begins with the acquisition of knowledge about the factors that make up a particular observed behavior. At this point, the psychological process of attention is important. The skill to be acquired must be broken down into its basic components and one must understand how these components are combined to form a whole in the correct execution of the task ( Fitts, 1954 ; Fitts and Posner, 1967 ).
• simple (b) Associative phase: Individual repeated practice takes place until there is an automatic response pattern. As one progresses through this point, the actions that are important for the implementation of a skill are learned and become automated, just as any superfluous or ineffective actions disappears. The individual sensory system acquires the exact symbolic and spatial data required for the appropriate execution of the skill ( Fitts, 1954 ; Fitts and Posner, 1967 ).
• simple (c) Autonomous/procedural phase: This is the final phase and it consists in perfecting acquired skills. The ability to judge which stimuli are important and unimportant improves and a lower level of conscious thought is required because the skill becomes automated ( Fitts, 1954 ; Fitts and Posner, 1967 ).

The other model corresponds to ( Tadlock, 2005 ) and is called Predictive Cycle. This model proposes that learning only requires conscious maintenance of the desired end result. The model consists of the following phases: Trial, error, implicit result analysis, and decision-making at the implicit level of the way in which execution of the next test must be changed for successful implementation. These steps are repeated again and again until the subject builds or remodels his/her neural network so that it can guide the activity without the need for conscious thought.

A number of factors are involved when acquiring and implementing skills, including attention and pressure. For the acquisition of a new skill one must pay attention to the steps to be undertaken. This process involves using working memory to allow for connecting the different steps involved. Procedural memory acquires the habit with the help of the attention span, but it implies a lesser performance. However, with practice, procedural knowledge is developed. Procedural knowledge operates away from working memory, which allows for the implementation of the most automated skills ( Anderson, 1993 ). Meanwhile, pressure can affect the performance of a task in two ways: choking or clutchness. The choking phenomenon occurs when experienced and skilled performers fail under stress. Auto-focus theories suggest that pressure causes an increase in anxiety and self-consciousness concerning correct execution. This ends up causing increased attention directed toward processes directly involved in the execution of the skill ( Beilock and Carr, 2001 ). On the other hand, the attention span allows the habit to be acquired in refers to giving a top performance on a given task when pressure is highest.

Because they are especially relevant, we will briefly outline brain components involved in the acquisition of new skills and habits, including the basal ganglia, cerebellum and limbic system.

As Christos and Emmanuel explain ( Constantinidis and Procyk, 2004 ), “basal ganglia are formed by several sub-structures: the striatum, the globus pallidus, the substantia nigra, and the subthalamic nucleus.” The basal ganglia are a collection of nuclei found on both sides of the thalamus, outside of and around the limbic system, but below the cingulate gyrus and within the temporal lobes. The striatum or striate nucleus is the main gateway for information to the basal ganglia. In turn, the striatum receives information from the cerebral cortex. Essentially, there are two parallel processing paths that depart from the striatum, each of which acts in opposition to each other in the control of movement and enables associations with other relevant functional structures ( Beilock and Carr, 2001 ). Both work together as a neuronal feedback loop. There are many circuits that reach the striatum from other brain areas, including the limbic cortex (associated with emotional processing); the ventral striatum (related to the processing of rewards), and other important motor regions involved in movement ( Alexander and Crutcher, 1990 ). Currently, striatal neuronal plasticity enables basal ganglia circuits to interact with other structures and thereby contribute to the processing of procedural memory ( Haber et al., 2000 ).

The cerebellum is involved in the execution of movements and the perfection of motor agility needed procedural skills. Damage to this area can impede one from relearning motor skills and recent studies have linked it to the process of automating unconscious skills during the learning phase ( Kreitzer, 2009 ).

The limbic system shares anatomical structures with a component of the neostriatum, which assumes primary responsibility for the control of procedural memory. There is a special protein membrane associated with the limbic system that runs through the nucleus basalis. Thus, activation of brain regions that work together during the operation of procedural memory can be followed through the protein membrane associated with the limbic system.

As a final note on procedural memory, whereas earlier theories proposed a passive role whereby memories were shielded from interfering stimuli during sleep ( Vertes and Eastman, 2000 ; Vertes, 2004 ), current theories suggest a more active role in which memories undergo a process of consolidation during sleep ( Ellenbogen et al., 2006 ). Furthermore, in human beings, this process of consolidation is thought to contribute to the development of procedural knowledge, especially when it occurs right after the initial phase of memory acquisition ( Karni et al., 1994 ; Gais et al., 2000 ; Stickgold et al., 2000a , b ; Saywell and Taylor, 2008 ). Within the scope of motor skills related to procedural memory, there is evidence to show that there is no improvement in skills if followed by short NREM sleep (stages 2–4 sleep), such as a short nap ( Walker et al., 2002 ). However, REM sleep (a sleep phase with an increased frequency and intensity of the so-called dream state) followed by a period of slow wave sleep has proven to be the most effective combination for procedural memory consolidation, especially immediately following skill acquisition ( Siegel, 2001 ).

Associative Memory: Classical and Operant Conditioning

Associative memory refers to the storage and retrieval of information through association with other information. The acquisition of associative memory is carried out with two types of conditioning: classical conditioning and operant conditioning. Classical conditioning is associative learning between stimuli and behavior. Meanwhile, operant conditioning is a form of learning in which new behaviors develop in terms of their consequences. Associationist philosophers have also worked with the latter model ( Hartley, 1749 ; Mill, 1829 ). We will look more closely at both.

The close association between two stimuli over time causes classical conditioning: first a conditioned stimulus and then an unconditioned stimulus. While a conditioned stimulus does not automatically trigger a response, an unconditioned stimulus does just that. By repeating a conditioned stimulus over time before an unconditioned stimulus, a conditioned stimulus acquires characteristics that simulate being necessary for an unconditioned stimulus. Pavlov’s Dog ( Pavlov, 1927 ) is a clear example. The dog produces saliva when it detects the presence of food (unconditioned stimulus). If the sound of a bell goes off (conditioned stimulus) during the act of giving the dog food, the dog will associate the sound of the bell with the presence of food. In successively repeating this, the dog will associate the unconditioned stimulus with the conditioned stimulus, thus producing saliva when just hearing the bell.

Although Skinner is considered to be the originator of operant conditioning, his research drew upon Thorndike’s law of effect. For operant conditioning, as has already been mentioned, positive consequences following a behavior promote its repetition. Conversely, if the behavior involves negative consequences, the behavior will be repeated less. Thorndike (1932) called this conditioning instrumental because it suggests that the behavior serves as a means to an end and emerges from trial and error. Skinner later coined the term that is now widely associated with this law of effect – reinforcement ( Skinner, 1938 ).

Non-associative Memory: Habituation and Sensitization

Non-associative memory is one of three types of non-declarative or implicit memory and refers to newly learned behavior through repeated exposure to an isolated stimulus.

New behavior can be classified into two processes: sensitization and habituation ( Alonso, 2008 ). Before delving into each process, it is worth noting that the simplicity in acquiring this type of memory has advanced knowledge of the learning process. This is due to the fact that both animals and human beings have these two processes, such that it is very likely that, in this regard, they share a molecular biological basis. Kandel (1976) proposed a model to explain habituation and sensitization’s operation mechanism. On one hand, for habituation, acetylcholine is progressively consumed, decreasing the effectiveness of the stimulus and thus the motor response. Furthermore, for sensitization, the presence of serotonin, secreted by another sensory nerve terminal, causes an excess of acetylcholine. Thus an enhanced motor response emerges. Let’s look at the two processes that take place in the acquisition of new behaviors— processes that are part of non-associative memory, habituation and sensitization.

Habituation, in this context, is linked to repetition. The repetition of a stimulus leads to a decrease in its response, which is known as habituation. Repeated exposure to a stimulus serves to stop responding to potentially important, but situationally irrelevant stimuli. Habituation could be due to a process of synaptic depression as a result of repeated activation. Thus, habituation is thought to be related to a decrease in the efficiency of synaptic transmission, a decrease that may be caused by a conductivity change in the membrane of the stimulated neuron’s iconic channels.

Unlike habituation, sensitization consists in an increase in response to a stimulus due to the repeated introduction thereof. Although the processes that produce sensitization are the same as those that produce habituation, sensitization’s effects are the opposite since it results in an increase of the original response. The process of sensitization may be due to a provision in transmission, whether it be presynaptic or postsynaptic.

Priming, the fourth modality of non-declarative or implicit memory, is an effect whereby exposure to certain stimuli influences the response given to stimuli presented later.

An example is in order. If you present a list of words to a person that contains the word ‘ball,’ and then the person is asked to participate in a task to complete words, they are more likely to respond with the word ball to the presentation of the word bowl than if they had not previously seen that word in the original list. Thus, the priming capacity can affect the choice of a particular word on a test to complete words, even long after conscious recollection of the primed words has been forgotten.

Another context where this can be seen is in asking a participant to identify an image from a small fragment. The participant is shown a larger portion of the image over time, giving them the ability to identify the image at the end. The participant will take longer to identify the image if it is the first time he/she sees it. But if he/she already saw it in a previous trial, he/she takes less time ( Kolb and Whishaw, 2003 ).

Future Directions

In spite of recent progress, a number of important questions remain to be tackled.

Many of these questions have to do with molecular processes of memory consolidation, retrieval, and decay. Take, for instance, the processes underlying the LTP of synaptic strength among neurons of the hippocampus ( Dong et al., 2015 ). In their review, Hardt et al. (2013) point out that while the “molecular processes involved in establishing LTP have been characterized well, the decay of early and late LTP is poorly understood.” One possibility that has recently been suggested is that LTP decay is mediated by AMPAR endocytosis, which in turn implies that inhibition of this process could preserve LTP and help to prevent memory loss ( Dong et al., 2015 ).

Other recent work shows the critical role of dopamine as a signal that promotes the stable incorporation of novel information into long-term hippocampal memory ( Otmakhova et al., 2013 ). Indeed, dopamine neurons can be activated by novelty in the absence of reward and it is thought that this activation occurs via a polysynaptic pathway that runs from the hippocampus to the dopamine cells of the VTA. However, many aspects of this process remain unclear (see Otmakhova et al., 2013 ).

Also requiring further investigation are the molecular processes involved in the regulation of protein synthesis related to memory. It is now thought that protein synthesis is not only involved in the consolidation of new memories, but must also be used to re-consolidate memories that have been degraded or “destabilized” as a result of retrieval. In a recent article, Jarome et al. (2016) indicate that CaMKII controls the re-consolidation process through the regulation of proteasome activity. Another mechanism for the regulation of protein synthesis involves MicroRNAs (miRNAs), a class of short, non-coding RNAs. By regulating components of pathways required for learning and memory, miRNAs modulate the influence of epigenetics on cognition in the normal and diseased brain ( Saab and Mansuy, 2014 ).

Another set of important questions relates to the long-standing hypothesis of “Hebbian learning”—the strengthening of synapses between neurons with correlated activity—and its role in memory. At the same time that we are learning more about the mechanisms involved in Hebbian plasticity, we are also learning about how these mechanisms are complemented by synaptogenesis and neuromodulatory processes. Recent research has shown that synaptogenesis is not only important during development, but also plays a central role in associative learning and memory. Synaptogenesis can be triggered by neuron–astrocyte or neuron–neuron contact, and mediated by cell-adhesion proteins including neurexin/neuroligin, Eph receptors, and cadherins, which activate intracellular signaling pathways involving cofilin, GTPases, and other proteins (for a review see Nelson and Alkon, 2015 ). Others have proposed that Hebbian processes, while important, are not sufficient for memory formation, and must be supported by the activation of neuromodulatory processes, especially in the case of associative aversive learning ( Johansen et al., 2014 ). Another study found evidence for both Hebbian and anti-Hebbian mechanisms of synaptic plasticity, indicating that the mechanisms of learning are highly adaptable ( Koch et al., 2013 ).

Finally, other questions relate to the special role of the amygdala in the emotional enhancement of memory consolidation. It has long been known that emotional arousal contributes to the selection and consolidation of memory. It has also been shown that previously weak or inconsequential information can be strengthened retroactively through an emotional learning experience ( Dunsmoor et al., 2015 ). Evidence suggests that it is the amygdala that is most responsible for the enhancement of memory ( de Voogd et al., 2016 ). As suggested by a review, these developments “will likely lead to an updated view of the amygdala as a critical nexus within large-scale networks supporting different aspects of memory processing for emotionally arousing experiences” ( Hermans et al., 2014 ). Moreover, this research is likely to have important implications for the treatment of psychological disorders ( Beckers and Kindt, 2017 ).

Conclusion and Glossary

There are three main forms of memory: sensory memory, short-term memory, and long-term memory ( Figure ​ Figure3 3 ). Sensory memory refers to the retention of information coming from the senses. Short-term memory refers to information processed in a short period of time. Working memory performs this processing. Working memory consists of four elements that process information: the central executive (attention control), the visuospatial sketchpad (creates and maintains a visuospatial representation), the phonological buffer (stores and consolidates new words), and the episodic buffer (stores and integrates information from different sources). Long-term memory allows us to store information for long periods of time. This information may be retrieved consciously (explicit memory) or unconsciously (implicit memory). Explicit memory consists of episodic memory (time-related events) and semantic memory (concepts and meanings). Implicit memory has, in turn, procedural memory (motor and executive skills), associative memory (classical and operant conditioning), non-associative memory (sensitization and habituation), and priming (a primary stimulus influencing a secondary one).

Memory classification.

Finally, the following glossary includes commentary about the terminology that, in our opinion, is essential for an introductory overview, enabling interested students and professionals to effectively approach the latest memory-related discoveries. This commentary is not intended as an exhaustive definition, but rather collects relevant information to situate the reader within a complex panorama.

Associative memory : refers to the storage and retrieval of information resulting from an association (i.e., resulting from an association with other information). Two types of conditioning are involved in its acquisition: classical conditioning and operant conditioning. Classical conditioning is a kind of associative learning between stimuli and behavior, and operant conditioning is a form of learning in which new behaviors develop in terms of their consequences.

Conceptual short-term memory/episodic buffer : This is a temporary storage system capable of integrating information from different sources that is probably controlled by the central executive. It is episodic in that it has episodes in which information is integrated through space and, potentially, extended through time.

Echoic memory : sensory memory that receives and processes auditory information.

Episodic memory : “involves the ability to learn, store, and retrieve information about unique personal experiences that occur in daily life. These memories typically include information about the time and place of an event, as well as detailed information about the event itself” ( Dickerson and Eichenbaum, 2010 ).

Explicit/declarative memory : refers to conscious memories of previously stored experiences, facts and concepts that are verifiable through a verbal reporting of them ( Tulving, 1972 ).

Haptic memory : sensory memory that receives and processes information from the sense of touch.

Iconic memory : visual-sensory memory that receives and processes visual stimuli.

Implicit/non-declarative memory : this encompasses all unconscious memories, as well as certain abilities or skills. There are four types of implicit memory: procedural, associative, non-associative, and priming memory.

Long-term memory : “refers to the unlimited, continuing memory store that can hold information over lengthy periods of time, even for an entire lifetime. Long-term memory is mainly preconscious and unconscious. Information in long-term memory is to a great extent outside of our awareness, but can be called into working memory to be used when needed. Some of this information is easy to recall, but some is much more difficult to access” ( Brodziak et al., 2013 ).

Non-associative memory : refers to newly learned behavior due to repeated exposure to a single stimulus. The new behavior can be classified into two processes: sensitization and habituation.

Perceptual memory : memory acquired through the senses. It includes a lot of individual experience; it ranges from the simplest forms of sensory memory to the most abstract knowledge.

Priming : an effect whereby exposure to certain stimuli influences the response to subsequently presented stimuli.

Procedural memory : a memory area involved in remembering executive and motor skills necessary to perform a task. It is an executive system that guides activity and usually works on an unconscious level. When necessary, procedural memories are automatically retrieved for use in the implementation of integrated procedures related to motor and intellectual skills.

Semantic memory : refers to the memory of meanings, interpretations and concepts related to facts, information and general knowledge about the world. Semantic memory gives meaning to words and phrases that would otherwise be meaningless and allows for learning based on past experience ( Kolb and Whishaw, 2003 ).

Sensory memory : “Sensory memory is the capacity for briefly retaining the large amounts of information that people encounter daily” ( Siegler and Alibali, 2005 ).

Short-term memory: is the ability to keep a small amount of information available for a short period of time. “Short-term memory should be distinguished from working memory, which refers to structures and processes used for temporarily storing and manipulating information. The relationship between short-term memory and working memory is presented variously by different theories. The notion of working memory is broader and more general because it refers to structures and processes used for temporarily stored and manipulated information” ( Brodziak et al., 2013 ).

Working memory : “The term working memory refers to a brain system that provides temporary storage and manipulation of the information necessary for such complex cognitive tasks as language comprehension, learning and reasoning” ( Baddeley, 1992 ).

Visual memory : constituted by iconic memory, visual short-term and long-term memory.

Visual short-term memory/visuospatial sketchpad : sketchpad’s main function is to create and maintain a visuospatial representation that persists through the irregular form found in eye movement and that characterizes our exploration of the visual world ( Luck, 2007 ).

Author Contributions

FG drafting of sensory and short-term memory; bibliographical review; introduction and conclusion. Revising critically and final approval of the version to be published. EC drafting of long-term memory (declarative and non-declarative); bibliographical review.

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

This work was supported by the Institute for Culture and Society (ICS) of the University of Navarra and by the “Programa de Ayudas de la Asociación de Amigos de la Universidad de Navarra.” We are grateful to Mind-Brain Group of ICS and, especially, to Nathaniel Barrett Ph.D., Gonzalo Arrondo Ph.D., and Javier Bernácer Ph.D. for their suggestions.

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Types of Memory

Aug 26, 2012

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Types of Memory. Information processing model. Sensory memory. Sensory memory. Senses Vision: iconic memory Auditory: echoic memory Purpose? Persistence: beyond physical duration Record until further processing Provides stability for senses. Photography: shutter speed. 1/30 1/500.

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• visuospatial sketch pad
• explicit memory
• spatial rehearsal
• senses vision

Presentation Transcript

Information processing model Sensory memory

Sensory memory Senses Vision: iconic memory Auditory: echoic memory Purpose? Persistence: beyond physical duration Record until further processing Provides stability for senses

Photography: shutter speed 1/30 1/500

Sensory memory Task: You will see a grid of 12 letters very quickly. I will ask you to write down as many of the letters as you can after they are flashed.

Iconic memory: Sperling (1960) Presentation time: 50ms Whole report: 4 out of 12 (37%) Auditory cue AFTER display Partial report: 3 out of 4 (75%) Rapidly decaying image!

Short-term memory and Long-term memorySTM & LTM Memory performance depends on three stages Encoding Storage Retrieval Form of information Verbal (acoustic) Visual (picture) Storage: What is capacity of STM? STM  LTM Rehearsal: use “inner voice” or“inner eye” Examine how we RETRIEVE information Recall Recognition

Working Memory (WM) WM: Short-term processing and storage of information Phonological loop: verbal rehearsal Visuospatial sketch pad: visuo-spatial rehearsal Central executive: controls processing and allocates resources Logie, Zucco, & Baddeley, 1990

Episodic vs. Semantic memory • Semantic memory • Memory for knowledge • What do people eat for breakfast? • Recall facts and knowledge • Episodic memory • Memory of events • What did you have for breakfast? • Recall a list of words • Distinguishes type of info learned, not when learned

Explicit vs. Implicit memory • Type of retrieval question • Explicit memory • Effortful, conscious recollection • Recall or recognition • Implicit memory • Remembering without awareness • Word fragment or identification • Distinguishes how info is retrieved

Implicit memory tests • Picture: What do you see? • Word: Fill in the fragment.

Memory & AmnesiaImplicit vs Explicit memory • Warrington & Weiskrantz (1970) • Implicit vs explicit memory tests (fig above) • Jacoby & Witherspoon (1982) (text: pp 287) • Anterograde amnesia & homophones • Study: Hear “book-read” or “saxophone-reed” • Test: Asked to spell 2nd word in pair

Amnesia & Movies • Memento (2000) • Regarding Henry (1991) • Fifty First Dates (2004) • The Bourne Identity (2002)

Case study approach to study memory: Amnesia • Retrograde amnesia • Can’t remember events prior to point of injury • “Soap opera amnesia” • Rare – and most can recover memory loss • Anterograde amnesia • Memory loss after point of damage • Cannot form new memories • E.g. H.M.; Korsakoff’s syndrome; viral encephalitis

Clive Wearing • Dense retrograde and anterograde amnesia patient • Born in 1938, contracted viral encephalitis in 1985 • Previously a very successful musician • Husband to 2nd wife; has children from 1st marriage • BBC 2005 – “Man with the 7s memory” • 20 yrs post injury – 67 yrs old • http://www.youtube.com/watch?v=wDNDRDJy-vo&feature=related • 1998 documentary • 13 yrs post injury – 60 years old • http://www.youtube.com/watch?v=Lu9UY8Zqg-Q&feature=related • http://www.youtube.com/watch?v=xCyvzI2aVUo&fea • http://www.youtube.com/watch?v=9BrCBq2FY_U&feature=related

Thought paper • What is Clive Wearing capable of and not capable of doing? • Provide examples for the below vocabulary in your answer • Short-term memory vs. long-term memory • Explicit vs. implicit memory • Episodic vs. semantic memory

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• 2.  RANDOM ACCESS MEMORY(RAM)  READ ONLY MEMORY(ROM)  CACHE MEMORY  BUBBLE MEMORY  SECONDARY MEMORY
• 3.  A MEMORY IS JUST LIKE A HUMAN BRAIN. IT IS USED TO STORE DATA AND INSTRUCTIONS. COMPUTER MEMORY IS THE STORAGE SPACE IN COMPUTER WHERE DATA IS TO BE PROCESSED AND INSTRUCTIONS REQUIRED FOR PROCESSING ARE STORED…..MEMORY IS PRIMARILY OF THREE MEMORY.
• 4.  A COMPUTER ‘S MAIN MEMORY USES VOLATILE RAM CHIPS ARE TWO TYPES- DYNAMIC AND STATIC RAM  STATIC RAM STORES BINARY INFORMATION USING CIOCKED SEQUENTIAL CIRCUITS.  DYNAMIC RAM STORES INFORMATION IN SIDE A CHIP IN THE FORM OF ELECTRIC CHARGES SUPPLIED TO THE CAPACITOR.
• 5.  ROM IS A NON VOLATILE MEMORY,IS A NON VOLATILE MEMORY CHIP IN WHICH DATA IS STORED PERMANENTLY.  THEY ARE TOW TYPES 1. PROGRAMMABLE READ ONLY MEMORY  2.ERASABLE PROGRAMMABLE READ ONLY MEMORY  3.ELECTRICALLY ERASABLE PROGRAMMABLE READONLY MEMORY.
• 6.  ITS TEMPORARILY STORES AND SUPPLIES THE DATA AND INSTRUCTIONS FROM THE MAIN MEMORY TO THE INTERNAL MEMORY TO SPEED UP THE PROCESS.THEY ARE FASTER THAN THE MAIN MEMORY WITH ACCESS TIME CLOSER TO THE SPEED OF THE CPU.
• 7.  BUBBLE MEMORY IS ATYPE OF NON - VOLATILE COMPUTER MEMORY THAT USES A THIN FILM OF A MAGNETIC MATERIAL TO HOLD SMALL MAGNETIZED ARAS KNOWN AS BUBBLES OR DOMAINS EACH STORING ONE BIT DATA .