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Split Brains

The brain's processing of information affected by hemispheric transfer..

Posted November 6, 2012 | Reviewed by Ekua Hagan

Split-brain surgery, or corpus calloscotomy, is a drastic way of alleviating epileptic seizures, the occurrence of sporadic electrical storms in the brain. The procedure involves severing the corpus callosum, the main bond between the brain’s left and right hemispheres.

After a split-brain surgery, the two hemispheres do not exchange information as efficiently as before. This impairment can result in split-brain syndrome, a condition where the separation of the hemispheres affects behavior and agency.

Michael Gazzaniga and Roger W. Sperry , the first to study split brains in humans, found that several patients who had undergone a complete calloscotomy suffered from split-brain syndrome. In patients with split-brain syndrome, the right hemisphere, which controls the left hand and foot, acts independently of the left hemisphere and the person’s ability to make rational decisions. This can give rise to a kind of split personality , in which the left hemisphere give orders that reflect the person’s rational goals , whereas the right hemisphere issues conflicting demands that reveal hidden desires.

Gazzaniga and Sperry's split-brain research is now legendary. One of their child participants, Paul S., had a fully functional language center in both hemispheres. This allowed the researchers to question each side of the brain. When they asked the right side what their patient wanted to be when he grew up, he replied, "an automobile racer." When they posed the same question to the left, however, he responded, "a draftsman." Another patient pulled down his pants with the left hand and back up with the right in a continuing struggle. On a different occasion, this same patient's left hand made an attempt to strike the unsuspecting wife as the right hand grabbed the villainous limp to stop it.

Split personality is a rare consequence of a split brain. In some cases, impaired interhemispheric communication leaves personality intact but still allows people to use the two hemispheres to complete independent intellectual tasks.

An MRI scan of the savant Kim Peek, who inspired the fictional character Raymond Babbitt (played by Dustin Hoffman) in the movie Rain Man , revealed an absence of the corpus callosum, the anterior commissure and the hippocampal commissure, the three cables for information transfer between hemispheres.

As a consequence of this complete split, Peek, who sadly died last year, was able to simultaneously read both pages of an open book and retain the information. He apparently had developed language areas in both hemispheres. Peek was a living encyclopedia. He spent every day with his dad in the library absorbing information. Among his most impressive feats was his ability to provide traveling directions between any two cities in the world.

Today, hemisphere interaction can be studied using devices that measure the electric or magnetic fields surrounding the skull. Unlike split-brain surgery, these techniques are non-invasive.

A team of researchers from UC Santa Barbara, led by Gazzaniga, recently tested information transfer using MEG. Language is processed in areas of the temporal lobe on the left side of the head. When you read with your left eye, the information first ends up in the right hemisphere and must be transferred to the left hemisphere via the corpus callosum to be processed.

To test the efficiency of the hemispheric transfer, the researchers showed a randomized list of words and nonsense words to the left or right eye of a number of research participants. They then measured how effectively the subjects would be able to distinguish words from nonsense words. The study showed that subjects were significantly more efficient in determining the nature of the string of letters when the information was fed directly to the left hemisphere via the right eye. Apparently, the brain has difficulties processing information that has had to travel long distances.

The researchers didn't compare both-eye exposure to single-eye exposure. At first glance, it may seem that it would be an advantage to get information from both eyes. However, one can also imagine that hemispheric transfer has a hampering effect on language processing. If this is true, you might want to wear a pirate eye patch covering your left eye when completing the verbal section of the GRE. At the very least, be careful not to shut your right eye while under time pressure.

K. W. Doron, D. S. Bassett, M. S. Gazzaniga. Inaugural Article: Dynamic network structure of interhemispheric coordination . Proceedings of the National Academy of Sciences, 2012; DOI: 10.1073/pnas.1216402109

Berit Brogaard, D.M.Sci., Ph.D. , is a professor of philosophy and the Director of the Brogaard Lab for Multisensory Research at the University of Miami.

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Roger Sperry’s Split Brain Experiments (1959–1968)

Editor's note: Sarah Walls created the above image for this article. You can find the full image and all relevant information here .

In the 1950s and 1960s, Roger Sperry performed experiments on cats, monkeys, and humans to study functional differences between the two hemispheres of the brain in the United States. To do so he studied the corpus callosum, which is a large bundle of neurons that connects the two hemispheres of the brain. Sperry severed the corpus callosum in cats and monkeys to study the function of each side of the brain. He found that if hemispheres were not connected, they functioned independently of one another, which he called a split-brain. The split-brain enabled animals to memorize double the information. Later, Sperry tested the same idea in humans with their corpus callosum severed as treatment for epilepsy, a seizure disorder. He found that the hemispheres in human brains had different functions. The left hemisphere interpreted language but not the right. Sperry shared the Nobel Prize in Physiology or Medicine in 1981for his split-brain research.

Sperry also studied other aspects of brain function and connections in mammals and humans, beyond split-brains, in 1940s and 1950s. In 1963, he developed the chemoaffinity hypothesis, which held that the axons, the long fiber-like process of brain cells, connected to their target organs with special chemical markers. This explained how complex nervous systems could develop from a set of individual nerves. Sperry then also studied brain patterns in frogs, cats, monkeys, and human volunteers. Sperry performed much of his research on the split-brain at California Institute of Technology, or Caltech, in Pasadena, California, where he moved in 1954.

Sperry began his research on split-brain in late 1950s to determine the function of the corpus callosum. He noted that humans with a severed corpus callosum did not show any significant difference in function from humans with intact corpus callosum, even though their hemispheres could not communicate due to the severing of the corpus callosum. Sperry postulated that there should be major consequences from cutting the brain structure, as the corpus callosum connected the two hemispheres of the brain, was large, and must have an important function. Sperry began designing experiments to document the effects of a severed corpus callosum. At the time, he knew that each hemisphere of the brain is responsible for movement and vision on the opposite side of the body, so the right hemisphere was responsible for the left eye and vice versa. Therefore, Sperry designed experiments in which he could carefully monitor what each eye saw and therefore what information is was going to each hemisphere.

Sperry experimented with cats, monkeys, and humans. His experiments started with split-brain cats. He closed one of their eyes and presented them with two different blocks, one of which had food under it. After that, he switched the eye patch to the other eye of the cat and put the food under the other block. The cat memorized those events separately and could not distinguish between the blocks with both eyes open. Next, Sperry performed a similar experiment in monkeys, but made them use both eyes at the same time, which was possible due to special projectors and light filters. The split-brain monkeys memorized two mutually exclusive scenarios in the same time a normal monkey memorized one. Sperry concluded that with a severed corpus callosum, the hemispheres cannot communicate and each one acts as the only brain.

Sperry moved on to human volunteers who had a severed corpus callosum. He showed a word to one of the eyes and found that split-brain people could only remember the word they saw with their right eye. Next, Sperry showed the participants two different objects, one to their left eye only and one to their right eye only and then asked them to draw what they saw. All participants drew what they saw with their left eye and described what they saw with their right eye. Sperry concluded that the left hemisphere of the brain could recognize and analyze speech, while the right hemisphere could not.

In the 1960s when Sperry conducted his split-brain research on humans, multiple scientists were studying brain lateralization, the idea that one hemisphere of the brain is better at performing some functions than the other hemisphere. However, researchers did not know which tasks each side of the brain was responsible for, or if each hemisphere acted independently from the other.

Sperry describes his research in cats in the article "Cerebral Organization and Behavior" published in 1961. To test how the cutting of the corpus callosum affected mammals, Sperry cut the corpus callosum of multiple cats and had them perform some tasks that involved their vision and response to a visual stimulus. After severing each cat´s corpus callosum, he covered one of the cat´s eyes to monitor with which eye the cat could see. Sperry could switch the eye patch from one eye to the other, depending on which visual field he wanted the cat to use. Next, Sperry showed the cats two wooden blocks with different designs, a cross and a circle. Sperry put food for the cat under one of the blocks. He taught the cats that when they saw the blocks with one eye, for instance, the right eye, the food was under the circle block, but when they saw it with the left eye, the food was under the block with a cross. Sperry taught the cats to differentiate between those two objects with their paws, pushing the correct wooden block away to get the food.

When Sperry removed the eye patch and the cats could see with both eyes, he performed the same experiment. When the cats could use both eyes, they hesitated and then chose both blocks almost equally. The right eye connects to the left hemisphere and the left eye connects to the right hemispheres. Sperry suspected that since he cut the corpus callosum in those cats, the hemispheres could not communicate. If the hemispheres could not communicate and the information from one eye only went to one hemisphere, then only that hemisphere would remember which block usually had food under it. From that, Sperry concluded that the cats remembered two different scenarios with two different hemispheres. He suspected that the cats technically had two different brains, as their hemispheres could not interact and acted as if the other one did not exist.

Sperry performed a similar experiment with monkeys, in which he also cut their corpus callosum. He wanted to test if both hemispheres could operate at the same time, even though they were not connected. That required separation of visual fields, or making sure that the right eye saw a circle, while the left eye saw a cross, like in the cat experiment, but without an eye patch and both eyes would see something at the same time instead of interchanging between the open eyes. Sperry solved that by using two projectors that were positioned side-by-side at an angle and showed mutually exclusive images. For example, the projector on the right showed a circle on the left and a cross on the right, while the projector on the left showed a cross on the left and a circle on the right. Sperry placed special light filters in front of each of the monkey´s eyes. The light filters made it so that each eye saw the images from only one of the projectors. That meant one of the eyes saw the circle on the right and the cross on the left, while the other eye saw the cross on the right and the circle on the left. From his experiments with cats, Sperry knew that there was no sharing of information from right and the left hemispheres, so he made the monkeys memorize two different scenarios at the same time.

The left eye saw a scenario where food would be dispersed when the monkey pressed the button corresponding to a cross, while the right eye saw a scenario where food would be dispersed when the monkey pressed a button corresponding to a circle. Ultimately, it was the same button, but the eyes saw it differently because of two projectors and special light filters. Sperry concluded that both hemispheres of the brain were learning two different, reversed, problems at the same time. He noted that the split-brain monkeys learned two problems in the time that it would take a normal monkey to learn one, which supported the assumption that the hemispheres were not communicating and each one was acting as the only brain. That seemed as a benefit of cutting corpus callosum, and Sperry questioned whether there were drawbacks to the procedure.

Sperry performed the next set of experiments on human volunteers, who had their corpus callosum severed previously due to outside factors, such as epilepsy. Sperry asked volunteers to perform multiple tests. From his previous experiments with cats and monkeys, Sperry knew that one, the opposite, hemisphere of the brain would only analyze information from one eye and the hemispheres would not be able to communicate to each other what they saw. He asked the participants to look at a white screen with a black dot in the middle. The black dot was the dividing point for the fields of view for a person, so the right hemisphere of the brain analyzed everything to the left of the dot and the left hemisphere of the brain analyzed everything that appeared to the right of the dot. Next, Sperry showed the participants a word on one side of the black dot for less than a second and asked them to tell him what they saw. When the participants saw the word with their right eye, the left hemisphere of the brain analyzed it and they were able to say what they saw. However, if the participants saw the word with their left eye, processed by right hemisphere, they could not remember what the word was. Sperry concluded that the left hemisphere could recognize and articulate language, while the right one could not.

Sperry then tested the function of the right hemisphere. He asked the participants of the same experiment that could not remember the word because it was in the left visual field to close their eyes and draw the object with their left hand, operated by the right hemisphere, to which he presented the word. Most people could draw the picture of the word they saw and recognize it. Sperry also noted that if he showed the word to the same visual field twice, then the person would recognize it as a word they saw, but if he showed it to the different visual fields, then the participants would not know that they saw the word before. Sperry concluded that the left hemisphere was responsible not only for articulating language, but also for understanding and remembering it, while the right hemisphere could only recognize words, but was not able to articulate them. That supported the previously known idea that the language center was in the left hemisphere.

Sperry performed another similar experiment in humans to further study the ability of the right hemisphere to recognize words. During that experiment, Sperry asked volunteers to place their left hand into a box with different tools that they could not see. After that, the participants saw a word that described one of the objects in the box in their left field of view only. Sperry noted that most participants then picked up the needed object from the box without seeing it, but if Sperry asked them for the name of the object, they could not say it and they did not know why they were holding that object. That led Sperry to conclude that the right hemisphere had some language recognition ability, but no speech articulation, which meant that the right hemisphere could recognize or read a word, but it could not pronounce that word, so the person would not be able to say it or know what it was.

In his last series of experiments in humans, Sperry showed one object to the right eye of the participants and another object to their left eye. Sperry asked the volunteers to draw what they saw with their left hand only, with closed eyes. All the participants drew the object that they saw with their left eye, controlled by the right hemisphere, and described the object that they saw with their right eye, controlled by the left hemisphere. That supported Sperry´s hypothesis that the hemispheres of brain functioned separately as two different brains and did not acknowledge the existence of the other hemisphere, as the description of the object did not match the drawing. Sperry concluded that even though there were no apparent signs of disability in people with a severed corpus callosum, the hemispheres did not communicate, so it compromised the full function of the brain.

Sperry received the 1981 Nobel Prize in Physiology or Medicine for his split-brain research. Sperry discovered that the left hemisphere of the brain was responsible for language understanding and articulation, while the right hemisphere could recognize a word, but could not articulate it. Many researchers repeated Sperry´sf experiments to study the split-brain patterns and lateralization of function.

• Sperry, Roger W. "Cerebral Organization and Behavior." Science 133 (1961): 1749–57. http://people.uncw.edu/puente/sperry/sperrypapers/60s/85-1961.pdf (Access December 8, 2017).
• Sperry, Roger W. "Hemisphere Deconnection and Unity in Conscious Awareness." American Psychologist 28 (1968): 723–33. http://people.uncw.edu/Puente/sperry/sperrypapers/60s/135-1968.pdf (Access December 8, 2017).
• Sperry, Roger W. "Split-brain Approach to Learning Problems." In The Neurosciences: A Study Program , eds. Gardner C. Quarton, Theodore Melnechuk, and Francis O. Schmitt, 714–22. New York: Rockefeller University Press, 1967. ttp://people.uncw.edu/puente/sperry/sperrypapers/60s/130-1967.pdf (Accessed November15, 2017).
• "The Split Brain Experiments." Nobelprize.org . https://www.nobelprize.org/educational/medicine/split-brain/background.html (Accessed May 3, 2017).

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Article Contents

Introduction, separated information processing in both hemispheres, lateralization of function, non-neural interhemispheric integration the concept of cross-cueing, the split-brain and concepts of neurological lesions, implications for understanding consciousness.

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Interaction in isolation: 50 years of insights from split-brain research

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Lukas J. Volz, Michael S. Gazzaniga, Interaction in isolation: 50 years of insights from split-brain research, Brain , Volume 140, Issue 7, July 2017, Pages 2051–2060, https://doi.org/10.1093/brain/awx139

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Fifty years ago, one of the first studies that showed the neuropsychological consequences of sectioning the corpus callosum, that great bundle of fibres that connects the two cerebral hemispheres, was published in Brain ( Gazzaniga and Sperry, 1967 ). With the help of several patients who have undergone this procedure and generously given of their time as willing participants in research, a gold mine of information about the way brains function has been ferreted out. Research studies in the ensuing years have both confirmed and extended the findings, not only in the original patient group, but other groups as well. The insights gained from testing these so called ‘split-brain’ patients have contributed to the evolving field of cognitive neuroscience and have helped establish information processing models for how the brain governs behaviour and cognition.

The original ‘split-brain’ patients tested in California had undergone a complete transection of the corpus callosum and the anterior and hippocampal commissures (with some minor variance occurring between subjects) to alleviate intractable, severe epilepsy, which it did. Twenty years before, testing of another group of similar split-brain patients in Rochester, New York ( cf. Akelaitis, 1941 ) had not revealed any discernible differences between pre- and post-surgical behaviour, suggesting that not much would be learned from this new group. Using a behavioural testing device (which had not been used in New York) that allowed information to be fed to either hemisphere independent of the other, however, revealed that these patients were to provide a unique opportunity to investigate the separate functions of the two cerebral hemispheres ( Fig. 1 ).

Tachistoscope. Presenting visual stimuli with a tachistoscope allows selective presentation of visual information to one hemisphere at a time. Patients were asked to fix their gaze on the centre of the translucent screen, upon which the examiner projects visual stimuli for 0.1 s. Information projected onto the left half of the screen is subsequently processed by the right hemisphere, whereas stimuli presented in the right visual field are processed by the left hemisphere. The short presentation interval prevents visual information on one side of the screen from being processed by both hemispheres due to eye movements. Modified from Gazzaniga (2000) , with permission.

‘In general the post-surgical studies indicate a striking functional independence of the gnostic activities of the two hemispheres. Perceptual, cognitive, mnemonic, learned and volitional activities persist in each hemisphere, but can proceed separately in each case outside the realm of awareness of the other hemisphere.’

The goal of this article is to outline some of the challenges in interpreting the experience of interacting with split-brain patients. After briefly summarizing some elementary and uncontroversial findings derived from split-brain patients, we will focus on more controversial points that remain the topic of ongoing debate. In particular, we will review the concept of cross-cueing, which is a crucial and tangible reality when interpreting split-brain results. This may resonate with any reader who has had the experience of working with neurological patients.

The starting point for many split-brain experiments is to provide information to one hemisphere at a time ( Fig. 1 ). This is most easily accomplished through the visual system, thanks to its tidy anatomy ( Fig. 2 ). If you stare straight ahead at a spot, information on the right side of space perceived by both eyes will end up in the left hemisphere and information on the left side of space will end up in the right hemisphere. This is true for all of us, including our split-brain patients. Since our hemispheres are connected, it is natural for our brains to stitch the two sides together and create a unified visual world ( Gazzaniga et al. , 1965 ). Yet, for the split-brain patient with no such connection, each hemisphere sees only the opposite half of the space.

Neuroanatomical basis for processing of visual information. When fixating the centre of the screen (cross), visual information presented on the left half of the screen (blue square) is processed by neurons located in the nasal half of the retina in the left eye and lateral half of the retina in the right eye. While the latter directly project into the right hemisphere, axons of retinal neurons in the nasal half of the left eye (blue) cross from the left to the right hemisphere in the optic chiasm. As a result, visual stimuli presented to the left visual field are processed by the right hemisphere, while stimuli presented to the right visual field (red circle) are processed by the left hemisphere.

This neat separation of visual input makes it possible to provide visual information to one hemisphere of split-brain patients without the knowledge of the other hemisphere. For example, when an object is shown in the right visual field, the visual information travels to the left hemisphere and the patient is effortlessly able to name it ( Fig. 3 A). When shown to the left visual field, however, the information travels to the right hemisphere, and when asked, the patient will typically answer that no object was seen ( Fig. 3 B). This phenomenon is easily explained by the fact that most people’s speech centre is located in their left hemisphere. When the hemispheres are separated, the left will be capable of naming an object, while the right hemisphere stays mute. Moreover, the left hemisphere will also eagerly answer the question intended for the right hemisphere. When it hears the question directed to the right hemisphere asking what the object was, the left hemisphere correctly and honestly reports that it did not see anything at all.

Separated information processing. ( A ) When two different letters are presented in each visual field, the patient will report the letter projected onto the right half of the screen (‘R’, processed by the verbally dominant left hemisphere). The letter presented on the left half of the screen (‘B’, processed by the right hemisphere) is not verbally reported, but can be identified via tactile information using the left hand (controlled by the right hemisphere). ( B ) If visual stimuli are exclusively presented in the left visual field (processed by the right hemisphere), they can again be identified by the patient via tactile information from the left hand (also processed by the right hemisphere). Intriguingly, the patient will verbally report that he did not see any stimulus, due to the lack of information in the verbal left hemisphere. Modified from Sperry et al. (1969) , with permission.

Now picture yourself listening to the completely normal looking person sitting in front of you saying that he did not see the object. He sounds absolutely sure about this. One might jump to the conclusion that the right hemisphere did not perceive the stimulus. Yet this interpretation drastically changes when the right hemisphere is asked to communicate non-verbally. For example, when instructed to point out the object from a group of objects with the left hand, patients reliably identify the object that had been presented to the right hemisphere. Not just better than chance. Every time.

From an anatomical perspective, this hardly seems surprising: the right hemisphere perceives and processes the visual input and then uses its loyal henchman, the left hand, to point it out. The left hand does this because it receives its neuronal input from corticospinal fibres that originate from the right hemisphere. Phenomenologically for the onlooker, however, the observation is far more challenging: the left hand is now confidently pointing out the object that the person just categorically and confidently denied seeing. This is where things get really interesting. Ask the person why he is pointing to that object. Since the left hemisphere and its speech centre do not know what the right hemisphere saw and do not know why the left hand is pointing to a particular object, one might think that the person would once again answer correctly and honestly by admitting ignorance with a simple ‘I don’t know’. This never happens. The left hemisphere always comes up with a story about why the left hand is doing what it is doing, ‘It is pointing to the apple because I like red’. The results of this very simple experiment led to numerous questions and more testing of the split-brain patients, resulting in more intriguing answers and inferences which are well summarized by the notion of the ‘left hemisphere interpreter’ ( Fig. 4 ; for a detailed account see Gazzaniga and LeDoux, 1978 ; Gazzaniga, 2000 ).

Example of the left hemisphere interpreter. In a classic test, a chicken claw was shown to the (speaking) left hemisphere and a snow scene was shown to the (silent) right hemisphere. Patient P.S. easily picked out related pictures from a set of eight options. His left hand chose a snow shovel and his right hand chose a chicken. When asked why he had picked those particular pictures, P.S. said, ‘Oh, that’s simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed’. Modified from Gazzaniga (2000) , with permission.

On the one hand, the strict separation of information processing seems to be a logical consequence of well understood basic neuroanatomy. At the same time, however, interpreting the consequences of two independent information processing systems housed in the same body challenges our intuitive understanding of fundamental aspects of psychology, such as conscious awareness of perception (when one hemisphere reports, ‘I didn’t see anything’) or agency (yet the other chooses the correct object) and causation (‘because I like red’), which ultimately led to the question how these independent systems can coexist and coordinate a single physical body despite the lack of direct, neural interaction. And there was the other nagging notion: can a flick of a knife really produce two separate-consciousness autonomous brains? If so, what exactly does that mean for, say, personal identity?

The fact that the left hemisphere jumps in to offer an explanation whenever asked, even if it does not know what its counterpart to the right is up to, may suggest that the right hemisphere is unable to process language at all. While, indeed, the right hemisphere is typically, at first, not capable of speech production, it does, however, understand both spoken and written language. Since auditory stimuli are typically processed bilaterally, the experimental design had to be adjusted to test lateralization of phoneme processing. For example, after verbally presenting a target word (perceived by both hemispheres) such as ‘chair’, a series of words was visually presented to the right hemisphere only. The left hand then successfully indicated that it recognized the target word by pointing to it ( Gazzaniga and Sperry, 1967 ). To accomplish this, the spoken word had to be interpreted by the right hemisphere in order to produce the correct response from the left hand, since only the right hemisphere could see the list of words from which to choose. In a similar fashion, the right hemisphere can also process the semantic meaning of short sentences. For example, changing the initial verbal target from a single word to a description (‘Used to tell the time’), also leads to a correct response with the left hand pointing to ‘clock’ from a list of words.

Despite the obvious dominance of the left hemisphere, various follow-up experiments have established and further characterized that both hemispheres possess the ability to process language independently. In a complementary fashion, the right hemisphere shows superior specialization for visuospatial processing, as observed in tasks involving part-whole relations, spatial relationships, apparent motion detection, mental rotation, spatial matching and mirror image discrimination (for further details see Gazzaniga, 2005 ).

More recent findings suggest that in the split-brain, the right hemisphere may be specialized to infer causality from physical interactions, whereas the left hemisphere may be involved in more abstract inference of causality ( Roser et al. , 2005 ). The right hemisphere is also better at recognizing familiar faces and human faces. The clinical observation that prosopagnosia typically occurs after lesions to the right hemisphere converges with results from split-brain research ( Turk et al. , 2002 ), as well as neuroimaging findings in both healthy subjects and neurological patients alike ( Rossion et al. , 2011 ). It also appears that the right hemisphere plays a major role in our ability to determine what the intentions of another person might be ( Young and Saxe, 2009 ). Even more startling the right hemisphere can develop speech following callosal section ( Gazzaniga et al. , 1979 , 1984 ; Baynes et al. , 1995 ).

The fact that the split-brain separately processes information in each hemisphere has been replicated numerous times for various domains and, by itself, constitutes an uncontroversial and accepted concept. The degree of hemispheric separation, however, is a topic of ongoing debate. Does surgically disconnecting (most) cortical interhemispheric fibres result in two distinct conscious systems? Are the two hemispheres each perceiving the world and processing information in a slightly different fashion, leading to two independent minds constructing and following their own respective goals?

A first objection might be that two completely separated neural systems should have trouble coordinating one body, given that each of these systems governs the motor function of half of the body. Indeed, some split-brain patients transiently experienced symptoms of an alien hand syndrome, where typically the left hand is perceived to be moving as if following its own goals with a reduced experience of agency over those movements ( Gazzaniga, 2015 ). Moreover, for some patients an intermanual conflict was observed. For example, when trying to arrange a set of blocks with both hands, one hand often undoes what the other has just arranged rather than cooperating to optimize task performance ( Gazzaniga, 2015 ). It is no surprise that the right hemisphere, with its specialized skills for visuospatial reasoning, runs circles around the left hemisphere outperforming it ‘hands down’ in this task. Yet very quickly after surgery, patients are able to walk and run while avoiding obstacles ( Holtzman et al. , 1981 ), even swim ( Gazzaniga, 2015 ), dance and play the piano ( Akelaitis, 1941 ).

Such behaviours critically rely on the coordinated interactions between the hemispheres and the movements they control. It seems almost impossible that two separated hemispheres should be able to swim or play piano, naturally leading to the question of whether the split-brain uses some alternative mysterious non-callosal pathway to transfer information. Could visual information from both hemi-fields be transferred via non-callosal fibres and used to adjust motor controls to avoid bumping into objects while walking or running? While in monkeys, visual information can indeed be exchanged between hemispheres via the anterior commissure, a similar mechanism has been ruled out in humans ( Gazzaniga, 2005 ).

A more likely explanation lies in behavioural ‘cross-cueing’ between hemispheres. A popular analogy illustrating the concept of cross-cueing lies in the coordinated behaviour displayed by conjoined twins. If two unquestionably independent brains control one body, as is the case if the conjunction is sufficiently high, we see a wonderful example of two distinct neural systems integrating information without direct pathways linking the two. Abby and Brittany Hensel are such a pair, each with different desires, likes and dislikes, and personalities. They are conjoined at the chest and torso with a single pair of arms and legs. Even though Abby controls one arm and leg and Brittany the other, they are athletically coordinated. By picking up on behavioural cues, for example when Brittany perceives a movement initiated by Abby (and vice versa), they are able to unconsciously and effortlessly coordinate their movements to a degree that allows them to do such things as play softball.

Split-brain patients might be in a related situation—in some instances only one hemisphere may have access to crucial information needed to perform a certain task. With the abundant amount of constant practice starting right after the surgery, it seems logical that split-brain patients quickly develop nuanced ways to integrate such crucial pieces of information, even in the absence of fibre bundles carrying it from one hemispheres to the other. Since patients are used to constantly relying on cross-cueing, these subtle behavioural cues, which allow them to accomplish complex behaviour, can turn into a profound problem for an experimenter who is trying to test the hemispheres in isolation.

In a manner similar to a patient with early dementia, who creatively dodges questions that would reveal his inability to recall recent events, a split-brain patient will use cueing mechanisms when faced with a task that requires integration of information between hemispheres. Neither of these patients, however, intend to trick the examiner. Their intent, like anyone’s, is simply to perform as well as they can when faced with a challenge. Over the decades, various findings seemed to support the notion of information integration across hemispheres in split-brain patients at first glance. Yet this support dissolved when meticulous re-examination prevented any possibility of cross-cueing ( Gazzaniga and Hillyard, 1971 ). Depending on the experimental design, this can be highly challenging or even impossible ( Seymour et al. , 1994 ).

Recently, Pinto et al. (2017) investigated the degree to which processing of visual information is segregated between hemispheres in two split-brain patients. In line with the canonical interpretation of independent visual processing, they observed that visual stimuli could not be compared across visual half-fields. The authors, however, also observed that some features, such as the presence or location of visual stimuli, were correctly reported throughout the entire visual field for responses obtained verbally or with either hand ( Pinto et al. , 2017 ). This seems at odds with two separated perceptual streams of information. For example, how can the patients verbally report or indicate with their right hand (both controlled by the left hemisphere) whether a visual stimulus was presented to the left visual half-field (i.e. the right hemisphere)? The authors conclude that a certain degree of information exchange has to occur between hemispheres through non-callosal fibres. They suggest that although the information is not sufficient to inform the other hemisphere about its details, there is enough to let it know if and where a stimulus was presented.

These findings can easily be explained by cross-cueing, even though the authors quickly discarded this explanation in their discussion. By characterizing cross-cueing as ‘behavioural tricks, such as touching the left hand with the right hand’ the authors reveal that they underestimate the potential range and subtlety of cueing behaviour, which has been flushed out over decades. In fact, their data and observations fall nicely in line with previous observations of non-neural communication occurring via cross-cueing.

As noted by the authors, the amount of information transferred from one hemisphere to the other by cross-cueing is limited. Accordingly, the patients answered the simple question of whether a visual stimulus was presented or not (almost) perfectly. With the more difficult question of the stimulus’s localization, the answers were not so perfect: though reported above chance level, there was a higher error rate (see Figure 2 in Pinto et al. , 2017 ). Thus, cueing binary information (stimulus/no stimulus) is easy for two separated hemispheres, even without a highly obvious manoeuvre such as touching hands. Informing the other hemisphere about the location of the stimulus is more difficult, however, as readily reflected in the increased error rates. The fact that patients localized stimuli above chance level, even in the crossed case (e.g. stimulus presented to the left hemisphere and response with left hand), can be explained by the experimental design: while an eye-tracking device made sure that a patient fixated on the centre of the screen during the presentation of the visual stimulus, the patients did not have to focus their gaze on the centre of the screen while consecutively indicating the stimulus location. Because split-brain patients have the capacity to cross-cue the location of visual stimuli by eye movements (a glance to the upper-left or right would be cue enough), this allowed them to cue the opposite hemisphere ( Gazzaniga, 1969 ).

Even without the cue of eye movements, intriguing previous data suggest that attentional capacities can be controlled by either hemisphere in split-brain patients, hence giving yet another alternative explanation for the above chance localization of visual stimuli ( Fig. 5 ; Holtzman et al. , 1981 ). For example, after a visual stimulus was exclusively perceived by the right hemisphere, it can direct the attention of the left hemisphere to the given spot in the consecutive relocation condition, by using eye movements or neural connections via collicular-cortical projections or the intact anterior commissure ( Holtzman et al. , 1981 ). In summary, cross-cueing directing hemispheric attention may well explain the findings, rendering the concluded direct inter-hemispheric transfer of visual information unnecessary. This explanation is also in perfect agreement with the observation that two stimuli simultaneously presented in different visual half-fields, could not be compared by the patients (in line with the canonical view of two independent processing systems).

Interhemispheric transfer of spatial location. In this experiment, patients were instructed to locate target stimuli by fixating them with their right eye, while the left eye was occluded. In the first condition, the target stimulus location was highlighted ( A and B ). Unsurprisingly, subjects correctly moved their right eye to the target location when the target was presented in the left visual field, processed by the right hemisphere (within-field trial). In the second between-field condition ( B ), the subject was required to move the eyes to the relative point in the right visual field (not processed by the right hemisphere). Split-brain subjects were able to do this, suggesting cross-integration of spatial information between hemispheres. In the second part of the experiment, information on the identity of the target was presented, either within the left visual field (processed by the right hemisphere, C ) or in the right visual field (not processed by the right hemisphere, D ). While patients had no problems correctly identifying the indicated target stimulus in within-field trials ( C ), they had to guess the target-identity in between-field trials ( D ), as reflected by chance-level accuracy. Hence, while crude information on the spatial localization of a stimulus can be cross-integrated between hemispheres ( B ), more complex information such as stimulus identity ( D ) is not integrated in split-brain patients. Modified from Gazzaniga (1995) , with permission.

Cross-cueing mechanism and mirror neurons

If cross-cueing indeed plays a prominent role in integrating information between hemispheres lacking direct neural connections, how does one hemisphere express content in a way that allows the other hemisphere to understand it? As mentioned above, an obvious possibility lies in initiating a motor action that is perceived by the other hemisphere, for example touching the right hand with the left or tapping a finger. But many more subtle possibilities exist. For example, some of the facial musculature is innervated bilaterally. Thus, a contraction instigated by one hemisphere can attract the other hemisphere’s attention. As discussed above, eye movements and direction of attention via subcortical pathways may be particularly suitable ways to convey the location of a stimulus.

The success of cross-cueing critically relies on the capacity of the recipient hemisphere to decipher the meaning of a given cue. This leads to the question of whether specific mechanisms are involved in the perception and interpretation of cues. Does each hemisphere possess neural circuitry that specializes in picking up, deciphering and potentially even anticipating actions initiated by the other hemisphere? A suitable candidate for this job may be mirror neurons, a set of neurons in the cortical motor system that are active each time an individual performs an action or observes another individual performing the same action ( Rizzolatti et al. , 1996 ). While the initial studies described the mirror mechanism for hand movements with neuronal representations in the ventral premotor cortex, similar neurons have been reported throughout a parieto-frontal network, reacting to a range of different actions, including movements of the mouth and face ( Rizzolatti and Sinigaglia, 2010 ). Could these specialized neurons also be activated in one hemisphere of a split-brain when it detects an action initiated by the other hemisphere? Indeed, when healthy subjects imitate actions, mirror neurons in the hemisphere not controlling the motor output show stronger activation than in the contralateral hemisphere’s network that performs the actual movement ( Aziz-zadeh et al. , 2006 ). Moreover, mirror neurons in the parietal cortex have been characterized as encoding the goal of a perceived action ( Rizzolatti and Sinigaglia, 2010 ), thus making them prominent candidates to decode action cues.

The sports’ world illuminates just how specialized the prediction of movements can be. For example, standing at bat, a skilled baseball player, unconsciously predicting a fastball’s trajectory from the pitcher’s movement, initiates his swing before the ball even leaves the pitcher’s hands. Similarly, the split-brain may rely on the mirror neuron network to become more and more efficient at interpreting and, in the case of sequences of cues, even anticipating such cues thrown to it by the other hemisphere. While this hypothesis remains pure speculation, it may explain how split-brain patients become more adept at using cross-cueing over time and some have even gained the capacity to produce simple speech, such as one-word utterances, from the formerly mute right hemisphere ( cf. Gazzaniga, 2000 ).

How could that formerly mute right hemisphere possibly learn to speak? This skill can emerge years after surgery in some patients and may partially rely on neural plasticity in the right hemisphere. As discussed above, the right hemisphere understands words and hence readily represents their semantic meaning. What could be holding back the right hemisphere’s verbal floodgates may be that it lacks the capacity to coordinate muscle activation in order to produce intelligible speech. Over those intervening years, every time a split-brain patient uses the left hemisphere to speak, the right hemisphere will perceive both intonation-related movements in the thorax, neck and face, and the auditory result. Using the capacity of the mirror neuron system, the right hemisphere might be able to emulate movements to produce speech-related motor output itself. Support for this hypothesis stems from the observation that some ‘audiovisual’ mirror neurons discharge both when seeing or hearing an action, such as when ripping paper or snapping a stick in two ( Kohler et al. , 2002 ). Such neurons may help to evolve the skill to generate motor commands that result in production of simple speech. How difficult it must be to accomplish this complex task is clear to anyone who has tried to speak a foreign language with a perfect accent, a major challenge even with both hemispheres on the job.

Beyond the insights into the functional specialization of the hemispheres and how much hemispheric integration is necessary to produce behaviour, the split-brain also offers a unique perspective on our understanding of brain lesions. In 1965, Norman Geschwind published his seminal paper entitled ‘ Disconnexion syndromes in animals and man ’ ( Geschwind, 1965 ), which reinvigorated the much older idea that the disconnection of communication pathways may lead to specific patterns of functional impairment, introduced by Karl Wernicke (1874). The prototypical example for a disconnection syndrome is conduction aphasia, where a person understands what they hear, can speak fluently, but may use the wrong words or parts of words and has difficulty or is unable to repeat spoken phrases. This condition is produced by lesions to the bundle of neural fibres connecting Broca’s area, which is responsible for the motor component of language and Wernicke’s area, responsible for the sensory component of language. Thus, the clinical observation linking lesions in communication pathways to specific deficits presented neuroscience a path worth pursuing, paving the way for the concept of distributed functional networks, a hot topic in contemporary neuroscience (for a review see Catani and ffytche, 2005 ).

While the split-brain is clearly an example of a disconnexion syndrome, it provides an opportunity that other examples of disconnection syndromes do not. This is the opportunity to study the presence of mental capacities, not the absence of mental capacity caused by lesions ( Gazzaniga, 2015 ). For example, in some patients, the corpus callosum was surgically sectioned in stages over a period of months, in the hope that the patient’s seizures could be controlled without sectioning the entire structure. Testing patients throughout this process revealed the functional organization of the corpus callosum: the more posterior regions transfer basic sensory information that relates to vision, audition and somatosensory information, while anterior regions are involved in the transfer of attentional resources and higher cognitive information ( cf. Gazzaniga, 2005 ). Moreover, split-brain research led to the development of several methodological advances that derived from questions specifically occurring in split-brain patients. One such question lies in accurately assessing the surgical result of the sectioning, that is, the actual extent of the corpus callosum sectioning. This led to the development of a specific neuroimaging approach that allows one to assess the extent of callosal disconnection in split-brain patients ( Gazzaniga et al. , 1985 ; Corballis et al. , 2001 ) and callosal lesions due to all kinds of pathologies ( Fig. 6 ).

Imaging the corpus callosum. The necessity to determine the extent of the callosotomy in split-brain patients motivated the advancement of neuroimaging methodology to investigate if the corpus callosum was entirely resected or if residual fibres allow information transfer between hemispheres. The first assessment of a split-brain patient via MRI in 1985 suggested two remaining interhemispheric connections in the anterior and posterior end of the corpus callosum (bright spots in white boxes). Reassessment of the same patient with advanced imaging technology (higher spatial resolution and 3D acquisition) in 2001 confirmed the remaining anterior connection, while showing that the posterior fibres were clearly severed. Modern imaging techniques allow reconstruction of callosal fibres from diffusion imaging data [diffusion spectrum imaging (DSI)] and hence a more direct assessment of corpus callosum integrity. Modified from Corballis et al. (2001) , with permission.

Besides the various insights on aspects of functional specialization of the hemispheres or the functional anatomy of the corpus callosum that were obtained from split-brain work, these extraordinary cases of separated hemispheres raise an even more general question: how much integration of information between specialized brain modules is necessary to give rise to our skilled behaviour and to create our unique experience of the world around us? It seems puzzling that the verbal IQ and problem solving capacities of split-brain patients are typically unaffected by the surgery. Moreover, patients do not report any difference in the nature of their personal experience—despite the fact that their hemispheres are separated, they report that they experience a single consciousness ( cf. Gazzaniga, 2000 ). Not surprisingly, theoretical frameworks of consciousness often include the split-brain as a test-case for their respective theory. Yet claims of support are made regardless of whether conscious experience is interpreted to result from the integration of regional resources, as in the Global Workspace Theory ( cf. Baars, 1997 ) or the Information Integration Theory ( cf. Tononi and Koch, 2015 ) or, in contrast, is hypothesized to stem from focal activity, as suggested by the local recurrent processing theory of consciousness for example ( cf. Lamme, 2006 ).

A set of observations from split-brain experiments may be particularly suitable to inform such theoretical frameworks of consciousness. In several domains of problem-solving, the left hemisphere shows fundamentally different strategic tendencies compared to the right hemisphere. For example, the right hemisphere adheres to factual knowledge when asked to identify previously presented stimuli and thus outperforms the left hemisphere, which falsely recognizes similar yet unseen objects ( Phelps and Gazzaniga, 1992 ). This observation is in line with the notion that the left hemisphere ‘gets the gist’ and tends to integrate information into theories, which can help to predict future events and offer a coherent interpretative framework. Interpretive qualities unique to the left hemisphere were also observed in a probability-guessing paradigm ( Wolford et al. , 2000 ) where it tries to find patterns, i.e. a ‘theory’ in random events. The left hemisphere is not shy to interpret the behaviour of or physiological responses evoked by emotional stimuli presented to the right hemisphere, even when it is bound to fail to come up with a veridical story due to the lack of critical information exclusively present in the right hemisphere. Why would the left hemisphere interpreter bother to do so? By constantly offering explanations for what it perceives, the left hemisphere interpreter may generate a feeling in all of us that we are integrated and unified ( Gazzaniga, 2000 ). Hence, the interpretive function that strings events together to form our seemingly coherent autobiographies is hosted by the left hemisphere.

Of course, the distinct interpretive capacities of both hemispheres are but a small piece in the puzzle of deciphering the neurobiological foundations that give rise to our conscious experience of the world. These findings also intriguingly illustrate the vast scope of impactful insights that can be gained from the persistent study of a unique group of neurological patients.

L.J.V. and M.S.G. thankfully acknowledge funding by the SAGE Center for the Study of the Mind, University of California.

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• Published: 01 August 2005

Forty-five years of split-brain research and still going strong

• Michael S. Gazzaniga 1  

Nature Reviews Neuroscience volume  6 ,  pages 653–659 ( 2005 ) Cite this article

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Forty-five years ago, Roger Sperry, Joseph Bogen and I embarked on what are now known as the modern split-brain studies. These experiments opened up new frontiers in brain research and gave rise to much of what we know about hemispheric specialization and integration. The latest developments in split-brain research build on the groundwork laid by those early studies. Split-brain methodology, on its own and in conjunction with neuroimaging, has yielded insights into the remarkable regional specificity of the corpus callosum as well as into the integrative role of the callosum in the perception of causality and in our perception of an integrated sense of self.

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Acknowledgements

This research was supported by National Institutes of Health grants to the author. It was also supported by a graduate reseach fellowship from the National Science Foundation to M. Colvin. I would like to thank my collaborators, M. Colvin, M. Funnell, M. Roser and D. Turk, for their scientific input as well as their assistance in reviewing this paper. I would also like to thank R. Townsend for her editorial assistance.

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Gazzaniga, M. Forty-five years of split-brain research and still going strong. Nat Rev Neurosci 6 , 653–659 (2005). https://doi.org/10.1038/nrn1723

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July 1, 1998

The Split Brain Revisited

Groundbreaking work that began more than a quarter of a century ago has led to ongoing insights about brain organization and consciousness

By Michael S. Gazzaniga

Interaction in isolation: 50 years of insights from split-brain research

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• PMID: 29177496
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Unified tactile detection and localisation in split-brain patients

Edward h.f. de haan.

a Department of Psychology, University of Amsterdam, Amsterdam, the Netherlands

b Amsterdam Brain & Cognition (ABC) Center, University of Amsterdam, the Netherlands

c Department of Experimental and Clinical Medicine, Marche Politechnical University, Ancona, Italy

H. Chris Dijkerman

d Department of Psychology, Utrecht University, the Netherlands

Nicoletta Foschi

e Epilepsy Center-Neurological Clinic, Azienda ‘Ospedali Riuniti’, Ancona, Italy

Simona Lattanzi

In ‘split-brain’ patients, the corpus callosum has been surgically severed to alleviate medically intractable, severe epilepsy. The classic claim is that after removal of the corpus callosum an object presented in the right visual field will be identified correctly verbally and with the right hand but not with the left hand. When the object is presented in the left visual field the patient verbally states that he saw nothing but nevertheless identifies it accurately with the left hand. This interaction suggests that perception, recognition and responding are separated in the two isolated hemispheres. However, there is now accumulating evidence that this interaction is not absolute. Recently, we (Pinto et al., 2017) showed that accurate detection and location of stimuli anywhere in the visual field could be performed with both hands. In this study, we explored detection and localisation of tactile stimulation on the body. In line with our previous results, we observed that split-brain patients can signal detection and localisation with either hand anywhere on the body (be it the arm or the leg) but they remain unable to match positions touched on both arms or legs simultaneously. These results add to the evidence suggesting that the effects of removal of the corpus callosum may be less severe than sometimes claimed.

1. Introduction

The corpus callosum is the main route for communication between the two cerebral hemispheres (e.g., Gazzaniga, 2000 , Innocenti, 1986 , Wahl et al., 2007 ). In ‘split-brain’ patients, the corpus callosum has been surgically resected to alleviate medically intractable, severe epilepsy. One of the Nobel Prize-winning discoveries in neuroscience is that lesioning the corpus callosum leads to a curious phenomenon. When an object is presented in the right visual field, the patient responds correctly verbally and with his/her right hand. However, when an object is presented in the left visual field the patient verbally states that he/she saw nothing but nevertheless identifies the object accurately with the left hand only Gazzaniga, 1967 , Gazzaniga et al., 1962 , Sperry, 1984 , Sperry, 1968 , Wolman, 2012 . This is concordant with the human anatomy; the right hemisphere receives visual input from the left visual field and controls the left hand, and vice versa ( Cowey, 1979 , Penfield and Boldrey, 1937 , Sakata and Taira, 1994 ). Moreover, the left hemisphere is generally the site of language processing ( Ojemann et al., 1989 , Vigneau et al., 2006 ). Thus, it appears that severing the corpus callosum causes each hemisphere to gain its own conscious agent ( Sperry, 1984 ). The left hemisphere is only aware of the right visual half-field and expresses this through its control of the right hand and verbal capacities, while the right hemisphere is only aware of the left visual field, which it expresses through its control of the left hand. This clinical observation features in many textbooks ( Gazzaniga, 1998 , Gray, 2002 ) and has influenced theoretical thinking about consciousness. Congruent with the idea that split-brain patients have two separate conscious agents, both the Global Workspace theory ( Baars, 1988 , Baars, 2005 , Dehaene and Naccache, 2001 ) and the Information Integration theory ( Tononi, 2004 , Tononi, 2005 , Tononi and Koch, 2015 ) imply that without massive interhemispheric communication two independent conscious systems appear.

On closer examination, the response x visual field interaction appears less than absolute. First, Sperry (1968) himself already observed that there are clear exceptions. Second, there are a number of studies that failed to observe this interaction and found that responding was well-above chance with both hands (e.g., Corballis, 1995 , Egly et al., 1994 , Levy et al., 1972 ). More recently, we ( Pinto et al., 2017 ) performed a quantitative study into this interaction, using sophisticated fixation control with an eye-tracker, a substantial number of trials in each condition, forced-choice responding, and a large number of different stimuli. The response type (left hand, right hand or verbally) was varied systematically. We found, in two split-brain patients, that although visual field played a large role in most tasks, a response type x visual field interaction was never observed. This result held across all tasks (detection, localization, orientation determination, labelling and visual matching), and all tested types of stimuli (isoluminant dots, simple shapes, oriented rectangles, objects). Pinto, de Haan, and Lamme (2017) and Corballis, Corballis, Berlucchi, and Marzi (2018) suggested that these effects are probably the result of intact subcortical routes. Savazzi et al. (2007) , for instance, showed that the superior colliculus is likely to play a role in visual interhemispheric transfer. However, others, such as Volz and Gazzaniga (2017) have suggested that these effects might be caused by confounds as ipsilateral arm control and/or cross-cueing.

Most of the studies on (the lack of) interhemispheric transfer of information have been carried out in the visual domain but the somatosensory system is also separated with the perception of the right half of body being carried out by the left hemisphere and vice versa (e.g., Penfield and Boldrey (1937) . Zaidel (1998) was one of the first to look at tactile perception. He investigated six patients with a complete commissurotomy using the Benton test of stereognosis looking separately at the left and the right hand. He observed deficits in stereognosis without primary somatosensory impairment in both disconnected hemispheres. Object naming was worse with left hand than with the right hand but both were above chance. Interestingly, there was surprisingly good performance in a cross-hemisphere condition where one hand explored the stimulus and the multiple-choice card was explored in the opposite visual field. Fabri, Polonara, Quattrini, and Salvolini (2002) used fMRI to investigate brain activations in response to touch and painful stimulation in three split brain patients. They observed contralateral activation in SI and the parietal operculum during unilateral tactile stimulation of the hand. In contrast to the healthy subjects ( Polonara, Fabri, Manzoni, & Salvolini, 1999 ), the patients showed no ipsilateral cortical activation ( Fabri et al., 1999 ). With painful stimuli both controls and the split-brain patients showed contra- and ipsilateral activation in the parietal operculum and in the insular cortex in one case and in the posterior parietal cortex in one other patient. In a follow-up study, again with the three split-brain patients, Fabri et al. (2005) investigated inter-manual tactile recognition performance. Tactile finger localization was flawless with the same hand but deteriorated to around 80% correct when the patients had to respond with the other hand. Split-brain patients were impaired compared to healthy controls but still good at verbally identifying objects in the right hand (93%) and even more impaired but still above chance in the left hand (30%). Inter-manual object comparisons with either two the same or two different objects in each hand was difficult (68% correct). Thus, also in the somatosensory domain, there is enough data to doubt the classic description of the split-brain. This study is aimed a fine-grained assessment of basic tactile perception in a split-brain patient. We adopted the same basic approach as in Pinto, de Haan, et al. (2017) and Pinto, Neville, et al. (2017) to look at simple detection, localisation and cross-hemisphere matching. Our working hypothesis was that we would replicate our observations of extensive interhemispheric transfer for detection and localisation but an absence of cross-hemispheric matching with tactile stimulation. Such a correspondence in interhemispheric transfer of both visual and tactile stimulation would further delineate the circumstances in which the two hemispheres continue to “communicate” in split-brain patients.

1.1. Case description

Patient DDC also participated in the Pinto, de Haan, et al. (2017) and Pinto, Neville, et al. (2017) studies. During surgery, his corpus callosum was completely removed and most of the anterior commissure. Note that other than the removal of the corpus callosum, DDC has no brain damage, and he falls within the normal IQ range. See Pizzini et al. (2010) and Corballis et al. (2010) for detailed descriptions of this patient.

2. Experiment 1: Detection threshold

The first experiment was designed to measure DDC's tactile detection thresholds on the dorsum of his hands while he responded either with the stimulated or the other hand. The objective was to find out whether or not each of his two hemispheres only perceive half of his body. In essence, this experiment is the tactile equivalent of the visual detection studies of Pinto, de Haan, et al. (2017) and Pinto, Neville, et al. (2017) .

2.1. Method

Thresholds were determined with von Frey hairs (VFA; Touch-Test™ sensory evaluators, North coast medical Inc.) using a descending staircase procedure [ Anema, van Zandvoort, de Haan, Kappelle, de Kort, Jansen & Dijkerman, 2009 ) starting with the thickest hair T (VFA 6.65 (= 300 g)]. In half of the trials, the hairs touched his skin while in the other half the experimenter (EdH) made the same hand movement but stopped short of touching the skin. The hand that was stimulated was positioned under a cardboard cover in order to obscure it from the patient's vision. In addition, he was asked to close his eyes during the whole experiment and to concentrate on his hands. There were four trials per hair, and we moved on to the thinner hair after 3 or more correct responses. The following hairs were used respectively: R [VFA 6.10 (= 100 g)], P [VFA 5.46 (= 26 g)], N [VFA 5.07 (= 10 g)], L [VFA 4.74 (= 6 g)], J (VFA 4.31 (= 2 g)], I [VFA 4.17 (= 1.4 g)], H [VFA 4.08 (= 1 g)], G (VFA 3.84 [= .6 g)], F [VFA 3.61 (= .4 g)] and E [VFA 3.22 (= .16 g)]. Testing proceeded until he made 2 or more errors and we took the previous hair as the threshold. Stimuli were applied to the back of the hand and each trial started with the experimenter counting to three in Italian. DDC indicated detection of being touched with a thumbs up gesture while an absence of touch was signalled with the thumb down. There were four separate blocks in which the stimulated hand and the hand with which he responded were systematically varied. A second experimenter (YP), who could not see whether the hand had been touched, registered the responses.

2.2. Results

DDC's accuracy thresholds in von Frey hair thickness are summarised in Table 1 . Overall, his performance (grand mean = 3.95) was slightly less sensitive than healthy subjects. Compared to 12 healthy controls [taken from Anema, van Zandvoort, de Haan, Kappelle, de Kort, Jansen and Dijkerman, 2009 : mean = 2.44 (= .02 g); cut-off = 3.22 (= .16 g)] his performance is just outside the normal range. In addition, he appears slightly more sensitive in the crossed conditions, i.e. when he was asked to respond with the other hand than the one that was stimulated but differences were minimal. We performed statistics on the results in the following way. Per condition (of hair thickness) hits and correct rejections were coded as 1 and misses and false alarms as 0. If one condition was not tested then we assigned an equal amount of 1's and 0's to that condition, i.e. chance performance. We did so because conditions were only omitted because it was beyond the threshold of the participant. Permutation testing revealed that performance was similar irrespective of which hand was touched ( p  = .4) and irrespective of with which hand the participant responded ( p  = .21). However, there was a significant interaction as the participant performed somewhat better in the crossed conditions (responding with the other hand than the stimulated hand) than in the uncrossed conditions (stimulated and responding hand are the same), p  = .011.

DDC's tactile detection thresholds (accuracy) in von Frey hair thickness.

2.3. Discussion

DDC shows slightly increased detection thresholds for tactile stimulation on either hand but, if anything, his performance is somewhat better in the crossed than the uncrossed conditions. We suggest that one reacts faster in the other hand condition because in the same hand condition the patient has to wait until the trial is completed and the experimenter has removed his hand. Unfortunately, we did not record reaction times, so we cannot check this suggestion in a quantitative manner. Whatever the explanation of this interaction, it clearly invalidates the claim that sensory information of touch can only be used by one hemisphere for manual output. Therefore, the classic interaction between side-of-stimulation x response-hand (where performance should be much better in the uncrossed conditions) is not observed. This finding suggests that response selection and action control remains unified in this split-brain patient.

3. Experiment 2: Tactile localisation

Having shown that the detection of tactile stimuli is not split in DDC, the next question we investigated was whether the localisation of tactile stimuli might also be unified across the two hemispheres. We carried out two separate, comparable tasks on the inner side of his arms and on the frontal side of his legs.

3.1. Method

DDC was asked to roll up the sleeves of his shirt up to above his elbow or the legs of his trousers. The to be stimulated arm or leg was positioned under a cardboard cover in order to obscure it from sight. Tactile stimulation was applied to the skin with the rubber tip of a pencil and was well above threshold. A response sheet (see Fig. 1 ) with the four stimulation sites on the arm (1a) or the leg (1b) was placed on top of the cardboard cover. The four stimulation sites were separated equidistantly on the underarm and the upper leg. Each of the four positions was stimulated seven times in a pseudo-random fashion (total number of trials is 28). Each trial started with the experimenter counting to three in Italian, and DDC indicated where he thought he had been touched by pointing to one of the four positions on the response sheet. There were four separate blocks in which the stimulated hand and the hand with which he responded were systematically varied. A second experimenter (YP), who could not see where his hand had been touched, registered the responses. His errors were calculated as the average distance from the correct position in terms of positions (maximum is 3).

The response sheets on which DDC had to indicate where he thought he had been touched on the arm (1a) and the leg (1b).

3.2. Results

For each trial, the distance between the correct and the indicated position was calculated on an interval scale (correct = 0, an adjacent position = 1, etc.). Subsequently, these distances were averaged per condition. The results are summarised in Table 2a , Table 2b . We performed permutation tests to determine statistics. His performance is well above chance-level in all four conditions (arms: all ps < .001, legs: all ps < .001). An important observation is that, again, the classic interaction between side-of-stimulation x response-hand is not observed (arms: p  = .77, legs: p  = .1). Moreover, there was no effect of with which hand the participant responded (arms: p  = .77, legs: p  = .33). When the legs were stimulated, accuracy did not depend on which leg was stimulated ( p  = .51). Also, there was no indication of a relatively better or worse performance in relation to the proximal or distal part of the underarm ( p  = .78). Average distance error per position 1: .31, position 2: .43, position 3: .43, and position 4: .29. However, there was an effect of which arm was stimulated ( p  = .0016), with better localization of stimuli on the left arm (average distance .19) than on the right arm (average distance .54).

Average localisation error in terms of position on his arm.

Average localisation error in terms of position on his leg.

3.3. Discussion

The results are clear cut. He performs well above chance level in all four conditions, and more importantly, for each hand his performance is almost identical whether he used his ipsi- or contralateral hand for responding. This suggests that apart from detection, tactile localisation is also unified in DDC. An interesting observation is that his localisation is relatively better on the left arm. This finding is reminiscent of our findings in DDC showing a relatively better localisation performance in his left compared to his right visual hemifield ( Pinto et al., 2017 ). Perhaps, this reflects a generalised (visual and tactile) right hemisphere advantage for spatial processing, or alternatively a noisier processing in the left hemisphere due to the epilepsy.

4. Experiment 3: Cross arms localisation: same / different

The observation that both detection and localisation of tactile stimuli are unified across the two hemispheres in the split-brain patient DDC raises the question whether the removal of his corpus callosum has had no effect on his somatosensory processing. It could be that both hemispheres have access to the sensory information from the whole body (perceptual unity) or that only response selection and action control (action unity) remain unified in this split-brain patient. Here, in the third experiment, we investigate whether he is able to compare where he has been touched simultaneously on both his arms.

4.1. Method

As in the previous experiment, DDC was asked to roll up the sleeves of his shirt up to above his elbow. Both arms were positioned under a cardboard cover in order to obscure it from sight (see Fig. 2 ). Simultaneous tactile stimulation was applied to the skin with the rubber tip of two pencils and was well above threshold. The distance between the four stimulation sites on each arm was equidistant. Each trial started with the experimenter counting to three in Italian, and then stimulated both arms at the same time. In half of the trials (36), the same positions were stimulated on both arms, and the twelve possible (all different permutations) “different” trials appeared three times. Thus, the total number of trials was 72. DDC reported verbally whether he thought he had been stimulated in a symmetrical fashion (“same”) or in two different positions (“different”) on both arms. A second experimenter (YP), who could not see where his arms had been touched, registered the responses.

Graphic representation of the stimulation sites on his two arms.

4.2. Results

DDC showed no sign of extinction and he indicated that he always felt the double stimulation. He scored below but not significantly different from chance ( p  = .19). The total number correct was 30/72. He is clearly not able to perform this task. Despite his poor performance, he maintained during the test session that he was quite confident about his responses.

4.3. Discussion

The absence of a corpus callosum has left DDC unable to compare simultaneous, tactile stimulation across the two arms. This impairment appears to be complete as he performs at chance level. Again, this finding is reminiscent of his inability to compare visual stimulation across fixation. Interestingly, he seems largely oblivious to this inability.

5. General discussion

This study was designed to investigate the classic observation of a stimulation-side x response-hand interaction in split brain patients with tactile instead of visual stimulation. The wiring of the somatosensory system is similarly crossed, with the perception of touch on the left half of the body being processed by the right hemisphere and vice versa. There is now substantial evidence from the visual domain that this interaction is not always observed (e.g., Corballis, 1995 , Pinto et al., 2017 , Savazzi et al., 2007 ). Notably, split brain patients appear able to signal detection and localisation of visual stimuli with both hands equally well. Here the main question was, thus, whether or not detection and localisation of touch on one half of the body can only be signalled by the ipsilateral hand.

Previous research with somatosensory stimulation had, at least, suggested that the processing of touch is not completely separated either (e.g., Fabri et al., 2005 , Zaidel, 1998 ). Our current findings corroborate this suggestion. In fact, there was no hand difference for detecting and localizing touch. Both hands can be used to signal detection and localization of touch anywhere on the body. Note that our findings are in line with several other findings that suggest that the processing of somatosensory information (of which touch is one aspect) is less than completely segregated in a split-brain patient. In other words, although our findings contradict some claims, they are certainly not extraordinary or revolutionary. Fabri et al. (2002) used fMRI to demonstrate contra- and ipsilateral activation in response to painful stimuli in healthy controls and split-brain patients, and Lepore, Lassonde, Veillette, and Guillemot (1997) showed that detection thresholds for temperature discrimination were similar for within- and between-side comparisons in split brain patients and comparable to the discrimination performance of healthy subjects. Our finding is also in line with a recent study by Dosso, Chua, Weeks, Turk, and Kingstone (2018) who looked at the interaction between proprioceptive perception of the left and the right hand positioned either in the left or the right visual half-field in two split brain patients. They concluded that each hemisphere can accurately represent the full visuomotor space, and suggested that this whole field perception is sub-served by subcortical connections between the hemispheres.

Some (e.g., Volz & Gazzaniga, 2017 ) have suggested that these observations do not represent the true split-brain state-of-affairs as the absence of an interaction could be due to confounding factors, such as “cross-cueing” or “ipsilateral hand control”. Cross-cueing is, in their view, something that the patients have developed over years of practice learning to cope with a split-brain. As localising the position where one has been touched is not an everyday requirement, we feel that this is not a likely explanation for touch localisation. Ipsilateral hand control is still controversial as far as it concerns the ability of one hemisphere to move the ipsilateral hand in a coherent fashion while the other hemisphere (that is dominant for that arm) has no intention to move that hand. For instance, observations during the Wada test ( Wada, 1960 ), where one hemisphere is temporarily anaesthetised in order to establish language dominance in the context of functional surgery, has systematically shown that the contralateral hand is paralysed after the drug takes effect. In addition, the pointing response that is required in Experiment 2 (taking the hand out of the stimulation box and then to move the index finger to the correct position on the drawing on top of the box) is too elaborate given the proximal ipsilateral innervation of the arm. Therefore, we suggest that these possible confounding factors cannot explain our current results. These results are in line with Polonara, Mascioli, Salvolini, Fabri, and Manzoni (2009) who showed that proximal body regions of each side (face, trunk, proximal limbs) and hand are represented in both hemispheres, and also argue against the “cross-cueing” or “ipsilateral hand control” hypothesis. Yet, although it may be difficult to explain our results with a simple cross-cueing account, more complex versions cannot be ruled out. Therefore, although our results advance this debate, they do not conclusively decide it.

Analogous to our observations in the visual domain ( Pinto et al., 2017 ), we found that DDC was unable to compare touched locations across the midline, performing this task at chance-level. This finding is in line with a study by Lassonde, Sauerwein, Chicoine, and Geoffroy (1991) who showed that after surgery the performance of three adult split-brain patients deteriorated to chance-level on a task where they had to indicate on which finger they were touched by touching the corresponding finger on their other hand with the thumb. Intra-manual matching, where the finger had to be touched with thumb of the touched hand, remained perfect. Clearly, the absence of the corpus callosum prevents detailed sensory information being transferred between the two hemispheres. Therefore, the ‘split brain paradox’, i.e. the demonstration that each hemisphere is able to signal the position of stimulation anywhere (in the visual field and on the body) while they are unable to compare these positions across the midline, has been firmly established in two different sensory domains. Based on the subjective report from the patients, who feel “normal” and unaltered after surgery (e.g., Bogen, 1965 ), it seems possible that they are able to respond consciously to stimulation anywhere in the visual world or their body, and that this information is provided via subcortical routes (e.g., Savazzi et al., 2007 , Pinto et al., 2017 , Corballis et al., 2018 ). This unified consciousness of vision and somatosensation does, however, not support the matching of information across the midline. Possible explanations are (1) that the information transfer via the subcortical connections is degraded (compared to callosal transfer), (2) that it is only at the response selection phase that unity is achieved, or (3) that this unified consciousness has access to but cannot integrate the information from both hemispheres in real-time. Future studies should be geared towards distinguishing between these options.

We argue that our current findings are not revolutionary or radically different from what has been previously claimed. For instance, Sperry, Gazzaniga, and Bogen (1969 , pages 279–280) have noted “Onset and presence or absence of tactile stimulation of the left hand can be reported verbally as can also a distinction between stimuli applied to the wrist or palm, thumb or palm, and thumb or little finger”. The importance of the current results is that they unequivocally, and quantitatively, show that tactile perception of presence and location of stimuli is unified in split-brain patients. Moreover, this information cannot be used for comparisons across the side of the body. Thus, although the patient knows, for both arms, which location is stimulated, he cannot indicate whether the same location was stimulated on both arms. This puzzling finding - if both locations are known to the patient, why can he not compare them? - neatly fits the model of the split-brain we recently put forward ( Pinto, de Haan, et al., 2017 ). In this model all perceptual information (from both fields, and the entire body) is available to one conscious agent, yet the information is not automatically integrated. That is, the subject experiences two independent streams of information, thereby hampering comparisons across these streams. Note that although previous studies have provided partial or qualitative support for the claims of our model, no study so far has collected the quantitative data needed to check our model. In the current study we investigated the “unified consciousness” part of the model, i.e. ability to report on presence and location of tactile stimuli across the entire body irrespective of response type (left hand or right hand). Moreover, we checked the “split perception” part, i.e. inability of the patient to compare the location of tactile stimuli across arms. Thus, the current study is the first to quantitatively verify crucial predictions of our model of the split-brain syndrome within one investigation.

In summary, in this study we carried out the tactile equivalence of the Pinto, de Haan, et al. (2017) and Pinto, Neville, et al. (2017) visual tests for the detection, location, and matching across the midline. In line with our previous results, we observed that split-brain patients can signal detection and localisation with either hand anywhere on the body (be it the arm or the leg) but they remain unable to match positions touched on both arms or legs simultaneously. Our study further clarifies the remaining unity of tactile perception in split-brain patients, and is in line with several previous studies into this domain. Further studies are needed to explore the extent of conscious unity in split-brain patients, and whether this unity extents to other processes in perception, memory and cognition.

Acknowledgements

This work was supported by an European Research Council Advanced grant FAB4V (#339374) to Edward de Haan. The Authors are grateful to Gabriella Venanzi for scheduling patient's exam.

Reviewed 18 September 2019

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Biopsychology: Evaluating Split-Brain Research

Last updated 10 Apr 2017

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Here are some key evaluation points on split-brain research.

It is assumed that the main advantage of brain lateralisation is that it increases neural processing capacity (the ability to perform multiple tasks simultaneously). Rogers et al. (2004) found that in a domestic chicken, brain lateralisation is associated with an enhanced ability to perform two tasks simultaneously (finding food and being vigilant for predators). Using only one hemisphere to engage in a task leaves the other hemisphere free to engage in other functions. This provides evidence for the advantages of brain lateralisation and demonstrates how it can enhance brain efficiency in cognitive tasks.

However, because this research was carried out on animals, it is impossible to conclude the same of humans. Unfortunately, much of the research into lateralisation is flawed because the split-brain procedure is rarely carried out now, meaning patients are difficult to come by. Such studies often include very few participants, and often the research takes an idiographic approach. Therefore, any conclusions drawn are representative only of those individuals who had a confounding physical disorder that made the procedure necessary. This is problematic as such results cannot be generalised to the wider population.

Furthermore, research has suggested that lateralisation changes with age.  Szaflarki et al. (2006)  found that language became more lateralised to the left hemisphere with increasing age in children and adolescents, but after the age of 25, lateralisation decreased with each decade of life. This raises questions about lateralisation, such as whether everyone has one hemisphere that is dominant over the other and whether this dominance changes with age.

Finally, it could be argued that language may not be restricted to the left hemisphere.  Turk et al. (2002)  discovered a patient who suffered damage to the left hemisphere but developed the capacity to speak in the right hemisphere, eventually leading to the ability to speak about the information presented to either side of the brain. This suggests that perhaps lateralisation is not fixed and that the brain can adapt following damage to certain areas.

• Biopsychology
• Split Brain Research

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Split Brain Research

Hemispheric lateralisation: This is the ideas that the brain’s two hemispheres are responsible for different functions; that particular functions (such as language) are the responsibility of one hemisphere but not the other- the function is lateralised.

Split-brain research: This involves individuals who have had surgical separation of their brain hemispheres, in order to relieve symptoms of epilepsy. Research can reveal to what extent other brain functions are lateralised. Sperry (1968) used a procedure where an image or word was projected to the patient’s right visual field (which would be processed by the left hemisphere) and another image was projected to the left visual field (processed by the right hemisphere). In a split-brain patient, information cannot be transmitted from one hemisphere to another, so the effects of this can be studied.

Findings from split-brain research: When a picture was shown to the right visual field, the patient could describe it easily. When the image was shown to the left visual field, the patient found it difficult to describe it, or couldn’t see anything there. This is likely due to the lack of language processing ability in the right hemisphere, which processes the left visual field. The left hemisphere was unable to receive the information due to the separation of the hemispheres.

When patients were presented with a word or object in the left visual field, then had to select that object (or a related one) from a bag using their left hand without looking, they were able to do this quite well. This is despite the fact that they couldn’t describe what the object was. They could still seemingly understand what the object was, showing that the right hemisphere is involved in understanding of objects.

When patients were presented with two words simultaneously (for example, ‘door’ ‘frame’), one in each visual field, they wrote the word in the left visual field (‘door’) with their left hand, and said the word in the right visual field (‘frame’). The left hand was much better at drawing images than the right hand, despite the fact that all of the patients tested were right-handed. This shows that the right hemisphere seems to be dominant for drawing skills.

When asked to match a face from a number of other faces, the picture presented to the left visual field (processed by the right hemisphere) was consistently selected, whilst the image presented to the right visual field (left hemisphere) was ignored. This shows that the right hemisphere seems dominant in face recognition. Images presented as composites of two faces led to the left hemisphere dominating when the patient was asked to verbally describe the face (the right-hand ‘half’ of the face was described, and the left-hand half ignored), and the right hemisphere dominated when the patient had to select a matching face from a number of others.

Evaluation:

• Sperry’s research supports the conclusion that the left hemisphere is more responsible for verbal and analytical tasks, whereas the right hemisphere is better at spatial and musical tasks. This has strengthened the understanding of how the brain works.
• Sperry’s procedure was closely controlled. Patients were given eye patches, and images were flashed up for a very brief time (fractions of a second), meaning there was no possibility of looking over and using the other visual field. This strengthens the internal validity of the studies.
• The sample used by Sperry was quite small (only 11 took part in all procedures), and their brains may have been affected by epileptic seizures. Therefore, it is hard to generalise the findings from the studies to the general population.

Scanning Techniques

• Functional magnetic resonance imaging (fMRI): this measures changes in blood flow and oxygenation in the brain whilst the individual is performing a task. Increased blood flow suggests that the area of the brain is working harder, so it can be determined which areas of the brain are involved with particular tasks.
• Electroencephalogram (EEG): measures the electrical activity of neurons in the brain, by recording the electrical impulses that take place during synaptic transmission. The individual wears a skull cap to do this. This can detect particular patterns of activity in the brain, for example what is happening during sleep.
• Event-related potentials (ERPs): activity from an EEG recording is analysed, in order to determine the specific responses relating to a particular task. Event-related potentials are therefore the results of this- types of brainwave which are triggered by particular events.
• Post-mortem examinations: the brain is studied and analysed following death, by looking at particular areas. This is often done for individuals who have had rare disorders or dysfunction in behaviour or cognitions. Their brain will be compared to a ‘control’ brain to see if there are differences in structure.
• fMRI is low-risk, involving no radiation, and produces very detailed images. However, it is expensive, and there is a time-lag- the image taken is 5 seconds behind the initial firing of neurons (therefore, this is poor temporal resolution ). Also, the activity of individual neurons cannot be seen.
• EEGs have been very useful in diagnosing conditions such as epilepsy, and the processes involved in activities such as sleep. Brain activity can be measured almost instantaneously (a single millisecond), unlike in fMRI. However, the information gained is very generalised, so the technique can’t be used to isolate exact neural activity.
• ERPs draw on EEGs to measure more specific activity in the brain. They have excellent temporal resolution and are widely used in identifying specific behaviours and functions. Many different ERPs have been successfully identified. However, there is a lack of standardisation in the methodology used, meaning the findings are in question, and eliminating all extraneous variables in order to isolate an ERP can be difficult.
• Post-mortems have greatly enhanced medical knowledge, for example Broca and Wernicke both made use of them before neuroimaging techniques were possible. However, it is hard to establish a cause-effect link when conducting post-mortem studies. Changes in brain structure may not be related to the disorder the patient had, but due to another issue.

Circadian Rhythms

A biological rhythm is a change in the boy’s processes, in response to environmental changes. These rhythms are influenced by external and internal factors. The internal factors are the body’s internal processes- the ‘body clock’, known as endogenous pacemakers . External factors are changes in the environment, known as exogenous zeitgebers . Rhythms that last around 24 hours are known as circadian rhythms .

Sleep/wake cycle: This is an example of a circadian rhythm. The exogenous zeitgeber in this case is the daylight (or lack of it), which contributes to feelings of drowsiness or being awake. Researcher Michael Siffre studied the effect of a complete lack of daylight on his own sleep/wake cycle, by living in a cave for several months at a time. After two months in one cave, he emerged believing the date to be mid-August, but it was actually mid-September. In each experiment, his body created a natural rhythm of just beyond the usual 24 hours, and he continued to sleep and wake on a regular cycle.

Aschoff and Wever (1976) found that participants who spent 4 weeks in a bunker without natural light showed circadian rhythms of 24-25 hours, except one participant who went up to 29 hours. This suggests the natural circadian rhythm is slightly shortened by the effects of daylight. Folkard et al (1985) found that when participants were deprived of sunlight for 3 weeks, and the length of day was manipulated by the researchers to 22 hours rather than 24 (by covertly adjusting the time on the clocks), only one participant easily adjusted to the shortened day. This suggests the strength of the body’s sleep/wake cycle, as it resisted environmental changes.

• Research into circadian rhythms has useful practical applications, for example how to manage the shift patterns of night workers so that they are more productive and make fewer mistakes. This increases the usefulness of the studies.
• Research has also shown when the effects of drugs on the body are at their most and least effective. Circadian rhythms seem to have an impact on how drugs affect the body, so guidelines can be developed for patients as to when they should take drugs for maximum impact. This is another useful practical application.
• Research in this area often uses small sample sizes (only 1, in the case of Siffre), so generalisation may be difficult. Also, participants had access to artificial light, which could have acted as a confounding variable- for example, turning off a light to go to sleep may have similar effects as the end of natural light at the end of a day. The internal validity of the research is therefore in question.

Infradian & Ultradian Rhythms

Infradian rhythms.

These take place over a longer time than 24 hours. For example, the menstrual cycle . This takes place over around 28 days, although this varies between women. Hormone levels rise during the cycle, which causes the release of an egg (ovulation), then the release of the hormone progesterone which thickens the womb lining to ready the body for pregnancy. If no pregnancy occurs, the womb lining comes away, resulting in menstruation.

There is evidence that the menstrual cycle can be affected by exogenous factors. McClintock et al (1998) found that collecting pheromones (chemicals released into the air and absorbed by others, affecting their behaviour) from women with irregular cycles and rubbing them on the lips of other women caused the ‘receiver’ of the pheromones to experience changes in their cycle, bringing them closer to the pheromone ‘donor’.

Seasonal affective disorder (SAD) is another infradian rhythm, characterised by changes to mood. Sufferers feel a lowered mood, and lowered activity levels, during the winter months when daylight is shorter. As their mood changes in a predictable way through the year, this is an example of a circannual rhythm. This is thought to be caused by the hormone melatonin , which has an effect on serotonin, a neurotransmitter linked with depression. More melatonin is released during the winter, as it is released when there is a lack of daylight.

Ultradian Rhythms

These take place more than once within a 24-hour period. An example is the different stages of sleep, of which 5 distinct stages have been identified through research involving an EEG.

• Stages 1 and 2: a light sleep, where a person can be easily woken. Brain wave patterns start to slow down, becoming more ‘rhythmic’ (alpha waves) at this time.
• Stages 3 and 4: delta waves take over, which are even slower than alpha waves. This is a deep sleep; from which it is hard to wave the person- sometimes known as ’slow wave sleep’.
• Stage 5: the pons (part of the brain) paralyses the body to stop the person from ‘acting out’ their dreams. The eyelids move in a fast, jerky fashion at this point, which is correlated with dreaming. Therefore, this stage is known as REM sleep (rapid eye movement).
• Research into the menstrual cycle is likely to be affected by many variables, such as diet, stress, amount of exercise, and so on. This means that the findings of studies such as McClintock may not be valid. Other studies have found no evidence of menstrual synchrony (women’s cycles moving closer to each other’s).
• Dement and Kleitman (1957) found that REM activity was strongly correlated with dreaming. Participants woken during REM sleep were able to describe dreams in vivid detail. This supports the effect of biological rhythms on the body and brain.
• An effective treatment for SAD has been developed as a result of research in this area. Sufferers are given a light box to simulate the effects of sunlight in the dark mornings and evenings of winter, which has led to a relief in symptoms for around 60% of sufferers. This increases the practical usefulness of research into infradian rhythms.

Endogenous Pacemakers

The suprachiasmatic nucleus (SCN): This is located in the brain’s hypothalamus in both hemispheres, and is influential in the maintenance of circadian rhythms. The SCN receives information about light from a structure called the optic chiasm, which sends messages from the eye to the visual area of the cerebral cortex. This can continue even if the person’s eyes are closed, allowing the body to adjust to changing daylight patterns.

Animal studies involving the SCN have shown that if the SCN connections are destroyed, the animals no longer have a sleep/wake cycle- this was observed in chipmunks by DeCoursey et al (2000). In addition, Ralph et al (1990) found that hamsters who received SCN cells through transplant from other hamsters bred to have a 20-hour sleep/wake cycle themselves defaulted to a 20-hour sleep/wake cycle.

The pineal gland and melatonin: The SCN passes information about daylight to the pineal gland, which is located behind the hypothalamus. This gland increases melatonin production, which induces sleep and is inhibited when a person is awake. As previously seen, melatonin is a possible cause of seasonal affective disorder.

Exogenous Zeitgebers

Environmental factors have an influence on biological rhythms through a process known as ‘entrainment’. These factors work with the body’s internal processes to affect rhythms such as the sleep/wake cycle.

Light: This resets the SCN, so has a key effect on the sleep/wake cycle. Hormone production and other processes are also influenced by light. Campbell and Murphy (1998) found that skin can detect light- when participants had light shone on to the back of their knees, this affected the duration of their sleep/wake cycles- even if it was dark outside. This suggests that light is perceived not just by the eyes, and has an effect on the body.

Social cues: Infants have no set sleep/wake cycle until about 6 weeks of age, and this process generally continues until around 16 weeks, when babies are entrained. This could be due to the schedules imposed on them by parents. Similarly, the effects of jet lag can be reduced by quickly adapting to local times for sleeping and eating (not going to bed when you feel tired, for example). This suggests that the body does respond to cues in the environment.

• Damiloa et al (2000) found that other organs in the body have their own circadian rhythms in cells known as ‘peripheral oscillators’. Changing feeding patterns in mice led to changes in the rhythms of the mice’s livers, for example. This supports that there are many influences on circadian rhythms, aside from the SCN.
• Animals used in this research are often exposed to great harm, for example in the DeCoursey study many of the chipmunks were killed by predators after their sleep/wake cycle was destroyed. This raises the question of whether the research is ethically justifiable.
• Evidence suggests that exogenous zeitgebers may not actually have much of an effect on biological rhythms. Miles et al (1977) reported that a man who was blind since birth and had a sleep/wake cycle of 24.9 hours could not have his cycle adjusted by any external factors such as social cues. Instead, he had to take sedatives at night and stimulants in the morning so that he could live in the ’24-hour world’. This weakens the influence of exogenous zeitgebers on biological rhythms.