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

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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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• 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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Unifying control over the body: consciousness and cross-cueing in split-brain patients

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Lukas J Volz, Steven A Hillyard, Michael B Miller, Michael S Gazzaniga, Unifying control over the body: consciousness and cross-cueing in split-brain patients, Brain , Volume 141, Issue 3, March 2018, Page e15, https://doi.org/10.1093/brain/awx359

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More than half a century of scientific work with split-brain patients has resulted in various profound insights on how the brain works. The generous dedication of these patients has truly transformed how we think about the brain and profoundly inspired the developing field of cognitive neuroscience. However, several questions raised by the work with split-brain patients still evoke controversy, one of the most prominent being whether separating the two cerebral hemispheres by cutting the corpus callosum may lead to two distinct conscious systems or not. In other words, is it possible to assume that a patient could be seen as two patients after the surgery?

In their recent study, Pinto and colleagues argued that their empirical data represent evidence for the notion that while (visual) perception in split-brain patients is divided, the resulting pair of perceptual systems are integrated into one conscious agent ( Pinto et al. , 2017 c ). In a review that preceded their original manuscript, Pinto et al. (2017 a ) referred to this idea as the ‘conscious unity, split perception model’. In our recent review, we addressed the interpretation offered by Pinto and colleagues and offered an alternative explanation: the reported experimental observations may well be explained by cross-cueing between the hemispheres, without the need for a single integrated conscious control ( Volz and Gazzaniga, 2017 ). Pinto and colleagues now argue against our perspective in a recent letter, stating that we did not present ‘substantive evidence’ for our interpretation ( Pinto et al. , 2017 b ). While we want to avoid a repetitive exchange on methodological aspects of the performed experiments, we do believe that something can be gained from the careful interpretation of split-brain findings, especially regarding their implications for our understanding of consciousness. Accordingly, we briefly address the points made by Pinto and colleagues in their response to the caveats we outlined regarding the interpretation of their work.

One of the most essential points that Pinto et al. addressed is the lack of a formal definition of cross-cueing, rendering a meaningful discussion rather difficult. We agree with the authors, as the portrayal of cross-cueing in their manuscript does not overlap too much with ours: ‘… cross-cueing (one hemisphere informing the other hemisphere with behavioural tricks, such as touching the left hand with the right hand) …’ ( Pinto et al. , 2017 c ). While such a simple description may potentially be accurate when testing a patient immediately after surgery, we have to emphasize the fact that the investigated patients underwent surgery many years prior to testing. Hence, the separated perceptual systems had ample time to learn how to compensate for the lack of commissural connections. For example, subtle cues may be given by minimal movements of the eyes or facial muscles, which might not even be visible to an external observer but are capable of encoding, for example, the location of a stimulus for the hemisphere that did not see it. Conversely, encoding the identity of a stimulus, i.e. what kinds of objects were presented to one visual field, seems far more complex. The resulting empirical prediction regarding Pinto et al. ’s experiments would hence be that information on the location of a stimulus may be readily transferred between hemispheres via cross-cueing (enabling accurate stimulus localization in the ‘incongruent condition’), while the identity of a unihemispherically presented stimulus remains lateralized (not allowing stimulus identification in the ‘incongruent condition’). Indeed, this is exactly what Pinto and colleagues observed: the split-brain patient was able to report the location but not the identity of a stimulus in the incongruent condition, e.g. correctly locating a stimulus with the right hand or verbally (i.e. using the left hemisphere) even if it was exclusively presented to the left visual field (i.e. to the right hemisphere) ( Pinto et al. , 2017 c ). The authors now discard the alternative cross-cueing explanation by pointing out that the reaction times in the ‘congruent’ and ‘incongruent’ conditions were not significantly different when stimulus location was indicated with the right or left hand ( Pinto et al. , 2017 b ). In other words, if cross-cueing were to explain these results, it would have to be nearly as fast as the intrahemispheric information processing in the ‘congruent condition’ . While this seems unlikely for a neural system that is accustomed to relying on interhemispheric integration via the corpus callosum (healthy volunteers or patients immediately after surgery), it is critical to note that the tested patients underwent surgery many years before these tests were carried out and had to rely on the efficient integration of information between hemispheres in the absence of callosal connections ever since. For example, when navigating through the world by walking or driving, making the location of an obstacle suddenly appearing in one visual field (i.e. visible to one hemisphere) accessible to the other hemisphere in order to enable a coordinated motor response (e.g. to avoid a collision) seems to constitute a crucial skill. Hence, as elaborated below, the empirical observations reported by Pinto et al. are neither surprising nor entirely novel.

Cross-cueing is not the only alternative explanation for the findings presented by Pinto and colleagues. Several seminal studies in split-brain patients have reported that crude information concerning the spatial location of stimuli can be cross-integrated (for further details see Gazzaniga, 2000 ). As early as 1968, Trevarthen concluded from research in split-brain monkeys that visual projections to the midbrain that subserve orientation in ambient space might be involved in the transfer of information on the location of objects in the split-brain via intact interhemispheric midbrain connections ( Trevarthen, 1968 ; Trevarthen and Sperry, 1973 ). Such a subcortical transfer of crude information about stimulus location may well have contributed to the accurate localization responses in the ‘incongruent conditions’ of Pinto et al. ’s Experiment 1.

Alternatively, it was suggested many years ago that either hand can be controlled by either hemisphere for simple pointing tasks ( Gazzaniga, 1964 , 1966 a , b ; Gazzaniga et al. , 1967 ). Thus, localizing the stimulus in the ‘incongruent condition’ via hand movements in Pinto et al. rsquo;s Experiment 1 may have principally been controlled by the hemisphere that also perceived the stimulus. Such an ipsilateral motor control might also account for the lack of a significant reaction time difference between the ‘congruent’ and ‘incongruent conditions’ in Pinto et al. rsquo;s experiment where subjects responded to a lateralized target’s colour with the right or left hand. Kingstone and Gazzaniga (1995) demonstrated how ipsilateral motor control of the hand could be misinterpreted as a purported transfer of conceptual information between the hemispheres in the absence of the corpus callosum ( Sergent, 1990 ). For example, when Patient JW was shown the word ‘arrow’ in one hemisphere and the word ‘bow’ in the other, his left hand would draw both a bow and arrow, indicating an apparent integration of concepts. However, that was only illusory. When instead, Patient JW was shown the words ‘hot’ and ‘dog’ in opposite hemispheres, his left hand would first draw a fire (corresponding to the word seen by his right hemisphere) and then draw a dog on top of it (the word seen by his left hemisphere indicating ipsilateral control). At no time, did the patient draw the emergent concept of a hot dog (see also Miller and Kingstone, 2005 ).

It should be noted that ipsilateral motor control could not explain the finding that split-brain patients made accurate verbal responses to stimuli presented in the left visual field (as in Experiments 2A, 2B, 3A, 3B and 4B of Pinto et al. , 2017 c ). However, when perception and cognition in commissurotomy patients are inferred from their verbal responses, special precautions must be taken to rule out the possibility of cross-cueing of information from the mute right hemisphere to the speaking left hemisphere. Gazzaniga and Hillyard (1971) showed that Patients LB and CC could respond verbally to one of two alternative numbers flashed to the left visual field, but the verbal reaction times for these binary choices were 200–300 ms longer than when the numbers were flashed to the right visual field (i.e. directly to the speaking hemisphere). Most strikingly, Patient LB could make accurate verbal reports to each of a set of numbers from 2 to 9 flashed one at a time to his left visual field, but with reaction time increasing linearly for larger numbers. Patient LB admitted to a (conscious) cross-cueing strategy wherein he (his speaking left hemisphere) counted subvocally until one of the numbers ‘stood out’. Whether this transfer of information between the hemispheres was mediated by subliminal muscular activity or intracranially (subcortically) was not clear. In general, whenever a commissurotomy patient is making a binary choice or selecting from a limited set of response alternatives, cross-cueing must be considered and ruled out, for example by examining reaction time measures. Unfortunately, Pinto and colleagues did not report reaction times for the above-mentioned experiments; if there were reliable reaction time differences between conditions this would strongly indicate a cross-cueing or alternative information transfer mechanism ( Gazzaniga and Hillyard, 1971 ). Empirically controlling for cross-cueing, e.g. via continuously recording eye movements and muscle activation throughout the experiment, or differentiating hemispheric contributions via recordings of neural activity during task performance using EEG, functional MRI, functional near-infrared spectroscopy (fNIRS) or comparable approaches may constitute ways to reveal the nature of task-specific information transfer between the surgically separated hemispheres.

In summary, Pinto and colleagues based fundamental claims regarding the nature of consciousness on behavioural observations that can be interpreted differently, in particular as consequences of cross-cueing, ipsilateral motor control, and/or subcortical information transfer. In other words, the conclusion that a unified conscious arises from largely separated systems only seems warranted if a number of alternative explanations has been ruled out. The existing evidence for two separate, independent streams of visual perception and hence visual consciousness in the surgically separated hemispheres seems compelling to us, and the data presented by Pinto et al. do not dissuade us. Consider the following two experiments: (i) Holtzman and Gazzaniga (1985) showed that split-brain patients could accurately perceive and remember two separate sequential visual patterns, one in each visual field, and match either of them to a subsequent probe sequence, while intact control subjects could not remember both sequences. (ii) Luck et al. (1994) showed that split-brain patients could identify a patterned visual target in a search array roughly twice as fast when the array was divided between the right and left visual fields (with each hemisphere seeing half the array) compared to when the array was presented to one visual field. Normal controls showed no such benefit of dividing the array. Pinto et al. argue that such results are not convincing evidence for separate spheres of consciousness because some experiments have shown better performance for divided-field arrays in normal, intact subjects. But the above-cited experiments did compare patients with normal controls, and the controls, unlike the patients, showed no evidence of a dual, independent processing capability.

Looking at all the evidence, we believe that the most parsimonious and logical conclusion is that the right hemisphere of the commissurotomy patients includes a stream of consciousness that is separate from that of the left hemisphere, but the two hemispheres may interact closely via cross-cueing and subcortical connectivity. The evidence for a separate conscious stream in the right hemisphere includes the following observations: while pictures and objects in the left visual field cannot be named through overt speech, they can be matched with written or spoken words, matched to conceptually related items, stored in short term memory for matching with subsequent probes ( Gazzaniga, 1995 ). Moreover, high-level cognitive judgements of the right hemisphere can initiate appropriate and accurate motor responses without the knowledge of the speaking hemisphere ( Gazzaniga, 2000 ). It is difficult to believe that such a high level of visual cognition could occur without a separate foundation of consciousness in the right hemisphere.

Finally, cross-cueing should not be simply viewed as ‘(un-conscious) cheating’ ( Pinto et al. , 2017 b ), but as an incompletely understood mechanism that allows for information integration in the absence of direct neural connections and hence considerably contributes to the quality of life of split-brain patients. Hence, while we argue for a separate stream of visual consciousness in the right hemisphere, we agree with Pinto et al. that the seemingly normal, bilaterally integrated behaviour following commissurotomy requires further explanation. Despite illustrating the ingenious capacity of human adaptation, cross-cueing itself may hold valuable insights on how the intact brain integrates information from highly specialized neural systems.

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One Head, Two Brains

How a radical epilepsy treatment in the early 20th century paved the way for modern-day understandings of perception, consciousness, and the self

In 1939, a group of 10 people between the ages of 10 and 43, all with epilepsy, traveled to the University of Rochester Medical Center, where they would become the first people to undergo a radical new surgery.

The patients were there because they all struggled with violent and uncontrollable seizures. The procedure they were about to have was untested on humans, but they were desperate—none of the standard drug therapies for seizures had worked.

Between February and May of 1939, their surgeon William Van Wagenen, Rochester’s chief of neurosurgery, opened up each patient’s skull and cut through the corpus callosum, the part of the brain that connects the left hemisphere to the right and is responsible for the transfer of information between them. It was a dramatic move: By slicing through the bundle of neurons connecting the two hemispheres, Van Wagenen was cutting the left half of the brain away from the right, halting all communication between the two.

In a paper he and a colleague published in the Journal of the American Medical Association in 1940, Van Wagenen explained his reasoning: He had developed the idea for the surgery after observing two epilepsy patients with brain tumors located in the corpus callosum. The patients had experienced frequent convulsive seizures in the early stages of their cancer, when the tumors were still relatively small masses in the brain—but as the tumors grew, they destroyed the corpus callosum, and the seizures eased up.

“In other words, as the corpus callosum was destroyed, generalized convulsive seizures became less frequent,” Van Wagenen wrote in the 1940 paper, noting that “as a rule, consciousness is not lost when the spread of the epileptic wave is not great or when it is limited to one cerebral cortex.” Based on the cases of the cancer patients—and some other clinical observations —Van Wagenen believed that destroying the corpus callosum of his patients would block the spread of the electrical impulses that lead to seizures, so that a seizure that began in the left hemisphere, for example, stayed in the left hemisphere.

The surgery worked for most of the patients : In his paper, Van Wagenen reported that seven of the 10 experienced seizures that were less frequent or less severe.

Between 1941 and 1945, Van Wagenen’s colleague, the University of Rochester psychiatrist A. J. Akelaitis, tested the patients to see if they had experienced any cognitive or behavioral changes as a result of the invasive procedure. After giving the patients a series of assessments—an I.Q. test, a memory test, motor-skills assessments, and interviews—he reported that most of the patients had the same levels of cognitive functioning after the surgery as before, and displayed no behavioral or personality changes. Though the brain hemispheres of split-brain patients had been disconnected, he wrote in a 1944 paper in the Journal of Neurosurgery , they were otherwise normal.

Or so it seemed.

When Michael Gazzaniga first learned about the Rochester patients as an undergraduate research intern in 1960, he was curious—and skeptical.

Gazzaniga’s timing was fortuitous: Roger Sperry, who headed the neuroscience lab where Gazzaniga worked at the California Institute of Technology, had begun split-brain research on cats and monkeys just a few years earlier. Sperry found that severing the corpus callosum of those animals had affected their behavior and cognitive functioning.

In one experiment with split-brain cats, for example, Sperry would cover one of the animal’s eyes and then teach it to differentiate between a triangle and a square. Once the cats learned to do that, Sperry switched the covering from one eye to the other and tested the them to see if they recalled their new knowledge. They didn’t. “The split-brain cat,” as one neurosurgeon wrote in an overview of Sperry’s work, “has to learn all over again.” As Sperry noted, this suggested that the two hemispheres were not communicating with each other, and that each was learning the task on its own.

If the Rochester patients’ left and right brains were also no longer communicating, Sperry and his colleagues believed, then they must be experiencing some sort of change, too.

The question was still bothering Gazzaniga by the time he returned to Sperry’s lab as a graduate student in 1961: What kind of change was it? Would human brains react the same way as those of the animals in Sperry’s lab?

“In monkeys,” Gazzaniga told me, “sectioning the corpus callosum led to the right hand not knowing what the left hand was doing. I wanted to know if we would see a similar result in humans.”

The researchers didn’t have to wait long to begin looking for the answer. In the summer of 1961, as Gazzaniga was preparing to return to Sperry’s lab as a graduate student, a young neurosurgeon at Caltech named Joseph Bogen approached Sperry about the opportunity to study a split-brain patient—and Sperry, who had been working exclusively with animals, seized the chance to work on his first human case.

The patient Bogen had in mind was a man in his late forties named William Jenkins, a World War II veteran who had been hit in the head with the butt of a German officer’s rifle after parachuting behind enemy lines. Jenkins’ doctors believed that this was the likely origin of the uncontrollable seizures he later developed; when he returned to the U.S. after the war and sought treatment, he discovered that no drugs worked to contain the seizures.

In 1961, as a last-ditch effort, Bogen suggested that he have split-brain surgery. Sperry assigned Gazzaniga to conduct some standard pre-operative neurological tests, and Bogen and a colleague performed the procedure in February of 1962. After a few months of post-surgery monitoring, Bogen found that the severity and frequency of Jenkins’ seizures had abated, but he still did not know if the surgery had produced other unintended consequences. So about a month after the surgery, Bogen sent Jenkins to Sperry and Gazzaniga for cognitive testing. In doing so, he kicked off a line of work that would turn the two men into pioneers of split-brain research, eventually earning Sperry a share of the Nobel Prize in 1981—and causing scientists to reconsider long-held ideas about the brain and the self.

The cognitive tests performed on the 10 original Rochester patients hadn’t tested each brain hemisphere separately; believing that this was one reason why the patients hadn’t shown any changes after surgery, Sperry and Gazzaniga decided to run tests for both the left and right sides of Jenkins’s brain.

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In one of the first split-brain studies that the pair designed, published in August 1962 in the Proceedings of the National Academy of Sciences , Gazzaniga invited Jenkins into the lab and had him stare straight ahead at a dot. As he was staring ahead, Gazzaniga flashed a picture of a square on a screen to the right of where his eyes were staring, meaning the image would be processed by Jenkins’ left brain. ( Because of the way the brain is wired , if a patient looks straight ahead, something quickly flashed to the left of his gaze will be processed by the right side of the brain, and vice versa. The brain’s hemispheres control activity mainly on the opposite side of the body—the left hemisphere controls the action of the right hand, for example, while the right hemisphere moves the left hand.)

When Gazzaniga asked Jenkins what he saw, Jenkins was able to describe the square. Then Gazzaniga tried the same thing on the other side, flashing the same image to the left of Jenkins’ gaze. When he asked Jenkins again what he saw, though, Jenkins said he saw nothing.

Intrigued, Gazzaniga pulled another image, this time of a circle, to flash on Jenkins’s right and left sides separately, as he had done with the square.

Instead of asking Jenkins to describe the object, though, he asked him to point to it. When the image was on Jenkins’ right side (left brain), he lifted his right hand (controlled by the left brain) to point to it. When the circle flashed on his left side (right brain), he lifted his left hand (controlled by the right brain) to point to it.

The fact that Jenkins was able to point to the circle with both hands told Gazzaniga that each of Jenkins’ hemispheres had processed the sight of the circle. It also meant that in the previous trial, both of Jenkins’s hemispheres had processed the square—even though Jenkins said, when his right brain processed the sight, that he saw nothing. At that point, scientists had known for about a century that language arises from the left hemisphere; given that, the researchers later reasoned, Jenkins could only talk about the square when its picture was flashed to his right eye (left brain). On the other side, even though Jenkins had seen the square, he could not speak about it.

Between 1962 and 1967, Sperry and Gazzaniga worked together to perform dozens of additional experiments with Jenkins and other split-brain patients. In one set of studies conducted in 1962 and 1963, Gazzaniga presented Jenkins with four multicolored blocks. Then, he showed Jenkins a picture of the blocks arranged in a certain order, and asked him to make the same arrangement with the blocks in front of him.

Because the right brain handles visual-motor capacity, Gazzaniga was unsurprised to see that Jenkins’ right hemisphere excelled at this task: Using his left hand, Jenkins was immediately able to arrange the blocks correctly. But when he tried to do the very same task with his right hand, he couldn’t. He failed, badly.

“It couldn’t even get the overall organization of how the blocks should be positioned, in a 2x2 square,” Gazzaniga later wrote of Jenkins’ left hemisphere in his memoir, Tales from Both Sides of the Brain . “It just as often would arrange them in a 3+1 shape.”

But more surprising was this: As the right hand kept trying to get the blocks to match up to the picture, the more capable left hand would creep over to the right hand to intervene, as if it realized how incompetent the right hand was. This occurred so frequently that Gazzaniga eventually asked Jenkins to sit on his left hand so it wouldn’t butt in.

When Gazzaniga let Jenkins use both hands to solve the problem in another trial, he again saw the two brain hemispheres at odds with one another. “One hand tried to undo the accomplishments of the other,” he wrote. “The left hand would make a move to get things correct and the right hand would undo the gain. It looked like two separate mental systems were struggling for their view of the world.”

The more information the split-brain researchers discovered, the more they wondered: If the two sides of the brain functioned so independently of each other, how do people—ordinary people and split-brain patients alike—experience a single, cohesive reality?

In a 1977 study with a 15-year-old split-brain patient from Vermont identified as P. S., Gazzaniga (then a professor at Dartmouth) and his graduate assistant Joseph LeDoux performed a visual test similar to the one Jenkins had undergone years earlier. The researchers asked P. S. to stare straight ahead at a dot, and then flashed a picture of a chicken foot to the brain’s left hemisphere and a picture of a snowy scene to the brain’s right hemisphere. Directly in front of the patient—so that he could process the sight with both hemispheres—was a series of eight other pictures. When the researchers asked him to point to the ones that went with the images he saw, P. S. pointed to the picture of a chicken head and a picture of a snow shovel.

So far, the results were as expected: Each hemisphere had led P. S. to choose an image that went along with the one that he had seen from that side moments earlier. The surprise came when the researchers asked him why he chose these two totally unrelated images.

Because the left hemisphere, which controls language, had not processed the snowy scene, they believed P. S. wouldn’t be able to verbally articulate why he chose the snow shovel. “The left brain doesn’t know why,” Gazzaniga told me. “That information is in the right hemisphere.” Neither hemisphere knew what the other had seen, and because the two sides of his brain were unable to communicate, P.S. should have been confused when Gazzaniga asked him why he had picked the two images he did.

But as Gazzaniga recalled in his memoir, P. S. didn’t skip a beat: “Oh, that’s simple,” the patient told them. “The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.”

Here’s what happened, as the researchers later deduced: Rather leading him to simply say, “I don’t know” to Gazzaniga’s question, P.S.’s left brain concocted an answer as to why he had picked those two images. In a brief instant, the left brain took two unconnected pieces of information it had received from the environment—the two images—and told a story that drew a connection between them.

Gazzaniga went on to replicate the findings of this study many times with various co-authors: When faced with incomplete information, the left brain can fill in the blanks. Based on these findings Gazzaniga developed the theory that the left hemisphere is responsible for our sense of psychological unity—the fact that we are aware of and reflect upon what is happening at any given moment.

“It’s the part of the brain,” Gazzaniga told me, “that takes disparate points of information in and weaves them into a storyline and meaning. That it’s central gravity.”

In addition to answering questions of brain specialization, split-brain research also examined some of the ways in which the left and right hemispheres are autonomous agents. Jenkins’ left and right hands started fighting over how to arrange the blocks, for example, because the two hemispheres are—as Gazzaniga told me—“two separate minds, all in one head.”

As he further explained in Tales from Both Sides of the Brain : “The notion that there is an ‘I’ or command center in the brain was an illusion.”

Among psychologists, the idea wasn’t exactly new; figures like Sigmund Freud and William James had previously theorized about a “divided self,” with Freud arguing that the mind is divided into the ego, the superego, and the id. But split-brain research was arguably one of the first scientific demonstrations that the divided self has a real, physical basis—a demonstration that, in turn, raised new questions about the relationship between the mind and the brain.

“The demonstration that you could in effect split consciousness by splitting anatomy—by just making a tiny change in anatomy … It was one of the most remarkable results in neuroscience, with huge implications,” said Patricia Churchland, a philosopher at the University of California, San Diego, whose work focuses on the relationship between philosophy and neuroscience. “If you thought that consciousness and mental states were independent of the brain, then this should have been a real wake-up call.”

Helping to illuminate the relationship between the mind and the brain, according to the cognitive psychologist Steven Pinker, is one of split-brain research’s most important contributions to modern psychology and neuroscience. “The fact that each hemisphere supports its own coherent, conscious stream of thought highlights that consciousness is a product of brain activity,” he told me. “The notion that there is a single entity called consciousness , without components or parts, is false.”

Today’s therapies for seizures are more advanced than those of the mid-20th century, and split-brain surgery is now exceedingly rare —Michael Miller, a neuroscientist at the University of California at Santa Barbara who did graduate work with Gazzaniga, told me the last one he heard of was performed around 10 years ago. Many of the split-brain patients that Gazzaniga, Sperry, and their colleagues studied have passed away.

Though the research on split-brain patients has slowed dramatically, Miller believes that the field still has something left to offer. He’s currently working on a study currently working with a patient to answer the question: Does each hemisphere of the brain reflect on and evaluate itself in a unique way?

“We know that the two hemispheres have different strategies for thinking,” Miller told me, “and we’re curious about how that might change their reflection of themselves. Does the left hemisphere think of itself as a sad person while the right one think of itself as a happy person? We are having each hemisphere evaluate itself to find out.”

Miller’s study uses a test called the “trait-judgment task”: A trait like happy or sad flashes on a screen, and research subjects  indicate whether the trait describes them. Miller has slightly modified this task for his split-brain patients—in his experiments, he flashes the trait on a screen straight in front of the subject’s gaze, so that both the left and right hemispheres process the information. Then, he quickly flashes the words “me” and “not me” to one side of the subject’s gaze—so that they’re processed only by one hemisphere—and the subject is instructed to point at the trait on the screen when Miller flashes the appropriate descriptor. (For example, if the screen reads “happy,” an unhappy left hemisphere would lead a subject to point when Miller flashes “not me” to the right side of the subject’s gaze, and to stay still when he flashes “me.”) If the subject reacts differently on each side—in this example, if the subject points to the screen when “me” is flashed to the right hemisphere—then Miller believes there must be a disconnect between the self-concept contained in each side of the brain.

Miller’s research is ongoing. But, he said, if the study finds that each hemisphere evaluates itself differently from the other, it could add a new layer of understanding to how divided the mind really is.

“Split-brain patients give you a unique glimpse into a state of consciousness you wouldn’t see otherwise,” Miller told me.

“There is something quite unique in interacting with a split-brain patient,” he added. “All the interactions you are engaging in are with left hemisphere, and you can suddenly manipulate things to interact with right hemisphere and it’s a completely different experience. A completely different consciousness.”

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Invisibilia

The roots of consciousness: we're of 2 minds.

After surgery to treat her epilepsy severed the connection between the two halves of her brain, Karen's left hand took on a mind of its own, acting against her will to undress or even to slap her. Amazing, to be sure. But what may be even more amazing is that most people who have split-brain surgery don't notice anything different at all.

But there's more to the story than that. In the 1960s, a young neuroscientist named Michael Gazzaniga began a series of experiments with split-brain patients that would change our understanding of the human brain forever. Working in the lab of Roger Sperry, who later won a Nobel Prize for his work, Gazzaniga discovered that the two halves of the brain experience the world quite differently.

When Gazzaniga and his colleagues flashed a picture in front of a patient's right eye, the information was processed in the left side of the brain and the split-brain patient could easily describe the scene verbally. But when a picture was flashed in front of the left eye, which connects to the right side of the brain, the patient would report seeing nothing. If allowed to respond nonverbally, however, the right brain could adeptly point at or draw what was seen by the left eye. So the right brain knew what it was seeing; it just couldn't talk about it. These experiments showed for the first time that each brain hemisphere has specialized tasks.

The Other Self

In this third episode of Invisibilia , hosts Alix Spiegel and Hanna Rosin talk to several people who are trying to change their other self, including a man who confronts his own biases and a woman who has a rare condition that causes one of her hands to take on a personality of its own.

I spoke with Gazzaniga about his seminal research and what it can tell us about the nature of the human brain and even human consciousness. He's the director of the SAGE Center for the Study of Mind at the University of California, Santa Barbara, and author of the upcoming book, The Consciousness Instinct . The interview has been edited for length and clarity.

Interview Highlights

It's incredible now to think that until you did those experiments, no one knew about brain lateralization. What does it feel like to make such a profound discovery?

Before we conducted our experiments, it seemed very clear that cutting the corpus callosum did not have any effect. Karl Lashley, an influential memory researcher, joked that the corpus callosum's role was simply "to keep the hemispheres from sagging."

So it was pretty stunning to witness a guy who was otherwise just like everybody else be completely unaware in his left hemisphere about what his right hemisphere was capable of. All of the information in half of his visual field could not be verbally described. And yet, the right hemisphere responding nonverbally was aware that the information had been presented. It boggles the mind. If you were witnessing that, trust me, you would just be stunned. You'd say, "I want to understand that more."

So what's the benefit of having the two halves of the brain specialized like that?

Well, people have been wondering about lateralization of the nervous system for a long time, and there are many theories, but it's basically not known. Up until you get to the human brain, if you look at monkeys and chimps, both sides of the brain serve basically the same functions. And then in humans, there starts to be this vast amount of lateral specialization. One simple idea that we've offered is that the human is really set with more capacities than fewer, and each one of those capacities takes up some kind of neural space.

If you start with a normal, intact brain with things duplicated on each side and you need more cortical space to add on all the new, higher functions of the human condition, you're gonna say, "Maybe let's recraft some of this space and just use one hemisphere, so we have more space for another capacity." But as I say, it's just speculation; it's not in the category of "we know how it works."

What are "functions of the human condition"?

Well, over time, as our experiments evolved, rather than just asking patients to identify what they saw, we asked them to select objects or drawings to match the images we showed them, and then we would ask them to explain themselves. For example, we showed the right eye of one patient a picture a chicken claw. The right hand had to pick a related drawing, and one was a chicken. So, the chicken claw obviously goes with the chicken. At the same time, we showed to the left eye a New England snow scene. The left hand had to pick a related image, and one was a shovel, so the left hand pointed to the shovel.

Afterward, we asked the patient, sort of confrontationally, "Why did you do that? Why did you point to the chicken and the shovel?" And the patient said, "Well, the chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed." And we realized — BOOM! — we do that all day long! We have all these separate systems, these impulses, these emotions, these behaviors, all this stuff, and we're constantly thinking about it and spinning it into a story that fits.

Once you're onto that as a big feature of the human condition, you could then see how you can take that kind of interpretive system and build larger stories about meaning and why we're doing things and our origins, and all the rest of it.

What can split-brain research teach us about normal brains?

One of the fundamental facts of split-brain research that people have to remember is that you can take any normal person and normal brain and disconnect the hemispheres and all of a sudden you have two consciousnesses. And through analysis and examination of all kinds of neurologic cases, you realized there are consciousnesses all over the brain!

So if you're looking at one system that somehow generates our subjective sense of being conscious — that's wrong. That's not how we should think about how consciousness evolved. You can take a conscious system and divide it in two just by disconnecting some neurons — that is a thing to go home and think about real hard.

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2.2 The story of the split brain patients

A surgical procedure that cuts through the corpus callosum has provided evidence to support the different specialisations of the left and right hemispheres of the brain. This procedure is used very rarely and always as a last resort when someone has frequent and major epileptic seizures that do not respond to drug treatment. The frequency and severity of their epileptic fits is very disabling and their quality of life is poor. The attacks can even be life threatening. In these patients epileptic activity would start in one area of the brain and then spread across the corpus callosum to all areas of the brain. By cutting these connections between the two hemispheres epileptic activity is contained in one hemisphere only. The operation usually leads to a significant decrease in the frequency and severity of the seizures without any apparent interference in normal functioning.

Early researchers were puzzled by the fact that people who had undergone this operation did not show any noticeable changes in behaviour, personality or their scores on intelligence tests despite such extensive surgery. In fact they wondered what the purpose of the corpus callosum was if you could cut through it with so little effect. However careful testing by Roger Sperry (1968) and colleagues did uncover behaviour that was far from normal. This work was to gain him a Nobel Prize for Medicine in 1981.

Sperry et al, devised a number of split brain experiments using people who had had split brain surgery as participants and comparing their responses to people who had not had this surgery. In one experiment the split brain participant was blindfolded and given objects to explore with their left hand. Information from the left hand goes to the right hemisphere but speech is generally controlled by the left hemisphere.

Participants were unable to tell the experimenter the name of the object they were holding in their left hand even though they could obviously recognise the object because they would make appropriate gestures with it. For example, if the object was a key they would hold it out as though putting it in a lock and turn it. Because the right hemisphere does not talk and could not transfer information to the left hemisphere the object cannot be named. However as soon as the participant touched the object with the right hand they were able to name it instantly.

In another experiment the participant would sit at a table with a screen in front of them. They would be asked to place their hands round the sides of the screen so that their hands were hidden from view. They would then be asked to fix their eyes on a spot in the centre of the screen.

A word is then flashed onto one side of the screen very briefly (approximately one tenth of a second). The word has to be flashed very quickly so that the participant does not have time to move their eyes and the information will only go to one of the brain hemispheres.

When a word is flashed on to the left-hand side of the screen the information will go to the right hemisphere of the brain. The information cannot be passed to the talkative left hemisphere so the participant cannot tell the experimenter what the word was.

However the participant can use their left hand to explore a pile of objects behind the screen and easily pick out the object that corresponds to the word that has been flashed up. They still won't be able to tell the experimenter what the left hand is doing as sensory information from the left hand is going to the silent right hemisphere only. Also they can't find the right object with their right hand as the right hand is controlled by the left hemisphere and the left hemisphere did not see the flashed word.

Activity 2: Sorting out right from left

Reading about split brain experiments can be a little confusing as you try and sort out right and left hands, hemispheres and sides of the screen. Taking some time to do this activity should help to make things clearer.

1 If a word is flashed on the right hand of the screen will a person with a split brain be able to:

Don't worry if you found this activity difficult. Many people find following a path from a word on the right side of the screen to the left hemisphere of the brain and then to the right hand far from easy.

In question 1 the word was flashed on the right side of the screen so the information will go to the left hemisphere of the brain. As speech is usually controlled by the left hemisphere the person should be able to name the word. As the left hemisphere also controls the right side of the body the person will be able to pick out a hidden corresponding with their right hand but not with their left hand.

So the answers to question 1 are

Activity 2b

2 If a word is flashed on the left hand of the screen will a person with a split brain be able to:

The word is flashed to the left side of the screen so the information will go to the right hemisphere. The person will not be able to name the word and will not be able to pick out a corresponding hidden object with their right hand. This is because the right hemisphere does not control speech or the right side of the body. The right hemisphere controls the left side of the body so the left hand will be able to select a corresponding object.

So the answers to question 2 are the opposite to those for question 1:

In split brain experiments the techniques used will limit information to one hemisphere only and the person behaves as if they have two separate brains with each hemisphere appearing to operate with no conscious awareness of what is happening in the other hemisphere.

Of course in everyday activities split brain people can operate normally because they can move their eyes and make sure that incoming information is available to both hemispheres. Occasionally odd behaviours do occur, especially in the early days after surgery. A patient might find that they are buttoning up a shirt with one hand and unbuttoning it with the other hand or that their left hand suddenly closes a book that they were engrossed in.

Split-Brain Patient

In subject area: Psychology

Split-brain patients are the people who have had their hemispheres surgically separated as a treatment for epilepsy.

From: Consciousness and Cognition , 2007

Chapters and Articles

You might find these chapters and articles relevant to this topic.

Cerebral Lateralization and Cognition: Evolutionary and Developmental Investigations of Behavioral Biases

Giulia Prete , Luca Tommasi , in Progress in Brain Research , 2018

1 Split-Brain Patients

The expression “ split-brain patient ” typically refers to individuals suffering from epilepsy, who underwent the surgical resection of the corpus callosum (CC), in an attempt to reduce the spread of epileptic foci between the cerebral hemispheres ( Zaidel and Iacoboni, 2003 ). This invasive treatment has been mostly abandoned today ( Prete and Tommasi, 2017 ), due to the introduction of pharmacological therapies that are more efficient than those available some decades ago. Nevertheless, it is still used in the most drug-resistant forms of epilepsy ( Englot et al., 2017 ). The surgery has been shown to effectively reduce the spread of epileptic activity between the hemispheres and improve the quality of life of patients ( Unterberger et al., 2016 ).

The CC is the largest bundle of white matter connecting the left and right hemispheres, and it is composed of different functional portions ( Fabri and Polonara, 2013 ). As shown in Fig. 1 , the most posterior portions of the CC are the splenium and the isthmus , and they connect occipital, parietal, and temporal areas across the two hemispheres. Frontal and temporal cortices are connected via the trunk , whereas prefrontal areas are connected through the genu and the rostrum ( Fabri and Polonara, 2013 ). The surgical section of the CC can either be complete (complete callosotomy), or it can involve only one or more specific portions of the fiber bundle (partial callosotomy). In some cases, additional interhemispheric commissures (i.e., anterior, hippocampal, posterior, and collicular commissures) are sectioned (commissurotomy; e.g., Uddin, 2011 ).

Fig. 1 . Schematic representation of the interhemispheric commissures (the corpus callosum is represented in gray ; portions of the corpus callosum are labeled in italics ).

The absence of callosal fibers can also result from a congenital condition, and in this case it is defined as callosal agenesis (e.g., D’Antonio et al., 2016 ). Even if callosal agenesis was previously considered to be “asymptomatic” (thanks to an interhemispheric reorganization due to cerebral plasticity), it has been found that patients with callosal agenesis show a syndrome similar to that of split-brain patients, both in the perceptual and in the motor domains ( Lassonde et al., 1995 ).

In the 1940s, Akelaitis described the positive clinical outcome of the first surgical resections of the CC carried out on epileptic patients by van Wagenen ( Akelaitis, 1941a,b ; Akelaitis et al., 1942 ; Mathews et al., 2008 ). Akelaitis (1941a,b) and Akelaitis et al. (1942) described the medical improvement in seizure control after complete or partial callosotomy: according to these pioneering observations, the intervention did not affect the patient's perceptual ( Akelaitis, 1941a ) and motor abilities ( Akelaitis et al., 1942 ), nor their psychiatric condition ( Akelaitis, 1941b ). Possibly the first paper describing the cognitive outcome of an epileptic patient who underwent the surgical resection of the CC was that published about two decades later, in 1962, by Gazzaniga, Bogen, and Sperry (previous cases were described for instance by Sperry in 1961 , but no cognitive effects had been noticed). The authors confirmed that the surgical intervention improved the clinical condition of the patients, by decreasing the frequency of seizures, but they presented the so-called classical disconnection syndrome (see Section 2 ).

Conducting research with split-brain patients constitutes a milestone for the neurosciences, but it is a hard and provides limited opportunities, due to the patients’ difficulties in maintaining a high level of attention, the effect of patient medications, and often an unfamiliarity with the use of computers used for presenting experimental paradigms (see Corballis and Häberling, 2017 ). Nevertheless, the research carried out with split-brain patients over the last decades has continued to help clarify hemispheric competences in disparate domains, such as language ( Bogen, 1997 ; Levy, 1983 ), music perception ( Prete et al., 2015c ), spatial abilities ( Corballis et al., 2010 ; Hausmann et al., 2003 ; Prete et al., 2017a, 2018b ), memory ( Zaidel, 1995 ), attention ( Berlucchi et al., 1997 ; Luck et al., 1994 ; Ptito et al., 2009 ), and moral reasoning ( Miller et al., 2010 ), among others.

Because the CC is the main connection between the left and right hemispheres, the first observations of split-brain patients were centered on the evaluation of the specific skills of each hemisphere, based on the idea that the functional separation was so sharp and strong to give rise to two “minds” or two “consciousnesses”: LeDoux et al. (1977) described the case of a split-brain patient who showed preserved linguistic skills in both of his disconnected hemispheres, so that the authors concluded that “ human conscious processes can be doubled by cerebral commissurotomy ” ( LeDoux et al., 1977 , p. 420). When detailing the clinical case, the authors reported that “On a day that this boy's left and right hemispheres equally valued himself, his friends, and other matters, he was calm, tractable, and appealing. On a day when testing indicated that the right and left sides disagreed on these evaluations, the boy became difficult to manage behaviorally. It is as if each mental system could read the emotional differences harbored by the other. When they were discordant, a feeling of anxiety, which appeared to be read out by hyperactivity and general aggression, was engendered. This clear example of surgically produced psychological dynamism, seen for the first time in P. S., raises the question whether such processes are active in the normal brain, where different mental systems, using different neural codes, coexist within and between the cerebral hemispheres” ( LeDoux et al., 1977 , p. 420).

The idea of a split consciousness was also proposed by Dimond (1978) who pointed out that the splenium was the site in which a general consciousness circuit takes place. In the same vein, Zaidel and Iacoboni (2003) wrote “ Soon after surgery there are episodes of intermanual conflict, in which the hands act at cross-purposes. Patients sometimes complain that their left hand behaves in a ‘foreign’ or ‘alien’ manner, and they routinely express surprise at apparently purposeful left-hand actions (autocriticism) ” (p. 320). The issue of one integrated vs two separated conscious entities in the human brain remained a central core to the neurosciences, so much so that after 40 years of research, the “unity of consciousness” is still one of the most debated issues in the split-brain literature (e.g., Bayne, 2008 ; Colvin et al., 2017 ; Volz and Gazzaniga, 2017 ).

Over time the idea of callosal fibers as mere connection between two independent hemispheres has been replaced by the softer interpretation of two cooperating halves of the brain that continue to interact even in the absence of callosal connections, thanks to subcortical bilateral projections (e.g., Funnell et al., 2000 ). Similarly, also the idea of a “dominant” hemisphere has been replaced with that of a possible superiority of one hemisphere over the other, but with the possibility that the processing of information can occur in each half of the (disconnected) brain (see Corballis and Häberling, 2017 ). Support for this view can be found in some studies with split-brain patients. For instance, split-brain patients were able to make perceptual judgments, such as matching of nonsense shapes, across the vertical meridian ( Zaidel, 1995 ), showing that unilateral information can reach the contralateral hemisphere in the absence of callosal fibers, even if spatial information is more efficiently processed by the right hemisphere ( Funnell et al., 1999 ). In summary, the findings highlighted by testing split-brain patients add important evidence about the role of interhemispheric connections, as well as about the specific competences of the two halves of the brain and the mechanisms involved in neuroplasticity.

1.1 The Callosal Disconnection Syndrome

The so-called callosal disconnection syndrome manifests itself in a combination of several impairments, mainly concerning bimanual coordination ( Berlucchi, 2012 ), spatial attention, and language impairment of the nonlinguistic hemisphere (e.g., Lausberg et al., 1999 ). The central core of this syndrome is rooted in the associationist theory proposed by Wernicke (1874) , and then revised by Geschwind (1965a,b) , according to which all cognitive functions emerge from white matter connections with different cerebral areas. In this view, cognitive, behavioral, and psychological dysfunctions occur as the result of white matter lesions. In this frame, the expression “disconnection syndromes” is used to define all of the disorders due to an acquired lesion involving neuronal projections, which leads to specific high-level disorders, including language disability (aphasia), motor disorder (apraxia), sensory processing deficit (agnosia), reading disorder (alexia), and so on ( Catani and ffytche, 2005 ). When referring to split-brain patients , the “callosal syndrome” is mainly defined as the linguistic inability of the right hemisphere, which is evident in higher order deficits in the left hemispace, such as the inability in reading, moving, and recognizing objects in the left hemispace ( Zaidel, 1983 ).

In the past, the classic view posited that callosal fibers simply allow the exchange of information between the two hemispheres: “ a copy of the visual world as seen in one hemisphere is sent over to the other ” through the callosum ( Gazzaniga, 1967 , p. 29). Similarly, Geschwind and Kaplan (1962) asserted that the callosal disconnection syndrome was the exact result of the interrupted exchange of information between the two sides of the brain. However, besides being the largest group of fibers connecting the two halves of the brain, the CC also plays a role in functional asymmetries. Only recently it has been found that the CC is not solely constituted of white matter, but it contains active cells: a series of functional magnetic resonance imaging (fMRI) studies highlighted functional activation in different portions of the CC depending on the nature of stimuli presented ( Fabri et al., 2014 ; Gawryluk et al., 2009 ). It is now considered that symptoms following callosal disconnection are attributable to the loss of a distributed balance mediated by the callosal fibers together with the other cortical and subcortical commissures. The notion of an equilibrating role of the callosum was initially put forward by Kinsbourne (2003) , based on evidence that callosal fibers are both excitatory and inhibitory and that some excitatory fibers activate inhibitory interneurons. On these grounds, the callosal disconnection syndrome could be seen as the result of a lack of response of the “uninformed” hemisphere, assuming that information reaches it anyway by means of subcortical pathways. With regard to the subcortical interhemispheric connections, Doty (1989) previously proposed that the serotonergic raphe system in the pons and the mesencephalon could be responsible for bilateral subcortical activation.

The prevalent interpretation nowadays is that callosal connections are mainly involved in interhemispheric communication, but they also have a functional role and they are crucial in determining cerebral functional asymmetries. For instance, Barnett and Corballis (2005) found that the right-to-left information transfer time was faster than the opposite route (left-to-right), and they attributed this finding to the faster axonal speed arising in the right rather than in the left hemisphere, due to the greater number of fast-conducting, myelinated fibers in the right hemisphere. This idea had been previously proposed by Marzi et al. (1991, 1997) who argued that callosal projecting neurons are more numerous in the right hemisphere than in the left hemisphere. Based on this observation, the authors also proposed an explanation for a number of impairments following right-hemispheric damage that were possibly attributable to the callosal projections, as the deficit in attention to and awareness of the left visual field (LVF), namely, spatial hemineglect ( Berlucchi and Vallar, 2018 ), and the inability to consciously perceive stimuli presented in the LVF when they are presented together with stimuli in the right visual field (RVF), namely, visual extinction ( Chen and Spence, 2017 ). According to the hypothesis proposed by Marzi et al. (1997) , a right-lateralized brain injury should cause a greater loss of callosal fibers, resulting in a stronger impairment of interhemispheric transmission. Thus, the information reaching the left hemisphere can project to the right, but the information reaching the right hemisphere cannot be projected to the left: the result is the extinction (or neglect) of the stimuli presented in the LVF ( Heilman et al., 1987 ). The model explained the case of right-damaged patients who did not show extinction as rare cases due to the preserved callosal projections despite the right hemisphere lesion.

The view of axonal fibers involved in functional asymmetries and lateralized attentional deficits (e.g., spatial hemineglect, extinction) has been widely referenced in a number of studies and neuropsychological models ( Corbetta and Shulman, 2011 ; De Schotten et al., 2005 ; He et al., 2007 ; Gaffan and Hornak, 1997 ). Furthermore, Corballis et al. (2005) described a case of alternating hemineglect present in a split-brain patient with a complete callosal resection, further supporting the role of the callosal projections in attentional processes: the patient showed slower reaction times for stimuli flashed in the LVF, but he did not show attentional bias when stimulus location was defined by continuous markers presented in both visual fields.

It should be noted that when we refer to complex perceptual stimuli, the disconnection syndrome could be weakly evident, unless it is studied with specific methodologies. In the visual domain, for instance, the most exploited paradigm is that of the divided visual field presentation ( Bourne, 2006 ). In this paradigm, a visual stimulus is presented in the left or in the RVF, for a duration shorter than that needed to make a saccadic movement (about 150   ms, computer-based, tachistoscopic presentation), and the observer is required to gaze ahead centrally, without moving their gaze directly to the location of the stimulus. When the stimulus is presented in a lateralized fashion, it is projected to the nasal portion of the retina, which is directly connected with the contralateral hemisphere (e.g., left eye/right hemisphere). This procedure allows researchers to be confident that a stimulus is directly processed by one hemisphere.

The computer-based presentation of lateralized stimuli has been widely exploited to investigate hemispheric skills in healthy observers. The performance of split-brain patients in this type of tasks gives researchers a unique opportunity to evaluate the ability of each hemisphere “in isolation.” Brown et al. (1999) recorded event-related potentials (ERPs) during a matching task in which letters and dots were presented unilaterally and bilaterally, in a group of six patients with either complete or partial (posterior) agenesis, in a commissurotomy patient, and in healthy controls. The authors found that none of the patients presented the early visual ERP components (P1/N1) related to visual perception without high-level cognitive processing, in the hemisphere ipsilateral to the stimulus presentation, showing that posterior callosal projections are necessary for an interhemispheric exchange of visual information. Interestingly, they also found that the commissurotomy patient was not capable at correctly comparing bilaterally presented letters, but that the patients with callosal agenesis—with an intact anterior commissure—carried out the task successfully, indicating that the anterior projections are sufficient to allow for a bilateral visual matching.

To conclude, the “callosal syndrome” is mainly evident in higher order deficits occurring in the left hemispace ( Zaidel, 1983 ). The classical view of the CC as a mere connection between the two hemispheres ( Seymour et al., 1994 ) is now out of date, in favor of an integrative view of interhemispheric communications taking place by both white matter connections and bilateral subcortical projections. Anatomical and functional studies on the interhemispheric commissures allowed neuroscientists to define the specific functional role of each portion of the CC ( Fabri et al., 2014 ; Gawryluk et al., 2009 ), and the evidence collected with patients with different degrees of callosal resection further confirmed these findings (e.g., Fabri and Polonara, 2013 ).

Split-brain patients constitute a small subpopulation of epileptic patients who have received the surgical resection of the callosal fibers in an attempt to reduce the spread of epileptic foci between the cerebral hemispheres. The study of callosotomy patients allowed neuropsychologists to investigate the effects of the hemispheric disconnection, shedding more light on the perceptual and cognitive abilities of each hemisphere in isolation. This view that callosotomy completely isolates the hemispheres has now been revised, in favor of the idea of a dynamic functional reorganization of the two sides of the brain; however, the evidence collected from split-brain patients is still a milestone in the neurosciences. The right-hemispheric superiority found in the healthy population concerning face perception has been further supported with split-brains, and it has been shown that the right disconnected hemisphere appears superior to the left hemisphere in recognizing and processing faces with similar characteristics as the observers’ (e.g., gender, identity, etc.). Even more controversial is the field of hemispheric asymmetries for processing facial emotion, some evidence suggesting a right-hemispheric superiority for all emotions, some others showing a complementary hemispheric asymmetry depending on the positive or negative emotional valence. Although the practice of callosotomy is mostly abandoned today in favor of pharmacological alternatives, further studies on the remaining split-brain patients could help advance our understanding of hemispheric specialization for social stimuli.

3 Conclusions

Evidence collected with split-brain patients provide us with a better understanding of the cerebral correlates of cognitive processes. The “split-brain literature” has been a very important resource for shedding more light on hemispheric asymmetries in the most disparate domains of perception and cognition. The growing introduction of pharmacological treatments for epilepsy has resulted in a reduction in the exploitation of invasive surgical resections of the callosal projections, even if callosotomy is still performed in the most drug-resistant forms of epilepsy ( Englot et al., 2017 ; Prete and Tommasi, 2017 ). The view according to which each disconnected hemisphere reflects—in an amplified fashion—the functioning of that hemisphere in the intact brain is now less supported than in the past decades ( Corballis and Häberling, 2017 ). The overall clinical condition of split-brain patients best explains some extreme evidence of asymmetry due, for instance, to the cerebral plasticity and to the effect of patient medications (e.g., Corballis and Häberling, 2017 ). Evidence collected in split-brains precipitated Roger Sperry's 1981 Nobel Prize in Physiology or Medicine for the discoveries on the functional specialization of the cerebral hemispheres and constitute a milestone for the neurosciences. With regard to facial processing, split-brain patients’ results have revealed a right-hemispheric superiority for the processing of facial features, primarily when they are shared between the observed face and the observer (e.g., own-race bias), as well as a right-hemispheric superiority in emotional detection. The scarce group of split-brain patients still available to be tested today could provide an invaluable contribution to the unresolved issues concerning facial processing and hemispheric asymmetries.

The Cross-Cultural Brain

E. Zaidel , J. Kaplan , in Consciousness and Cognition , 2007

Publisher Summary

Everyone has two fully functioning brains inside their heads. These two brains are the left and right cerebral hemispheres, each of which is capable of perceiving the world, controlling body movements, forming memories, understanding some language, and having self-awareness. E half of the brain can carry its own mental weight partly because of the research conducted on a special population known as split-brain patients . Split-brain patients are the people who have had their hemispheres surgically separated as a treatment for epilepsy. In this surgery, the corpus callosum, the huge bundle of fibers that connects the two hemispheres, is cut. This leaves the left and right hemispheres entirely independent and unable to communicate with one another. The left and right hemispheres, of a split-brain patient, each seems to carry on their own detailed and rich, yet separate, mental lives. The differences between a split-brain person and a person with an intact corpus callosum are only a matter of degree. In the split brain, although the corpus callosum is cut, disconnecting the left and right cerebral cortices, the lower parts of the brain remain connected, which allows for some transfer of information.

Split-Brain Patients

M.E. Roser , M.S. Gazzaniga , in Encyclopedia of Neuroscience , 2009

Split-brain patients , in whom the cortical commissures, principally the corpus callosum, have been cut, provide a unique window into functional specialization of each cerebral hemisphere. Early testing of these patients, using various methods for lateralizing stimulus input and responses, confirmed hemispheric specializations suspected from previous studies of patients with lateralized brain damage. The following decades produced many examples of functional differences between the two hemispheres in the attentional, perceptual, and cognitive domains. Comparisons of partial- and complete-callosotomy patients have yielded information about functionally specific pathways through the corpus callosum. Division of the brain has also provided insight into the nature of consciousness in each hemisphere.

Split-Brain, Split-Mind

Nicole L. Marinsek , ... Michael B. Miller , in The Neurology of Conciousness (Second Edition) , 2016

A Split-Brain Conundrum and the Left Hemisphere Interpreter

The most remarkable and perplexing finding to emerge from split-brain research is the fact that, despite evidence that split-brain patients have a divided consciousness, these patients feel unified. As Zaidel writes, “Their walk is coordinated, their stride is purposeful, they perform old unilateral and bilateral skills, converse fluently and to the point, remember long-term events occurring before surgery, are friendly, kind, generous, and thoughtful to the people they know, have a sense of humor, and so on down a whole gamut of what it takes to be human” ( Zaidel, 1994, p. 9–10 ). After complete commissurotomy, split-brain patients do not exhibit distress or internal conflict, nor do they report any subjective feelings associated with a dual consciousness. So then, if both hemispheres are conscious and the conscious experiences of the two hemispheres differ, why do split-brain patients feel united?

One possibility is that the consciousness of the hemispheres is not completely split. As Roger Sperry (1984) suggested, the consciousness of split-brain patients may be Y-shaped; that is, consciousness may be divided at the hemispheric level but not at the subcortical level. Subcortical connections may help unify the conscious experiences of the hemispheres by supporting the subcortical transfer of some types of information, such as emotional valence or mood ( Gazzaniga, 2000 ). Similar inputs to the two hemispheres may also help unify their conscious experiences. Bilateral afferents, such as facial sensation, audition, pain, temperature, pressure, and proprioception, provide identical information to each hemisphere, which may lead to the recruitment of similar cognitive processes ( Sperry, 1984 ). In non-experimental settings, the two hemispheres also receive similar visual information since patients can freely explore their environments. Additionally, the two hemispheres of split-brain patients can communicate with each other via external cueing, such as by using hand gestures or language, for example. All of these facts may compensate for the lack of interhemispheric communication due to commissurotomy and may serve to lessen the division between the conscious experiences of the two disconnected hemispheres of a split-brain patient ( Gazzaniga, 2012 ).

Alternatively, split-brain patients may not feel conflicted because the left hemisphere plays a dominant role in cognition. As we have seen, the language and problem-solving abilities of left hemisphere far surpass those of the right. The relatively impoverished consciousness of the right hemisphere may prevent it from exerting control, which may in turn contribute to the feeling of a unified consciousness. However, we would expect that the right hemisphere would at least occasionally disagree with the intents and actions of the left hemisphere. Since emotions have been shown to transfer to the opposite hemisphere subcortically ( Gazzaniga, 2000 ), the dominant left hemisphere should be able to detect the discomfort of the right hemisphere in these cases. In one series of experiments, researchers instructed a split-brain patient’s disconnected hemispheres to make subjective judgments about a list of words ( LeDoux et al., 1977 ). They found that “on a day that this boy’s left and right hemispheres equally valued himself, his friends, and other matters, he was calm, tractable, and appealing. On a day when testing indicated that the right and left sides disagreed on these evaluations, the boy became difficult to manage behaviorally” (p. 420). Although this finding is limited to one patient, it suggests that conflicting conscious experiences, goals, and intentions may produce noticeable subjective feelings. However, these feelings may be wrongly interpreted as a change in mood or temperament.

Finally, and perhaps most importantly, the left-brain interpreter may preserve the subjective unity of split-brain patients. The left-brain interpreter refers to a cognitive system that is exclusive to the left hemisphere. The interpreter “makes sense of all the information bombarding the brain, interpreting our responses—cognitive or emotional—to what we encounter in our environment, asking how one thing relates to another, making hypotheses, bring order out of chaos, creating a running narrative of our actions, emotions, thoughts and dreams. The interpreter is the glue that keeps our story unified and creates our sense of being into a coherent, rational agent” ( Gazzaniga, 2008 ). The interpreter may serve to reconcile salient conflicts or feelings of disunity between the hemispheres. For example, in one experiment the left hemisphere was shown a picture of a chicken claw and right hemisphere was shown a picture of snow scene ( Gazzaniga, 2000 ). Next, the split-brain patient was asked to point, with each hand, to a card that was related to the picture it just saw. With his right hand, the patient pointed to a chicken, which matched the chicken claw, and with his left hand he point to a shovel, which matched the snow scene. When the experimenter asked the patient why he selected each item, the patient’s speaking left hemisphere rightly reported that the chicken matched the chicken claw but said, “You need the shovel to clean out the chicken shed.” The left hemisphere made an explanation for the actions of the right hemisphere based on the information that was available to it. By doing so, the patient could maintain the illusions that his actions were willful and his mind was unified and in control. On another occasion, experimenters commanded the right hemisphere of a split-brain patient to stand. The patient stood and the experimenter asked him why he did so. Again, the speaking left hemisphere created an explanation for his behavior, explaining he was thirsty and wanted to get a drink. The left hemisphere interpreter extinguishes conflict and uncertainty and, in doing so, maintains the feelings of unity and willful control.

The left hemisphere interpreter may not only rationalize behavior, but may also rationalize any distress signals it receives from the right hemisphere. If so, we may expect that, even if the right hemisphere feels distress and the emotional valence of the distress crosses over to the dominant left hemisphere, the left hemisphere interpreter may attribute the discomfort to some external cause.

Hemispheric Specialization and Cognition

M.T. Banich , in Encyclopedia of Neuroscience , 2009

Evidence from Split-Brain Patients

In the 1960s, research by Nobel laureate Roger Sperry and colleagues with split-brain patients dramatically demonstrated the relative specialization of the cerebral hemispheres. In these split-brain patients, the main nerve fiber tract connecting the cerebral hemispheres, the corpus callosum, is severed for the treatment of intractable epilepsy. As a result, higher order information, such as that about an item’s identity (e.g., a car, the letter ‘A,’ and the face of Bill Clinton), cannot be transferred from one hemisphere to the other. Thus, information directed to a single hemisphere is functionally isolated to that hemisphere. This situation provides a unique opportunity to examine the relative specialization of the cerebral hemispheres because each hemisphere’s capabilities can be examined in isolation from those of its partner. As a result, research with split-brain patients has yielded much important information about hemispheric specialization. Absolute differences have been demonstrated only for a couple of functions, namely speech output and phonological processing, which are under sole control of the left hemisphere. Both hemispheres can perform all other tasks, albeit with differing levels of ability and in different manners. Whereas the left hemisphere has a rich ability to perform most all language tasks, the vocabulary of the right hemisphere is much more limited, as is its ability to process complicated grammatical functions. On the other hand, the right hemisphere is superior at processing most types of spatial relationships, especially those involving three-dimensional relations or complicated geometries.

The Split-Brain Phenomenon Revisited: A Single Conscious Agent with Split Perception

Yair Pinto , ... Victor A.F. Lamme , in Trends in Cognitive Sciences , 2017

The Five Hallmarks of the Split-Brain Syndrome

In humans, nearly all communication between the cerebral hemispheres occurs via the corpus callosum [1–3] . In split-brain patients ( callosotomized; see Glossary), the corpus callosum is surgically severed, normally at an adult age, to alleviate otherwise intractable seizures ( Box 1 ). Thus, in split-brain patients, communication between the left and the right cerebral hemisphere is almost completely abolished. Although these patients behave normally and report to feel unchanged after the operation [4–6] , research has revealed a multitude of marked, and sometimes dramatic, changes ( Figure 1 , Key Figure; see callosal agenesis for comparison).

Midline Connections

In split-brain patients, the corpus callosum is surgically removed after age 12. The corpus callosum is by far the largest of the commissures – white matter tracts that connect homologous structures on both sides of the central nervous system – and possesses a complex architecture (e.g., [117,118] ). A large part of the corpus callosum, extending from the genu to the posterior part, connects the prefrontal cortices. Fibers from the parietal lobes cross mainly in dorsal areas of the splenium and isthmus, while the temporal lobes are largely connected via the posterior and ventral regions of the corpus callosum. The medial cortical surface is largely connected via the dorsal corpus callosum, while the fibers from the ventral regions of the brain cross ventrally (e.g., [119,120] ). The visual cortices are mainly connected via the splenium [121] . With respect to subcortical structures, there is evidence that the claustrum connects to the contralateral prefrontal cortex, precentral gyrus, and postcentral gyrus and claustrum via the body of the corpus callosum [122] .

The anterior commissure entails connections between the orbitofrontal, temporal, parietal, and occipital lobes [123] , and even the insular cortices. In terms of subcortical structures, the anterior commissure connects the olfactory bulbs, the septal area, the amygdalae, and the overlying entorhinal cortices [124] . In addition, a small number of fibers connecting both claustra pass through the anterior commissure [122] . In some split-brain patients this commissure is removed as well.

The posterior commissure connects the precentral and postcentral gyri, the superior parietal region in the left hemisphere to the temporal region, and lateral occipital and superior parietal regions of the right hemisphere [119] . The subcortical connections that run through the posterior commissure originate in the thalamic, superior colliculus and the habenular nuclei.

Other smaller commissures include the hippocampal commissure (connecting the subicular and parahippocampal cortices [124] ), the commissure of Probst (connecting the dorsal nucleus of the lateral lemniscus and the inferior colliculus), the commissure of the inferior colliculi (connecting the two inferior colliculi), the commissure of the superior colliculi (connecting the two superior colliculi), the habenular commissure (which connects the habenular nuclei), the middle commissure (connecting the thalamus), and the anterior and posterior cerebellar commissures (connecting the two cerebellar hemispheres). Finally, many fibers decussate in lower brain structures, such as the pons. Examples are the white matter tracts from the two hemispheres of the cerebellum to the cortical hemispheres (e.g., [125] ). In general, the smaller commissures and decussations are intact in split-brain patients.

Figure 1 . Key Figure: The Five Hallmarks of the Split-Brain Syndrome

The classical view of split-brain patients asserts that conscious unity is disrupted in this syndrome. The evidence for this view comes from five hallmarks. First, a marked response type   ×   visual field interaction occurs in split-brain patients [1,5,6,8,9,11] . They can only respond accurately to stimuli in the right visual field with the right hand or verbally, and to stimuli in the left visual field with the left hand. Therefore, when a stimulus appears in the left visual field, the patient verbally reports that he/she saw nothing, yet draws the image with his/her left hand. This supports the notion that each hemisphere controls half the body, and consciously perceives half the visual field. The second hallmark is extreme hemispheric specialization [8,10,12–14,36–45] . The left hemisphere is, among other things, better at language, maths, and detailed processing. The right hemisphere is better at visuospatial tasks, time perception, and causal inferencing. This again suggests that each hemisphere operates independently of the other, and thereby creates consciousness autonomously. The third striking phenomenon is that split-brain patients confabulate wildly when asked to explain actions of their left hand (controlled by the mute right hemisphere) [1,15] . The notion here is that the left hemisphere creates an independent conscious agent, who is unaware of why the right hemisphere chooses its actions. Therefore, this agent resorts to ad hoc confabulations. Fourth, in split-brain patients, each hemisphere seems to have its own focus of attention [16–19] . Since attention and consciousness are thought to be tightly linked [64–67] , this again supports the classical notion that consciousness is not unified in split-brain patients. Fifth, split-brain patients cannot compare stimuli across the midline [10,20–23,85–87] . This makes sense if two independent conscious agents each view half of the visual field, and cannot communicate their perceptions to each other.

One task has been particularly useful in documenting the cognitive changes observed in split-brain patients [6–10] . In this task, a visual stimulus is either presented to the left visual field or the right visual field. This is ensured by monitoring eye fixations and movements, and by presenting stimuli for less than 0.15   s (the minimum amount of time needed to initiate and execute an eye movement). The reason for this set up is that although both eyes project information to both cerebral hemispheres, all visual information to the left of fixation (i.e., the left visual field) is exclusively processed by the right cerebral hemisphere, while all visual information to the right of fixation (the right visual field) is processed solely by the left hemisphere. Another key aspect of this task is the response type. The patient either reacts with the left hand, the right hand, or verbally. The idea behind this is that the right hemisphere controls the left side of the body, including the left hand, and the left hemisphere controls the right hand and verbal responses. Thus, this task controls both which hemisphere receives input and which hemisphere produces output.

The prototypical split-brain patient shows five related phenomena on variations of this task. Arguably, these five aspects encompass the main differences between split-brain patients and healthy adults. The first, and most salient, observation in split-brain patients is the response   ×   visual field interaction [1,5,6,8,9,11] . When a stimulus is presented to the left visual field, the patient can only respond adequately with his/her left hand, and vice versa for the right field and hand. The second aspect concerns the hemispheric specialization, with each of the two hemispheres being better at certain tasks [8,10,12–14] . The third aspect focusses on post hoc confabulations after actions with the left hand [1,15] . The fourth characteristic concerns the observation that each hemisphere may have its own independent focus of attention [16–19] . Finally, there is abundant evidence that shows that split-brain patients are incapable of comparing stimuli across the visual midline [10,20–23] .

Altogether these five observations have led to what we dub the classical models of split-brain patients. There are two primary classical models. The first model revolves around the notion that only the left hemisphere gives rise to consciousness, while the right hemisphere only processes information in an unconscious manner. The right hemisphere may prime the left hemisphere toward certain behavior, but this will only affect consciousness after it has been molded and interpreted by the left hemisphere. This is the so-called partial consciousness model [1,11] . The second classical model posits that in a split-brain patient each hemisphere has its own consciousness, independent of the other hemisphere [5,6,24] . Thus, according to this ‘ split consciousness ’ model, a split-brain patient houses two independent conscious agents. This model has been concisely argued by Sperry [24] who wrote that the two hemispheres acted as if they were ‘two separate conscious entities or minds running in parallel in the same cranium, each with its own sensations, perceptions, cognitive processes’ (p. 318).

By contrast, we will argue that despite their prominence, these classical models face serious challenges. This is because three of the five hallmarks (hemispheric specialization, post hoc confabulations, and split attention) also exist in healthy adults with unified consciousness . Thus, these hallmarks cannot constitute proof for disturbances in conscious unity. Moreover, one hallmark (inability to compare across the midline) also fits the more modest explanation that visual processing is unintegrated. In general, a more extreme explanation (destroyed conscious unity) should only be preferred if simpler explanations do not suffice. Finally, the strongest proof for a breakdown of unified consciousness, the response   ×   visual field interaction, does not hold for all split-brain patients. We posit a new model of the split-brain syndrome, which claims that both hemispheres give rise to a single conscious agent, and discuss its wider implications.

Five hallmarks characterize the split-brain syndrome: a response   ×   visual field interaction, strong hemispheric specialization, confabulations after left-hand actions, split attention, and the inability to compare stimuli across the midline.

These hallmarks underlie the classical notion that split brain implies split consciousness. This notion suggests that massive interhemispheric communication is necessary for conscious unity.

Closer examination challenges the classical notion. Either the hallmark also occurs in healthy adults or the hallmark does not hold up for all split-brain patients .

A re-evaluation of the split-brain data suggests a new model that might better account for the data. This model asserts that a split-brain patient is one conscious agent with unintegrated visual perception.

This new model challenges prominent theories of consciousness, since it implies that massive communication is not needed for conscious unity.

What Split-Brain Patients Tell Us about Consciousness

Split-brain patients demonstrate that an intact corpus callosum and, correspondingly, interhemispheric communication are not essential for consciousness. Considering the great abundance of interhemispheric functional connections in the healthy brain ( Doron et al., 2012; Salvador et al., 2005 ), it is surprising that consciousness survives callosotomy, in which nearly all interhemispheric communication is lost. But it does—the two disconnected hemispheres of a split-brain patient appear to possess all of the defining attributes of consciousness ( Damasio and Meyer, 2009 ), such as wakefulness, emotion, attention, and purposeful behavior ( Gazzaniga, 2000; LeDoux et al., 1977 ). However, although consciousness persists after hemispheric disconnection, the conscious experience of split-brain patients may differ from that of healthy individuals since each disconnected hemisphere (especially the right) possesses only a subset of the intact brain’s gamut of cognitive functions.

One possible explanation for the preserved consciousness of split-brain patients is that each hemisphere’s thalamocortical connections remain intact following commissurotomy. Several theories of consciousness, such as the global workspace model ( Newman et al., 1997 ) or dynamic core model ( Tononi and Edelman, 1998 ), posit that consciousness arises through reentrant neural activity of thalamocortical loops ( Llinas et al., 1998; Newman et al., 1997; Tononi and Koch, 2008; Tononi, 2004; Ward, 2011 ). In line with these theories, each hemisphere of a split-brain patient may remain conscious following commissurotomy because the connections between the cortex and thalamus in each hemisphere remain intact. In a related vein, each hemisphere may remain conscious following surgery because of spared cortico-cortical connections. In support of this view, recent fMRI ( Monti et al., 2013 ) and EEG ( Boly et al., 2012 ) studies have shown that loss of consciousness is associated with a decrease in cortico-cortical connectivity, even when thalamocortical connectivity remains unchanged. Spared subcortical and cortico-cortical connections may enable bilateral functional connectivity after split-brain surgery ( Uddin et al., 2008 ). In a recent study, O’ Reilly et al. (2013) measured rhesus monkeys’ resting state functional connectivity with fMRI before and after callosotomy. They found that corpus callosum sectioning abolished nearly all interhemispheric functional connectivity, but these effects were mitigated if the anterior commissure was spared. In sum, these theories suggest that both hemispheres of a split-brain patient are conscious because subcortical and intra-hemispheric cortico-cortical connections remain intact following commissurotomy.

Another possible explanation for the preserved consciousness of split-brain patients stems from Giulio Tononi’s integrated information theory (IIT) of consciousness, which posits that consciousness is proportional to the capacity of a system to integrate information, such that greater information integration produces richer conscious experiences ( Tononi and Edelman, 1998; Tononi, 2004, 2008 ; Oizumi et al., 2014 ). Integrated information, denoted by phi, is determined by a system’s degree of differentiation (a measure of possible conscious experiences) and integration (the degree to which information across modules is unified). According to the theory, the human brain has a large phi—and therefore a rich conscious experience—because it consists of many specialized modules (high differentiation) that can form countless functional assemblies (high integration). The brain’s capacity to integrate information decreases after commissurotomy because interhemispheric connections are lost (less integration) and each hemisphere has fewer specialized modules and possible conscious states than an intact brain (less differentiation). Therefore, according to IIT, the consciousness of each hemisphere should correspondingly decrease. In a computation model of split-brain patients’ capacity for information integration, Tononi (2004) estimates that phi equals 72 in the united brain but only 61 in each disconnected hemisphere. The model predicts that phi decreases after commissurotomy, but not drastically so, because many cognitive functions are present in both cerebral hemispheres. The model is consistent with research on split-brain patients, with one exception: as we have seen, both hemispheres appear to be conscious following commissurotomy, but the conscious experience of the left hemisphere is superior to that of the right. In relation to the integrated information model, these findings may suggest that the disconnected left hemisphere has a greater capacity to integrate information—and therefore a greater phi—than the right hemisphere.

Related terms:

• Anterior Cingulate Cortex
• Cognitive Function
• Cognitive Process
• Consciousness
• Visual Search
• Functional Connectivity
• Facial Expression
• Superior Colliculus
• Prefrontal Cortex

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Split brain: divided perception but undivided consciousness

Affiliations.

• 1 Department of Psychology, University of Amsterdam, Amsterdam, The Netherlands.
• 2 Amsterdam Brain and Cognition (ABC) Center, University of Amsterdam, The Netherlands.
• 3 Donders Institute for Brain, Cognition and Behaviour, Radboud University, Nijmegen, The Netherlands.
• 4 School of Psychology, University of Auckland, Auckland, New Zealand.
• 5 Epilepsy Center-Neurological Clinic, Azienda 'Ospedali Riuniti', Ancona, Italy.
• 6 Department of Experimental and Clinical Medicine, Marche Politechnical University, Ancona, Italy.
• PMID: 28122878
• DOI: 10.1093/brain/aww358

In extensive studies with two split-brain patients we replicate the standard finding that stimuli cannot be compared across visual half-fields, indicating that each hemisphere processes information independently of the other. Yet, crucially, we show that the canonical textbook findings that a split-brain patient can only respond to stimuli in the left visual half-field with the left hand, and to stimuli in the right visual half-field with the right hand and verbally, are not universally true. Across a wide variety of tasks, split-brain patients with a complete and radiologically confirmed transection of the corpus callosum showed full awareness of presence, and well above chance-level recognition of location, orientation and identity of stimuli throughout the entire visual field, irrespective of response type (left hand, right hand, or verbally). Crucially, we used confidence ratings to assess conscious awareness. This revealed that also on high confidence trials, indicative of conscious perception, response type did not affect performance. These findings suggest that severing the cortical connections between hemispheres splits visual perception, but does not create two independent conscious perceivers within one brain.

Keywords: consciousness; epilepsy; neurosurgery; split-brain; visual fields.

© The Author (2017). Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For Permissions, please email: [email protected].

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• Consciousness post corpus callosotomy. Meador KJ, Loring DW, Sathian K. Meador KJ, et al. Brain. 2017 Jul 1;140(7):e38. doi: 10.1093/brain/awx106. Brain. 2017. PMID: 28460007 No abstract available.

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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