corpus callosum severed experiment

The renowned American neuropsychologist Michael Gazzaniga, who began his career working with Roger Sperry, has developed several devices for analyzing functional differences between the two hemispheres in split-brain patients. The idea behind these devices is to deliver stimuli in such a way that they reach only one hemisphere, and then to observe how this hemisphere manages to process these stimuli on its own.

To study language, Gazzaniga asked his subjects to focus on a point at the centre of a screen. He then projected images, words, and phrases onto the screen, to the left or right of this point. By flashing these items quickly enough that the subjects’ eyes had no time to move, Gazzaniga was able to “talk” to just one of the hemispheres at a time. Information projected in the subjects’ left visual field was received by the right hemisphere, while information projected in the right visual field was received by the left.

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One Brain. Two Minds? Many Questions

For several decades, split-brain research has provided valuable insight into the fields of psychology and neuroscience. These studies have progressed our knowledge of hemispheric specialization, language processing, the role of the corpus callosum, cognition, and even human consciousness. Following a recent empirical paper by Pinto et al. (2017a) and review by Volz and Gazzaniga (2017) , a debate has ensued about the nature of conscious perception of visual stimuli in split-brain patients. This exchange is an ideal platform for generating discussion about both the implications of recent findings and the interpretation of results from split-brain studies in general.

From its beginnings fifty years ago, split-brain research has continually proved to be a vital field within the greater scope of psychology and neuroscience. Split-brain research refers to research and insights garnered from studying patients who have had their corpus callosum, a bundle of fibers connecting the two hemispheres of the brain, severed, in most cases to treat severe epilepsy. This unique condition, combined with a novel technique of presenting information to each hemisphere independently, led to a field that has been prominent for five decades, and still continues to produce new and exciting revelations in neuroscience. However, the field also continues to spark debate and controversy. This is best demonstrated by a recent exchange in journal Brain .

In a 2017 empirical paper, Pinto and colleagues offer evidence against a dominant view in split-brain research: that after severing the corpus callosum visual information cannot be transferred through other fibers ( Pinto et al., 2017a ). Going even further, they interpret results indicative of conscious reporting across hemispheres as suggesting the two hemispheres are not separately conscious following the surgery. In their recent review, Volz and Gazzaniga (2017) , argue against these interpretations by Pinto et al. Together, these papers triggered a debate within the field leading to further responses in the form of letters to the editor from Pinto et al. (2017b) , Volz et al. (2018) , and Corballis et al. (2018) . Here, I summarize each component of the current debate, and also argue why the exchange as a whole can serve as a valuable teaching tool.

I will start by summarizing sections of the review by Volz and Gazzaniga (2017) that give context to both this exchange and the field as a whole. A group of patients in Rochester, New York in 1939 were the first to undergo surgery designed to treat severe epilepsy by severing the corpus callosum, but these first patients were not actually the first group of split-brain patients that we think of today. That is because though they were studied extensively, these patients appeared not to be significantly different after the surgery compared to before ( Akelaitis, 1941 ). This conclusion was accepted by many for two decades, until a novel experimental design was able to present information to each hemisphere in isolation, which for the first time gave experimenters the ability to observe the two hemispheres individually ( Gazzaniga and Sperry, 1967 ; Volz & Gazzaniga, 2017 ). I am including this not just as an interesting anecdote, but also because it is a great example of how difficult it can be to design an experiment in split-brain research. In this line of research, it is of the utmost importance that each hemisphere receives information independently. Because of the nature of the condition and the way patients learn to adapt to their new circumstance after surgery, this is not trivial, and therefore relevant for the debate at hand.

Because of the straightforward nature of the visual system when compared with our knowledge of how the other senses are processed, it is commonly used to deliver stimuli in split-brain experiments ( Volz and Gazzaniga, 2017 ). To explain briefly how this works, when an image is shown in right visual field, it is ‘seen’ and processed by the left hemisphere and vice versa. Meaning, if a split-brain patient were to see information only in one half of their visual space, it would be processed only by the contralateral hemisphere ( Volz and Gazzaniga, 2017 ). Interestingly though, when an object is shown in the right visual field and the patient is asked what was seen they can and do answer correctly, but when shown an object in left visual field and asked the same question, the patient will often answer that nothing was seen ( Volz and Gazzaniga, 2017 ). This is because the left hemisphere houses most language capabilities. So, when something is presented in the right visual field (to the left hemisphere) patients are able to respond verbally; however, when an image is presented in the left visual field, though the patient may not be able to respond verbally, they are able to non-verbally. For example, participants can use their left hands (controlled by the right hemisphere) to point out what was seen from a group of objects ( Volz and Gazzaniga, 2017 ).

In their 2017 empirical paper, Pinto et al. (2017a) nicely summarize this phenomenon postulating that the left hemisphere can only perceive the right side of visual space with expression through verbal language and the right hand, while the right hemisphere can only perceive the left side of visual space with expression through the left hand. However, following this summary, Pinto et al. (2017a) also mention that though this is widely taught and believed, there are no quantitative data supporting the idea, only clinical observations.

Now I will outline the empirical findings by Pinto et al. (2017a) that have sparked the current controversy. The researchers studied two split-brain patients, and though some of their results replicate past findings, others appear to challenge the status quo in the field. While two patients may seem like a small number, Pinto et al. justify this by explaining that there are very few split-brain patients remaining today. It is also worth noting that both patients were tested at least a decade after surgery. In their first experiment, Pinto and colleagues (2017a) examined if the patients could detect a stimulus and indicate its location when presented in only one visual half field. They asked patients to respond with their left hands, right hands, and verbally. Researchers observed near perfect accuracy for detection of the stimulus, regardless of response type (left hand, right hand, verbal), and well above chance accuracy for indicating location ( Pinto et al., 2017a ). Even more interesting, however, is that there was no observed interaction between response type and stimulus location (left visual field, right visual field).

This led to further testing to determine if the results above could be due to transfer of visual information across the two hemispheres. In follow-up experiments only one of the patients was asked to compare stimuli across and within visual half fields, as well as name and match pictures within visual half fields. The patient could not compare stimuli across half fields but was able to within half fields. Additionally, the same patient showed better performance when labeling objects presented to the right visual field, and matching objects presented to the left ( Pinto et al., 2017a ). These findings, consistent with previous research, suggest that visual processing is indeed independent for each hemisphere in split-brain patients. However, the authors note there was still no interaction between response type and visual field. This leaves the question of how patients were able to correctly report what was processed regardless of which side did the processing. To test if this phenomenon was due to conscious or unconscious processes, the experimenters asked the patient to complete similar testing, but this time with confidence ratings. Based on confidence ratings being higher for correct responses, the researchers concluded that the patient was indeed consciously aware of his reporting. Again, there was no interaction between response type and stimulus location ( Pinto et al., 2017a ).

The authors entertain several interpretations of their data, but ultimately, they take the stance that that visual perception remains divided in split-brain patients, but that in reporting what was perceived, consciousness is undivided. They refer to this as “‘split phenomenality’ combined with ‘unity of consciousness’” ( Pinto et al., 2017a ). This interpretation lies in direct contrast with both previous theories of processing in split-brain patients and dominant theories of consciousness.

Pinto and colleagues (2017a) go into a lengthy explanation as to why cross-cueing should be ruled out. First, they define cross-cueing as “one hemisphere informing the other hemisphere with behavioral ticks, such as touching the left hand with the right hand” and that it can only transfer “one bit of information” ( Pinto et al., 2017a ). Using this definition, they claim cross-cueing is not likely responsible for their results. They reason that: 1) cross-cueing could not transfer the amount of information needed for correct responses, 2) there were significant differences in performance on visual tasks between hemifields (this refers to the experiment in which the patient was better at matching objects shown in the left visual field but better at labeling objects shown in the right visual field), 3) the experiment was set up to prevent hands from touching each other, 4) in an experiment of reaction times with a colored circle appearing in either the left or right visual field there were no significant time differences between ipsilateral and contralateral responses, which would be expected if cross-cueing were to take place as it should slow down ipsilateral responses. After this lengthy discussion on cross-cueing, the authors conclude with one final possibility that because testing began several years after the operation and both patients were operated on as young adults, it could be that over time patients develop new structural connections to transfer information across hemispheres ( Pinto et al., 2017a ).

Switching back to the review by Volz and Gazzaniga (2017) , after summarizing basics in the field, the authors take the time to discuss recent findings focusing primarily on the empirical paper by Pinto et al. (2017a) . Volz and Gazzaniga (2017) describe cross-cueing as one hemisphere using knowledge gained by perceiving behavioral cues from the other to overcome a challenge or complete a task that would require information to be shared between hemispheres. The authors also note that this is not done actively or consciously and the cues can often be exceptionally subtle. This emphasis on subtle cues marks a difference in definition of cross-cueing between the two sets of authors, which is noted in the review. Volz and Gazzaniga (2017) critique Pinto et al.’s (2017a) willingness to write-off cross-cueing far too quickly. Although Pinto et al. (2017a) used eye tracking technology to ensure the patient was fixating (maintaining visual gaze on a specific location) during stimulus presentation, fixation was not monitored while the patient was responding. According to Volz and Gazzaniga (2017) this meant that cross-cueing could occur in the form of an eye movement when asked to indicate the location of the stimulus.

Pinto and colleagues (2017b) subsequently responded to Volz and Gazzaniga’s review in a letter to the editor of Brain . In this letter they once again assert why they believe cross-cueing is an unlikely explanation, responding more specifically to points brought up in the review. They contend that even cross-cueing cannot explain the lack of an interaction between response type and location. Though they do give way that an alternative explanation broached by Volz and Gazzaniga (2017) (transfer through subcortical routes) could be more likely, they assert that there is a larger problem in the whole interpretation framework, namely that the term cross-cueing is not clearly defined ( Pinto et al. 2017b ). In a subsequent reply to Pinto et al. (2017b) , Volz and colleagues (2018) concede that the lack of a formal definition of cross-cueing is a significant issue, but still reassert their stance. They emphasize that due to the passing of time between the patients’ surgery and testing, they could have learned much more subtle and efficient ways to transfer information through behavioral cues. In a final response in the form of a letter to the editor, a third party weighs in. Corballis and colleagues (2018) cite the ongoing debate and argue that it is a mistake to focus so heavily on cross-cueing. Instead the authors assert that both groups should return to the idea of subcortical routes. The authors provide anatomical evidence citing a ‘second visual system’ pathway involving midbrain structures. This pathway is believed to go through the superior colliculi, the pulvinar nuclei, and subsequently to the parietal lobes with a subcortical interhemispheric connection at the collicular commissure ( Trevarthen and Sperry, 1973 ; Corballis et al., 2018 ). In addition to the anatomical evidence, Corballis et al (2018) summarize results from previous behavioral experiments involving split-brain patients that support this possibility. Overall the authors make a strong case for subcortical connections as a possible explanation for Pinto and colleagues’ (2017a) observations.

The above exchange serves as an example of a lively and provocative conversation in neuroscience emerging from competing interpretations of published data. The value of this exchange as a teaching tool comes not from which interpretation (if any) the reader chooses to accept, but rather from understanding why these different interpretations exist, and how each group of authors was able to use scientific evidence to support their ideas. In a classroom setting, research is often presented as producing facts, but it is important to remember that different scientists can draw different conclusions from the same data. This means that our interpretations of scientific work are just as much a part of science as the actual evidence. Though this may seem obvious to researchers, it is something that is often overlooked by students.

The current debate in split-brain research brings the audience’s attention to critical components of scientific research in general, including experimental design and interpretation, as well as communication within the field. Though the separate sets of authors may disagree, they communicate effectively and publicly, and in doing so demonstrate that there can be wide variation in interpretation of scientific evidence which can largely affect the implications of a study as well as guide future research.

In addition to being a great teaching tool for the aspects mentioned above, this exchange is also useful in that it can introduce students to a variety of publication types. The inclusion of an empirical paper, a review, and responses in the form of letters to the editor, teaches students that scientific research is not done in isolation, and shows how and when to use different forms of publication.

I believe there is a place for this set of papers in almost any introductory psychology or neuroscience class, as well as cognitive neuroscience classes. Additionally, this exchange could be especially useful in upper level psychology and neuroscience classes with a focus on evaluating scientific literature, interpretation, or experimental design. The authors’ emphasis on critical thinking and interpretation creates a springboard for classroom discussion and ideas for future directions in the field.

If I were to teach this exchange in a classroom, I would have students read these manuscripts in the order I have presented them here: starting with the review by Volz and Gazzaniga which contains relevant background of the field, followed by the empirical by paper by Pinto et al. (2017a) . I would then ask the students to discuss if they believe the criticism in the review was fair and why (or why not). Afterwards, I would follow up the discussion with the three letters to the editor and ask the students to decide which interpretation they side with and why, or to come up with their own interpretation supported by empirical evidence.

Regardless of how this set of papers is taught, it has the potential to stimulate thought and discussion. It will be exciting to see how this debate continues to develop over time.

The author would like to thank all involved in the University of St. Andrews MRes in Neuroscience program, especially Dr. Stefan Pulver.

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• Published: 14 March 2012

The split brain: A tale of two halves

• David Wolman 1  

Nature volume  483 ,  pages 260–263 ( 2012 ) Cite this article

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• Medical research
• Neurosurgery

Since the 1960s, researchers have been scrutinizing a handful of patients who underwent a radical kind of brain surgery. The cohort has been a boon to neuroscience — but soon it will be gone.

In the first months after her surgery, shopping for groceries was infuriating. Standing in the supermarket aisle, Vicki would look at an item on the shelf and know that she wanted to place it in her trolley — but she couldn't. “I'd reach with my right for the thing I wanted, but the left would come in and they'd kind of fight,” she says. “Almost like repelling magnets.” Picking out food for the week was a two-, sometimes three-hour ordeal. Getting dressed posed a similar challenge: Vicki couldn't reconcile what she wanted to put on with what her hands were doing. Sometimes she ended up wearing three outfits at once. “I'd have to dump all the clothes on the bed, catch my breath and start again.”

In one crucial way, however, Vicki was better than her pre-surgery self. She was no longer racked by epileptic seizures that were so severe they had made her life close to unbearable. She once collapsed onto the bar of an old-fashioned oven, burning and scarring her back. “I really just couldn't function,” she says. When, in 1978, her neurologist told her about a radical but dangerous surgery that might help, she barely hesitated. If the worst were to happen, she knew that her parents would take care of her young daughter. “But of course I worried,” she says. “When you get your brain split, it doesn't grow back together.”

In June 1979, in a procedure that lasted nearly 10 hours, doctors created a firebreak to contain Vicki's seizures by slicing through her corpus callosum, the bundle of neuronal fibres connecting the two sides of her brain. This drastic procedure, called a corpus callosotomy, disconnects the two sides of the neocortex, the home of language, conscious thought and movement control. Vicki's supermarket predicament was the consequence of a brain that behaved in some ways as if it were two separate minds.

After about a year, Vicki's difficulties abated. “I could get things together,” she says. For the most part she was herself: slicing vegetables, tying her shoe laces, playing cards, even waterskiing.

But what Vicki could never have known was that her surgery would turn her into an accidental superstar of neuroscience. She is one of fewer than a dozen 'split-brain' patients, whose brains and behaviours have been subject to countless hours of experiments, hundreds of scientific papers, and references in just about every psychology textbook of the past generation. And now their numbers are dwindling.

Through studies of this group, neuroscientists now know that the healthy brain can look like two markedly different machines, cabled together and exchanging a torrent of data. But when the primary cable is severed, information — a word, an object, a picture — presented to one hemisphere goes unnoticed in the other. Michael Gazzaniga, a cognitive neuroscientist at the University of California, Santa Barbara, and the godfather of modern split-brain science, says that even after working with these patients for five decades, he still finds it thrilling to observe the disconnection effects first-hand. “You see a split-brain patient just doing a standard thing — you show him an image and he can't say what it is. But he can pull that same object out of a grab-bag,” Gazzaniga says. “Your heart just races!”

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Michael Gazzaniga reflects on five decades of split-brain research

Work with the patients has teased out differences between the two hemispheres, revealing, for instance, that the left side usually leads the way for speech and language computation, and the right specializes in visual-spatial processing and facial recognition. “The split work really showed that the two hemispheres are both very competent at most things, but provide us with two different snapshots of the world,” says Richard Ivry, director of the Institute of Cognitive and Brain Sciences at the University of California, Berkeley. The idea of dichotomous consciousness captivated the public, and was greatly exaggerated in the notion of the 'creative right brain'. But further testing with split-brain patients gave a more-nuanced picture. The brain isn't like a computer, with specific sections of hardware charged with specific tasks. It's more like a network of computers connected by very big, busy broadband cables. The connectivity between active brain regions is turning out to be just as important, if not more so, than the operation of the distinct parts. “With split-brain patients, you can see the impact of disconnecting a huge portion of that network, but without damage to any particular modules,” says Michael Miller, a psychologist at the University of California, Santa Barbara.

David Roberts, head of neurosurgery at Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire, sees an important lesson in split-brain research. He operated on some of the cohort members, and has worked closely with Gazzaniga. “In medical school, and science in general, there is so much emphasis on large numbers, labs, diagnostics and statistical significance,” Roberts says — all crucial when, say, evaluating a new drug. But the split-brain cohort brought home to him how much can be gleaned from a single case. “I came to learn that one individual, studied well, and thoughtfully, might enable you to draw conclusions that apply to the entire human species,” he says.

Today, the split-brain patients are getting on in years; a few have died, one has had a stroke and age in general has made them all less fit for what can be taxing research sessions of sitting, staring and concentrating. The surgery, already quite rare, has been replaced by drug treatments and less drastic surgical procedures. Meanwhile, imaging technologies have become the preferred way to look at brain function, as scientists can simply watch which areas of the brain are active during a task.

But to Miller, Ivry, Gazzaniga and others, split-brain patients remain an invaluable resource. Imaging tools can confirm, for example, that the left hemisphere is more active than the right when processing language. But this is dramatically embodied in a split-brain patient, who may not be able to read aloud a word such as 'pan' when it's presented to the right hemisphere, but can point to the appropriate drawing. “That gives you a sense of the right hemisphere's ability to read, even if it can't access the motor system to produce speech,” Ivry says. “Imaging is very good for telling you where something happens,” he adds, “whereas patient work can tell you how something happens.”

A cable, cut

Severing the corpus callosum was first used as a treatment for severe epilepsy in the 1940s, on a group of 26 people in Rochester, New York. The aim was to limit the electrical storm of the seizure to one side of the brain. At first, it didn't seem to work. But in 1962, one patient showed significant improvement. Although the procedure never became a favoured treatment strategy — it's invasive, risky, and drugs can ease symptoms in many people — in the decades since it nevertheless became a technique of last resort for treating intractable epilepsy.

To Roger Sperry, then a neurobiologist and neuropsychologist at the California Institute of Technology, and Gazzaniga, a graduate student in Sperry's lab, split-brain patients presented a unique opportunity to explore the lateralized nature of the human brain. At the time, opinion on the matter was itself divided. Researchers who studied the first split-brain patients in the 1940s had concluded that the separation didn't noticeably affect thought or behaviour. (Gazzaniga and others suspect that these early sections were incomplete, which might also explain why they didn't help the seizures.) Conversely, studies conducted by Sperry and colleagues in the 1950s revealed greatly altered brain function in animals that had undergone callosal sections. Sperry and Gazzaniga became obsessed with this inconsistency, and saw in the split-brain patients a way to find answers.

The duo's first patient was a man known as W. J., a former Second World War paratrooper who had started having seizures after a German soldier clocked him in the head with the butt of a rifle. In 1962, after W.J.'s operation, Gazzaniga ran an experiment in which he asked W.J. to press a button whenever he saw an image. Researchers would then flash images of letters, light bursts and other stimuli to his left or right field of view. Because the left field of view is processed by the right hemisphere and vice versa, flashing images quickly to one side or the other delivers the information solely to the intended hemisphere (see 'Of two minds').

For stimuli delivered to the left hemisphere, W.J. showed no hang-ups; he simply pressed the button and told the scientists what he saw. With the right hemisphere, W.J. said he saw nothing, yet his left hand kept pressing the button every time an image appeared. “The left and right didn't know what the other was doing,” says Gazzaniga. It was a paradigm-blasting discovery showing that the brain is more divided than anyone had predicted 1 .

Suddenly, the race was on to delve into the world of lateralized function. But finding more patients to study proved difficult. Gazzaniga estimates that at least 100 patients, and possibly many more, received a corpus callosotomy. But individuals considered for the operation tend to have other significant developmental or cognitive problems; only a few have super-clean cuts and are neurologically healthy enough to be useful to researchers. For a while, Sperry, Gazzaniga and their colleagues didn't know if there was ever going to be anyone else like W.J..

But after contacting neurosurgeons, partnering with epilepsy centres and assessing many potential patients, they were able to identify a few suitable people in California, then a cluster from the eastern part of the United States, including Vicki. Through the 1970s and the early 1980s, split-brain research expanded, and neuroscientists became particularly interested in the capabilities of the right hemisphere — the one conventionally believed to be incapable of processing language and producing speech.

Gazzaniga can tick through the names of his “endlessly patient patients” with the ease of a proud grandparent doing a roll call of grandchildren — W.J., A.A., R.Y., L.B., N.G.. For medical confidentiality, they are known in the literature by initials only. (Vicki agreed to be identified in this article, provided that her last name and hometown were not published.)

On stage last May, delivering a keynote address at the Society of Neurological Surgeons' annual meeting in Portland, Oregon, Gazzaniga showed a few grainy film clips from a 1976 experiment with patient P.S., who was only 13 or 14 at the time. The scientists wanted to see his response if only his right hemisphere saw written words.

In Gazzaniga's video, the boy is asked: who is your favourite girlfriend, with the word girlfriend flashed only to the right hemisphere. As predicted, the boy can't respond verbally. He shrugs and shakes his head, indicating that he doesn't see any word, as had been the case with W.J.. But then he giggles. It's one of those tell-tale teen giggles — a soundtrack to a blush. His right hemisphere has seen the message, but the verbal left-hemisphere remains unaware. Then, using his left hand, the boy slowly selects three Scrabble tiles from the assortment in front of him. He lines them up to spell L-I-Z: the name, we can safely assume, of the cute girl in his class. “That told us that he was capable of language comprehension in the right hemisphere,” Gazzaniga later told me. “He was one of the first confirmation cases that you could get bilateral language — he could answer queries using language from either side.”

The implications of these early observations were “huge”, says Miller. They showed that “the right hemisphere is experiencing its own aspect of the world that it can no longer express, except through gestures and control of the left hand”. A few years later, the researchers found that Vicki also had a right-hemisphere capacity for speech 2 . Full callosotomy, it turned out, resulted in some universal disconnections, but also affected individuals very differently.

In 1981, Sperry was awarded a share of the Nobel Prize in Physiology or Medicine for the split-brain discoveries. (“He deserved it,” Gazzaniga says.) Sperry died in 1994, but by that point, Gazzaniga was leading the charge. By the turn of the century, he and other split-brain investigators had turned their attention to another mystery: despite the dramatic effects of callosotomy, W.J. and later patients never reported feeling anything less than whole. As Gazzaniga wrote many times: the hemispheres didn't miss each other.

Gazzaniga developed what he calls the interpreter theory to explain why people — including split-brain patients — have a unified sense of self and mental life 3 . It grew out of tasks in which he asked a split-brain person to explain in words, which uses the left hemisphere, an action that had been directed to and carried out only by the right one. “The left hemisphere made up a post hoc answer that fit the situation.” In one of Gazzaniga's favourite examples, he flashed the word 'smile' to a patient's right hemisphere and the word 'face' to the left hemisphere, and asked the patient to draw what he'd seen. “His right hand drew a smiling face,” Gazzaniga recalled. “'Why did you do that?' I asked. He said, 'What do you want, a sad face? Who wants a sad face around?'.” The left-brain interpreter, Gazzaniga says, is what everyone uses to seek explanations for events, triage the barrage of incoming information and construct narratives that help to make sense of the world.

The split-brain studies constitute “an incredible body of work”, said Robert Breeze, a neurosurgeon at the University of Colorado Hospital in Aurora, after listening to Gazzaniga's lecture last year. But Breeze, like many other neuroscientists, sees split-brain research as outdated. “Now we have technologies that enable us to see these things” — tools such as functional magnetic resonance imaging (fMRI) that show the whereabouts of brain function in great detail.

Miller, however, disagrees. “These kinds of patients can tell us things that fMRI can never tell us,” he says.

Subject of interest

Seated at a small, oval dining-room table, Vicki faces a laptop propped up on a stand, and a console with a few large red and green buttons. David Turk, a psychologist at the University of Aberdeen, UK, has flown in for the week to run a series of experiments.

Vicki's grey-white hair is pulled back in a ponytail. She wears simple white sneakers and, despite the autumn chill, shorts. She doesn't want to get too warm: when that happens she can get drowsy and lose focus, which can wreck a whole day of research.

During a break, Vicki fetches an old photo album. In one picture, taken soon after her surgery, she is sitting up in the hospital bed. Her hair is starting to grow back as black stubble and she and her daughter have wide smiles. Another page of the album has a slightly faded printout of a 1981 paper from The Journal of Neuroscience glued into it: the first published report involving data gleaned from Vicki, in which researchers describe how she, like P.S., had some capacity for language in her right hemisphere 4 .

I have a hard time saying it's all over.

When pressed to share the most difficult aspect of her life in science, the perpetually upbeat Vicki says that it would have to be an apparatus called the dual Purkinje eye tracker. This medieval-looking device requires the wearer to bite down on a bar to help keep the head still so that researchers can present an image to just the left or right field of view. It is quite possible that Vicki has spent more of her waking hours biting down on one of those bars than anyone else on the planet.

Soon, it is time to get back to work. Turk uses some two-sided tape to affix a pair of three-dimensional glasses onto the front of Vicki's thin, gold-rimmed bifocals. The experiment he is running aims to separate the role of the corpus callosum in visual processing from that of deeper, 'subcortical' connections unaffected by the callosotomy. Focusing on the centre of the screen, Vicki is told to watch as the picture slowly switches between a house and different faces — and to press the button every time she sees the image change. Adjusting her seat, she looks down the bridge of her nose at the screen and tells Turk that she's ready to begin.

Deep connections

Other researchers are studying the role of subcortical communication in the coordinated movements of the hands. Split-brain patients have little difficulty with 'bimanual' tasks, and Vicki and at least one other patient are able to drive a car. In 2000, a team led by Liz Franz at the University of Otago in New Zealand asked split-brain patients to carry out both familiar and new bimanual tasks. A patient who was an experienced fisherman, they found, could pantomime tying a fishing line, but not the unfamiliar task of threading a needle. Franz concluded that well-practised bimanual skills are coordinated at the subcortical level, so split-brain people are able to smoothly choreograph both hands 5 .

Miller and Gazzaniga have also started to study the right hemisphere's role in moral reasoning. It is the kind of higher-level function for which the left hemisphere was assumed to be king. But in the past few years, imaging studies have shown that the right hemisphere is heavily involved in the processing of others' emotions, intentions and beliefs — what many scientists have come to understand as the 'theory of mind' 6 . To Miller, the field of enquiry perfectly illustrates the value of split-brain studies because answers can't be found by way of imaging tools alone.

In work that began in 2009, the researchers presented two split-brain patients with a series of stories, each of which involved either accidental or intentional harm. The aim was to find out whether the patients felt that someone who intends to poison his boss but fails because he mistakes sugar for rat poison, is on equal moral ground with someone who accidentally kills his boss by mistaking rat poison for sugar 7 . (Most people conclude that the former is more morally reprehensible.) The researchers read the stories aloud, which meant that the input was directed to the left hemisphere, and asked for verbal responses, so that the left hemisphere, guided by the interpreter mechanism, would also create and deliver the response. So could the split-brain patients make a conventional moral judgement using just that side of the brain?

No. The patients reasoned that both scenarios were morally equal. The results suggest that both sides of the cortex are necessary for this type of reasoning task.

But this finding presents an additional puzzle, because relatives and friends of split-brain patients do not notice unusual reasoning or theory-of-mind deficits. Miller's team speculates that, in everyday life, other reasoning mechanisms may compensate for disconnection effects that are exposed in the lab. It's an idea that he plans to test in the future.

As the opportunities for split-brain research dwindle, Gazzaniga is busy trying to digitize the archive of recordings of tests with cohort members, some of which date back more than 50 years. “Each scene is so easy to remember for me, and so moving,” he says. “We were observing so many astonishing things, and others should have the same opportunity through these videos.” Perhaps, he says, other researchers will even uncover something new.

Other split-brain patients may become available — there is a small cluster in Italy, for instance. But with competition from imaging research and many of the biggest discoveries about the split brain behind him, Gazzaniga admits that the glory days of this field of science are probably gone. “It is winding down in terms of patients commonly tested.” Still, he adds: “I have a hard time saying it's all over.”

And maybe it's not — as long as there are scientists pushing to tackle new questions about lateralized brain function, connectivity and communication, and as long as Vicki and her fellow cohort members are still around and still willing participants in science. Her involvement over the years, Vicki says, was never really about her. “It was always about getting information from me that might help others.”

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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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split-brain syndrome , condition characterized by a cluster of neurological abnormalities arising from the partial or complete severing or lesioning of the corpus callosum , the bundle of nerves that connects the right and left hemispheres of the brain .

Although it is not fully understood whether the processing of specific tasks is dependent on both hemispheres of the brain, the two hemispheres appear to each have some control over certain tasks. The left hemisphere, for example, is generally responsible for analytical tasks, such as calculating and reading. In many individuals, it is also the dominant centre for speech and language (though the right hemisphere is involved in language processing to a minor extent). In general, the right hemisphere is more efficient at dealing with spatial tasks, such as navigating a maze or reading a map, than the left hemisphere. The two hemispheres, however, routinely communicate with one another through the corpus callosum. This connection further serves as the conduit through which certain sensory signals are transmitted from one side of the body to the contralateral (opposite) side of the brain and through which motor control is effected in the reverse direction (i.e., the right hemisphere controls the left side of the body, and vice versa).

Among the first to characterize split-brain syndrome was American neurobiologist Roger Wolcott Sperry , who in the 1960s studied human split-brain subjects and contributed to the discovery that the left and right hemispheres of the brain carry out specialized duties. For this work, Sperry received a share of the 1981 Nobel Prize for Physiology or Medicine.

The primary cause of split-brain syndrome is intentional severing of the corpus callosum, partially or completely, through a surgical procedure known as corpus callosotomy. Rarely performed in the 21st century (having been replaced largely by drug treatments and other procedures), this operation is reserved as a last measure of treatment for extreme and uncontrollable forms of epilepsy in which violent seizures spread from one side of the brain to the other. By preventing the propagation of seizure activity across the hemispheres, corpus callosotomy can greatly improve the patient’s quality of life . However, following the operation, patients develop acute hemispheric disconnection symptoms that last for days or weeks and chronic symptoms that often are permanent.

Less-common causes of split-brain syndrome include stroke, infectious lesion, tumour, or ruptured artery. Many of these events result in varying degrees of spontaneous damage to the corpus callosum. The syndrome can also be caused by multiple sclerosis and in rare instances by agenesis of the corpus callosum, in which the connection fails to develop or develops incompletely. (Lesions in the corpus callosum also occur in patients with Marchiafava-Bignami disease, a rare alcoholism-related condition, but the more global brain damage associated with this disease leads to stupor , seizures, and coma, rather than the features typical of split-brain syndrome.)

Many patients with split-brain syndrome retain intact memory and social skills. Split-brain patients also maintain motor skills that were learned before the onset of their condition and require both sides of the body; examples include walking, swimming, and biking. They can also learn new tasks that involve either parallel or mirrored movements of their fingers or hands. They cannot, however, learn to perform new tasks that require interdependent movement of each hand, such as learning to play the piano, where both hands must work together to produce the desired music. Eye movements also remain coordinated.

Since information cannot be directly shared between the two hemispheres, split-brain patients display unusual behaviours, particularly concerning speech and object recognition. For instance, when blindfolded a split-brain patient may not be able to name a familiar object that is held in the left hand, because information for the sense of touch is relayed from the left side of the body to the right hemisphere, which typically has a weak language centre. Without an intact corpus callosum, a person cannot access verbal information in the left hemisphere as long as the object remains in the left hand. For the same reason, the patient may have difficulty using the left hand to execute verbal commands; the inability to respond to commands with motor activity is a form of apraxia . To compensate for deficiencies in touch recognition by the left hand and left-hand apraxia, the patient (still blindfolded) may hold the object in the right hand, which relays information to the left hemisphere, providing access to the patient’s dominant verbal bank and enabling him to speak the name of the object. Upon hearing the name of a given object, the patient may also use the left hand to retrieve it; this presumably is because auditory information is processed by both hemispheres. The diffuse nature by which sounds and smells are processed across the brain appears to underlie other problems experienced by split-brain patients. For example, patients are unable to name odours presented to the right nostril, though the left hand can point out the source. Some symptoms of chronic disconnection can improve with time.

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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.

• invisibilia
• consciousness
• medical treatments
• neuroscience

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Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness

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• Published: 12 May 2020
• Volume 30 , pages 224–233, ( 2020 )

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• Edward H. F. de Haan 1 ,
• Paul M. Corballis 2 ,
• Steven A. Hillyard 3 ,
• Carlo A. Marzi 4 ,
• Anil Seth 5 ,
• Victor A. F. Lamme 1 ,
• Lukas Volz 6 ,
• Mara Fabri 7 ,
• Elizabeth Schechter 8 ,
• Tim Bayne 9 ,
• Michael Corballis 2 &
• Yair Pinto 1  

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Recently, the discussion regarding the consequences of cutting the corpus callosum (“split-brain”) has regained momentum (Corballis, Corballis, Berlucchi, & Marzi, Brain , 141 (6), e46, 2018 ; Pinto et al., Brain, 140 (5), 1231–1237, 2017a ; Pinto, Lamme, & de Haan, Brain, 140 (11), e68, 2017 ; Volz & Gazzaniga, Brain , 140 (7), 2051–2060, 2017 ; Volz, Hillyard, Miller, & Gazzaniga, Brain , 141 (3), e15, 2018 ). This collective review paper aims to summarize the empirical common ground, to delineate the different interpretations, and to identify the remaining questions. In short, callosotomy leads to a broad breakdown of functional integration ranging from perception to attention. However, the breakdown is not absolute as several processes, such as action control, seem to remain unified. Disagreement exists about the responsible mechanisms for this remaining unity. The main issue concerns the first-person perspective of a split-brain patient. Does a split-brain harbor a split consciousness or is consciousness unified? The current consensus is that the body of evidence is insufficient to answer this question, and different suggestions are made with respect to how future studies might address this paucity. In addition, it is suggested that the answers might not be a simple yes or no but that intermediate conceptualizations need to be considered.

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Split-brain syndrome and extended perceptual consciousness.

History of the Corpus Callosum

Unit 5 Lesson: A Very Brief Introduction to Neuroimaging

Avoid common mistakes on your manuscript.

Introduction

The term “split-brain” refers to patients in whom the corpus callosum has been cut for the alleviation of medically intractable epilepsy. Since the earliest reports by van Wagenen and Herren ( 1940 ) and Akelaitis ( 1941 , 1943 ) on the repercussions of a split-brain, two narratives have emerged. First and foremost is the functional description, pioneered by Gazzaniga, Sperry and colleagues (Gazzaniga, Bogen, & Sperry, 1963 ; Gazzaniga, Bogen, & Sperry, 1962 ; Sperry, 1968 ), in which the intricacies, the exceptions, the effects of different testing conditions, and the experimental confounds have been delineated by decades of extensive research with a relatively small group of patients (Berlucchi, Aglioti, Marzi, & Tassinari, 1995 ; Corballis, 1994 ; Corballis et al., 2010 ; Corballis, 2003 ; Luck, Hillyard, Mangun, & Gazzaniga, 1989 ; Pinto, Lamme, & de Haan, 2017b ; Volz, Hillyard, Miller, & Gazzaniga, 2018 ). It is important to note that even in this small group there are differences. In some patients all commissures were severed (“commissurotomy”), in others only the corpus callosum was cut (“callosotomy”) and some patients fall somewhere in between these two boundaries. Now, the search term “split-brain” results in a total of 2848 publications in the database of the Web-of-Science and 29,300 hits on Google Scholar, indicating a wealth of detailed information. The other depiction of split-brain patients entails the first-person perspective of the split-brain. In other words, “what is it like” to be a split-brain patient? It is especially this perspective that has captured the attention of the general press, popular science books and basic textbooks. By its nature, this second narrative lacks the detail of the functional description of the phenomenon, but it captures the intriguing question of how unity of consciousness is related to brain processes. Dominant in this description is the idea that in a split-brain each hemisphere houses an independent conscious agent. This notion, and particularly the concept of an isolated but conscious right hemisphere that is unable to express its feelings, desires or thinking due a lack of language, has captured the imagination (Gazzaniga, 2014 ).

It is important to clarify what we mean by unified consciousness. Here, we use the term in the sense of “subject unity” as defined by Bayne (Bayne & Chalmers, 2003 ; Bayne, 2008 ; Bayne, 2010 ). Subject unity is present if all the experiences generated in a system belong to one subject. In other words, if a system contains a first person perspective, then subject unity is preserved if that system only contains one such perspective, but subject unity is absent if the system contains multiple first person perspectives. Thus, in our definition of conscious unity, consciousness in a split-brain is split if each cortical hemisphere houses an independent conscious agent.

The view that consciousness is split in a split-brain has significantly impacted cognitive neuroscience at large. For instance, currently dominant theories about conscious awareness - the Integrated Information Theory (Tononi, 2005 ; Tononi, 2004 ) and the Global Neuronal Workspace Theory (Dehaene & Naccache, 2001 ; Dehaene, Kerszberg, & Changeux, 1998 ) - may be critically dependent on the validity of this view. Both theories imply that without massive communication between different subsystems, for instance cortical hemispheres, independent conscious agents arise. Thus, if the split consciousness view is invalid, these theories may be critically challenged.

The idea of split consciousness in a split-brain had its origin in the early split-brain studies (Gazzaniga, 1967 ; Gazzaniga, 1975 ; Gazzaniga et al., 1962 ; Sperry, 1968 ). These studies tested patients primarily in the two perceptual domains where processing is largely restricted to the contralateral hemisphere, that is vision and touch. In these early studies, stimuli, for instance objects, that were presented to the left hemisphere either physically in the right hand or as an image in the right visual half-field, could be readily named (as the left hemisphere is dominant for language) or pointed out with the right hand (which is controlled by the left hemisphere). The patient’s behavior became intriguing when the stimuli were presented in the left visual field or in the left hand. Now the patient, or at least the verbal left hemisphere, appeared oblivious to the fact that there had been a stimulus at all but was nevertheless able to select the correct object from an array of alternatives presented to the left hand or the left visual half-field (see Fig.  1 ). In a particularly dramatic recorded demonstration, the famous patient “Joe” was able to draw a cowboy hat with his left hand in response to the word “Texas” presented in his left visual half field. His commentary (produced by the verbal left hemisphere) showed a complete absence of insight into why his left hand had drawn this cowboy hat. Another astonishing example involved the same patient. MacKay and MacKay ( 1982 ) flashed a digit in the left visual field and trained the patient to play a version of ‘20 questions’ across hemispheres. The left hemisphere guessed the answer vocally, and the right hemisphere provided responses by pointing ‘up’ (meaning ‘guess a higher number’) or ‘down’ with the left hand. In this way the patient managed to vocalize the right answer. This suggests two independent conscious agents communicating with each other (one steering the left hand, the other agent controlling vocal expressions). However, note that an alternative interpretation is possible. Perhaps the patient knows the answer but finds it hard to vocalize. The ‘20 questions’ then simply help him in finding the correct vocalization.

One of the most well-known split-brain findings is that the patient claims verbally not to have seen the stimulus in the left visual field, yet indicates the identity of it with their left hand. This suggests that the left hemisphere (controlling verbal output) is blind to the left visual field, while the right hemisphere (controlling the left hand) does perceive it

Thus, these early observations suggested that there is no meaningful communication between the two hemispheres in split-brain patients. This led to the hypothesis that there might be two separate conscious agents, a left hemisphere that is able to talk to us and can explain what it sees and feels, and a mute right hemisphere that cannot communicate in language but that can nevertheless show that it has perceived and recognized objects and words. However, over time this view has eroded somewhat due to several anomalies (even right from the start) that may challenge this view.

Common Ground

An early observation, suggesting some remaining unification concerned what Joe Bogen called the “social ordinariness” of split-brain patients. Apart from a number of anecdotal incidents in the subacute phase following the surgery, these patients seem to behave in a socially ordinary manner and they report feeling unchanged after the operation (Bogen, Fisher, & Vogel, 1965 ; Pinto et al., 2017a ; R. W. Sperry, 1968 ; R. Sperry, 1984 ), although their extra-experiment behavior has not been systematically observed in great detail (Schechter, 2018 ). While the right hemisphere appears to be better at recognizing familiarity from faces, self-face recognition, that is the ability to realise immediately that a presented photograph represents you, appears to be available equally to both hemispheres in a split-brain patient (Uddin et al., 2008 ; Uddin, 2011 ). Thus, it seems unlikely that a mute but conscious right hemisphere would not have made itself known one way or the other. Thus, right from the start a paradox arose. The controlled lab tests suggested that consciousness is split in split-brain patients. Yet, everyday experiences of the patient and their close ones suggests that only one person exists in a split-brain. Additionally, it has been suggested that the two separate consciousnesses-hypothesis presumes that in the intact brain (before surgery) both hemispheres were conscious but connected via the corpus callosum, and they only became dissociable due to the operation. This casts doubt on the viability of the two consciousness view.

Crucially, the lab tests themselves were not always supportive of the split-consciousness view. Multiple experimental results showed that capacity for communication between the hemispheres varies both across patients and across tasks. For instance, a central observation in split-brain patients concerns the inability to compare visual stimuli across the midline. In other words, when one stimulus is presented to the left visual hemifield and the other to the right hemifield, the patient cannot accurately indicate whether both stimuli are the same or not, although they can do so when both stimuli are presented within one visual field. This is consistent with the notion that each hemisphere independently perceives the contralateral visual field, and that an intact corpus callosum is necessary for integration. Although there are indeed many examples of split-brain patients who are incapable of comparing stimuli across the midline, prominent examples can also be found of patients who can compare stimuli across the midline (Johnson, 1984 ; but see Seymour, Reuter-Lorenz, & Gazzaniga, 1994 ). This points to an important problem in the field, namely, individual differences. One aspect that may be important for individual differences is handedness differences. Variations in handedness may lead to differences in language capabilities in the right hemisphere, and could even underly differences in inter-hemispheric integration.

Moreover, under certain circumstances nearly all tested split-brain patients seem able to compare, or integrate, particular types of stimuli across the two visual half-fields (see Fig. 2 ). An early demonstration of across hemifields integration is the study by Eviatar and Zaidel ( 1994 ). They showed that split-brain patients could accurately indicate the identity and shape of upper- and lower-case letters in either hemifield, regardless of with which hand they responded, for instance accurately identifying the letter A in the left visual field with the right hand. Yet these patients were mostly unable to compare these same stimuli across visual fields. In another experiment, two tilted lines were presented with a gap between them. The lines were positioned in such a way that extending them across the gap would either cause the lines to coincide or to run in parallel. When split-brain patients indicate whether the lines are parallel or coincident, they are highly accurate, even when both line segments are located in different half-fields (Corballis, 1995 ; Pinto, de Haan, Lamme, & Fabri, n.d. ; Sergent, 1987 ; Trevarthen & Sperry, 1973 ). Another example of visual integration across the midline involves apparent motion. When two dots are presented in succession at a short distance (2 to 14 visual degrees), split-brain patients are able to accurately indicate whether the dots create apparent motion, or that they were presented simultaneously or with delays too long to provoke apparent motion. Critically, they are able to do so even when one dot appears in the left visual field, and the other in the right visual field (Knapen et al., 2012 ; Naikar & Corballis, 1996 ; Ramachandran, Cronin-Golomb, & Myers, 1986 ). Clearly, under specific conditions, there is meaningful communication between the two hemispheres in the absence of the corpus callosum.

Although most split-brain patients cannot compare visual features such as shape and object identity across the midline, other features, such as good continuation of lines, and apparent motion, are integrated without a corpus callosum

Another observation that suggests some form of unity across the two visual half fields concerns detection and localization of stimulation, for instance, a brief flash (see, for example, an early study on the response times to light flashes with the ipsi- or contralateral hand: Clarke & Zaidel, 1989 ). Several investigations (Corballis, Corballis, Fabri, Paggi, & Manzoni, 2005 ; Pinto et al., 2017a ; Trevarthen & Sperry, 1973 ) have demonstrated convincingly that split-brain patients can accurately report the presence and location of stimuli for any position in the whole visual field, with either hand, and even verbally. Accurate detection and localization appears to be possible for all patients and all stimuli (different shapes, figures, equiluminant stimuli) tested so far. Thus, when patients in earlier studies said that they saw “nothing” when a stimulus was presented in the left visual half-field, they may have meant that they could see it but that they could not identify or retrieve the name of the object.

Other findings point to a crucial difference between the degree of lateralization of visual-perceptual processing and producing overt responses. Perception appears to be more split, while responding remains largely unified. Whether a stimulus appears in the left or the right visual hemifield strongly impacts performance of split-brain patients. However, response type (left hand, right hand or verbally) seems to have a much smaller, or no effect at all. For instance, Pinto et al., 2017a ) found that the split-brain patient was much better at matching pictures to sample stimuli in the left visual field. Yet, for the exact same stimuli matching pictures to verbal labels was vastly superior when the stimuli appeared in the right visual field. Crucially, response type did not play any role. The patient was better in matching pictures to sample for stimuli in the left visual field, even if they responded verbally or with the right hand. Similarly, Levy, Trevarthen, and Sperry ( 1972 ) presented split-brain patients with chimeric or composite faces, that is, one half-face in each visual field. Subsequently the patient either matched the chimeric face to sample, or attached a verbal label to it. Verbal matching was mostly based on the half-face in the right visual field, while matching to sample was mainly driven by the half-face in the left visual field. But crucially, the latter was the case, independent of whether the patient responded with the left or the right hand.

Thus, it seems that in split-brain patients perceptual processing is largely split, yet response selection and action control appear to be unified under certain conditions. This, by itself, does not prove whether a split-brain houses one or two conscious agents. One explanation could be that the split-brain houses two agents, each having their own experiences, who synchronize their behavioral output through various means. Another possible explanation is that a split-brain houses one agent who experiences an unintegrated stream of information who controls the entire body, comparable to watching a movie where sight and sound are out-of-sync. At any rate, these findings challenge the previously mentioned classic split-brain description, which is still found in reviews and text books (Gray, 2002 ; Wolman, 2012 ). In this classic characterization the patient indicates that they saw nothing when a stimulus appeared in the left visual field. Yet, to their own verbal surprise, the left hand correctly draws the stimulus. The aforementioned examples of unity in action control suggests that these effects may depend on the type and complexity of the response that is required.

Interpretations

There are three, not-mutually exclusive, hypotheses concerning the mechanisms involved in, seemingly, preserved unity in the split-brain. The first notion is that information is transferred subcortically. The second idea is that ipsilateral motor control underlies unity in action control. The third idea claims that information transfer is based on varies forms of inter-hemispheric collaboration, including subtle behavioral cues. The first proposal (Corballis Corballis, Berlucchi, & Marzi, 2018 ; de Haan et al., 2019 ; Pinto, Lamme, & de Haan, 2017b ; Pinto et al., 2017a ; Savazzi et al., 2007 ; Mancuso, Uddin, Nani, Costa, & Cauda, 2019 ) suggests that the multitude of subcortical connections that are spared during surgery are responsible for the transfer of information. As was initially pointed out by Trevarthen ( 1968 ) and Trevarthen and Sperry ( 1973 ) and recently stressed by Pinto, de Haan, and Lamme ( 2017a ) and Corballis et al. ( 2018 ), there are many commissures (white matter tracts that connect homologous structures on both sides of the central nervous system) and decussations (bundles that connect different structures on both sides) that link nuclei that are known to be involved in perceptual processing. The importance of these commisural connections for transferring visual information in split-brain patients has been highlighted by Trevarthen and Sperry ( 1973 ). Moreover, the role of these connections in a split-brain has recently been demonstrated by bilateral fMRI activations in the first somatosensory cortex, after unilateral stimulation of trunk midline touch receptors (Fabri et al., 2006 ) and in the second somatic sensory area after unilateral stimulation of hand pain receptors (Fabri, Polonara, Quattrini, & Salvolini, 2002 ). Uddin and colleagues used low-frequency BOLD fMRI resting state imaging to investigate functional connectivity between the two hemispheres in a patient in whom all major cerebral commissures had been cut (Uddin et al., 2008 ). Compared to control subjects, the patient’s interhemispheric correlation scores fell within the normal range for at least two symmetrical regions. In addition, Nomi and colleagues suggested that split-brain patients might rely particularly on dorsal and ventral pontine decussations of the cortico-cerebellar interhemispheric pathways as evidenced by increased fractional anisotropy (FA) on diffusion weighted imaging (Nomi, Marshall, Zaidel, Biswal, Castellanos, Dick, Uddin & Mooshagian, 2019). Interhemispheric exchange of information also seems to occur in the domain of taste sensitivity, activation of primary gustatory cortex in the fronto-parietal operculum was reported in both hemispheres after unilateral gustatory stimulation of the tongue receptors (Mascioli, Berlucchi, Pierpaoli, Salvolini, Barbaresi, Fabri, & Polonara, 2015 ). Note that patients may differ with respect to how many of these connections have been cut, and this might also explain some of the individual variance among patients. Moreover, in all patients subcortical structures remain intact. For instance, the superior colliculus is known to integrate visual information from both hemispheres and project information to both hemispheres (Meredith & Stein, 1986 ; Comoli et al., 2003 ). Such structures may support attentional networks, and may enable the right hemisphere to attend to the entire visual field. In turn, attentional unity could help in unifying cognitive and motor control, which may subserve ipsilateral motor control.

The second point concerns the ipsilateral innervation of the arms. Manual action is not strictly lateralized, and the proximal (but not the distal) parts of the arm are controlled bilaterally, although the ipsilateral contribution remains undetermined. This could explain why split-brain patients may respond equally well with both hands in certain experimental conditions (Corballis, 1995 ; Gazzaniga, Bogen, & Sperry, 1967 ; Pinto, de Haan, & Lamme, 2017a ). First, there is substantial evidence that bilateral cortical activations can be observed during unilateral limb movements in healthy subjects. In addition, ipsilesional motor problems in arm control have been observed in patients with unilateral cortical injuries, and finally there is evidence from electrocorticography with implanted electrodes for localization of epileptic foci showing similar spatial and spectral encoding of contralateral and ipsilateral limb kinematics (Bundy, Szrama, Pahwa, & Leuthardt, 2018 ). While these observations argue convincingly for a role in action control by the ipsilateral hemisphere, they do not prove that a hemisphere on it’s own can purposefully control the movements of the ipsilateral hand. Thus, the role of ipsilateral arm-hand control in explaining split-brain findings is currently not settled.

The third hypothesis argues that in addition to whatever direct neural communication may exist between the hemispheres, they may inform one another via strategic cross-cueing processes (Volz & Gazzaniga, 2017 ; Volz et al., 2018 ). The split-brain patients underwent surgery many years prior to testing, and the separated perceptual systems have 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. A cross-cueing mechanism might also allow one hemisphere to convey to the other which one of a limited set of known items had been shown (Gazzaniga & Hillyard, 1971 ; Gazzaniga, 2013 ).

Finally, it is possible to entertain combinations of the different explanations. For instance, it is conceivable that in the subacute phase following split-brain surgery the hemispheres are ineffective in communicating with each other. During this initial phase, phenomena such as an “alien hand” - that is a hand moving outside conscious control of the (verbal) person - may be present. In the ensuing period, the patients may have learned to utilize the information that is exchanged via subcortical connections, ipsilateral motor control or cross-cueing to coordinate the processing of the two hemispheres. In such a way, the patient may counteract some of the effects of losing the corpus callosum.

What do We Need to Know?

This paper aims to contribute to the agenda for the next decade of split-brain research. Full split-brain surgery is rare these days, and it is important that we try to answer the central questions while these patients are still available for study. In order to examine the variations between patients it would be useful to test as many of the available patients as possible with the same tests.

One important goal is to map out precisely how much functionality and information is still integrated across hemispheres in the split-brain, and what the underlying principles are. For instance, in some cases the two hemispheres seem to carry out sensory-motor tasks, such as visual search, independently from one another (Arguin et al., 2000 ; Franz, Eliassen, Ivry, & Gazzaniga, 1996 ; Hazeltine, Weinstein, & Ivry, 2008 ; Luck, Hillyard, Mangun, & Gazzaniga, 1994 ; Luck et al., 1989 ), while in other cases functions such as attentional blink, or attentional cueing, seem to be integrated across hemispheres (Giesbrecht & Kingstone, 2004 ; Holtzman, Volpe, & Gazzaniga, 1984 ; Holtzman, Sidtis, Volpe, Wilson, & Gazzaniga, 1981 ; Pashler et al., 1994 ; Ptito, Brisson, Dell’Acqua, Lassonde, & Jolicœur, 2009 ). An important challenge is to unveil why some cognitive functions can be carried out independently in the separated hemispheres while other functions engage both hemispheres. Furthermore, it is now clear that accurate detection and localization is possible across the whole visual field, and there is some evidence that even more information concerning visual images can be transferred between hemispheres. Although we have some understanding of what types of information can be transferred in the visual domain, our knowledge base in the somatosensory domain is much more limited. This is probably due to a bias throughout cognitive neuroscience and psychology, leading to a strong focus on vision in split-brain research. It is important to collect converging evidence by investigating the somatosensory system which is also strongly lateralized. Note that in somatosensory processing transfer between hemispheres (about 80% correct for the bimanual conditions) has been observed for basic same-different matching of real objects (Fabri, Del Pesce et al., 2005 ).

Another important goal is to obtain a more detailed description of the perceptual, cognitive and linguistic capabilities of the disconnected right hemisphere. For understanding unity of mind, two capabilities specifically are crucial. First, experiments investigating aspects of the conscious mind often go beyond simple visual processing, and future studies will thus critically depend on testing high-level cognitive abilities of both hemispheres. Specifically, language abilities, crucial for understanding questions and instructions, will likely play a pivotal role. Thus, the first question is to what extent the right hemisphere is capable of language processing. Note that complicated instructions (Gazzaniga, Smylie, Baynes, Hirst, & McCleary, 1984 ; Pinto et al., 2017a ; Zaidel, 1983 ), for instance relating to mental imagery (Johnson, Corballis, & Gazzaniga, 2001 ; Kosslyn, Holtzman, Farah, & Gazzaniga, 1985 ; Sergent & Corballis, 1990 ), seem to be well within the reach of the right hemisphere. Moreover, right hemisphere language capabilities seem to improve over time (Gazzaniga, Volpe, Smylie, Wilson, & LeDoux, 1979 ; Gazzaniga et al., 1996 ). Longitudinal language tests (for instance with a Token test: De Renzi & Vignolo, 1962 ) would further illuminate the extent of right hemisphere language processing.

Second, unveiling to what extent each hemisphere is capable of subserving consciousness at all seems relevant for unity of mind as well. If the disconnected right hemisphere can produce full-blown consciousness, then questions regarding unity of mind are clearly more pertinent then if the right hemisphere only produces minimal amounts of consciousness. Right hemisphere consciousness can be studied through novel neural paradigms (Bekinschtein et al., 2009 ; Casali et al., 2013 ; Pitts, Metzler, & Hillyard, 2014 ; Shafto & Pitts, 2015 ). For instance, Bekinschtein et al. employed EEG to measure if the brain detected irregularities (as indicated by an event-related potential [ERP] signal called the P3) in different states of consciousness. They found that when consciousness was reduced, local irregularities were still detected - for instance after three high auditory tones a low tone evoked a P3. However, global irregularities - several times a low tone followed three high tones, then on the critical trial three high tones were followed by another high tone - did not evoke a P3 when consciousness was reduced. Crucially, when consciousness was unimpaired both local and global irregularities evoked a P3 response. Right hemisphere consciousness may also be studied in other patient groups where interhemispheric communication is hampered. One particularly interesting group are post-hemispherotomy patients (Lew, 2014 ). These patients have been surgically treated to disconnect an entire hemisphere (usually for intractable epilepsy), but unlike hemispherectomy patients the disconnected hemisphere remains in place in the cranium and remains vascularized.

Clearly, the central question, whether each hemisphere supports an independent conscious agent, is not settled yet. Novel paradigms in this respect could lead to progress. For instance, a pivotal question is whether each hemisphere makes its own decisions independent of the other hemisphere. If each hemisphere produces its own autonomous conscious agent then this should be the case. That is, if two agents are asked to freely choose a random number, then the odds that they consistently pick the same number are small. And vice versa, if each hemisphere makes its own conscious decisions, independent of the other hemisphere, then this seems to rule out unity of mind. Note that each hemisphere making its own decisions is different from information processing occurring independently per hemisphere. Unconscious information processing is almost certainly split across hemispheres in a split-brain. However, this does not prove that consciousness is split or unified. Even in a healthy brain, where consciousness is unified, many unconscious processes run independently, and in parallel.

One way to tackle the central question is by having the hemispheres respond to questions in parallel. Overt behavior most likely does not allow for this, due to bilateral motor control processes sketched earlier. However, perhaps parallel responding is possible if the hemispheres produce covert responses. For instance, the patient could be asked to pick one of four options and indicate their choice by carrying out certain content-specific mental imagery tasks. This imagery can then be decoded in parallel from each hemisphere using neuroimaging techniques (see Owen et al., 2006 for a similar approach with vegetative state patients). If each hemisphere harbors an autonomous conscious agent, then it is highly unlikely that the two hemispheres will consistently make the same choices. Thus, if the choices are uncorrelated across hemispheres, then this may critically challenge the unified mind view.

Another way to tackle the question of unified consciousness in the split-brain is to employ ERPs as markers of concurrent conscious processing in the left and right hemispheres. For instance, in one study (Kutas, Hillyard, Volpe, & Gazzaniga, 1990 ) visual targets were presented either separately to the left or right visual field or to both visual fields simultaneously. It was found that the P300 - a signal possibly reflecting conscious processing of a visual target (Dehaene & Changeux, 2011 ; Dehaene, Charles, King, & Marti, 2014 ; Salti, Bar-Haim, & Lamy, 2012 ) - was reduced for bilateral targets. This suggests some type of integration of conscious processing. Studies employing ERPs may indicate whether conscious processing is unified, while unconscious processing is split, which would be suggestive of unified consciousness.

Conclusions

In summary, the pivotal issue in split-brain research is whether dividing the brain divides consciousness. That is, do we find evidence for the existence of one, or two conscious agents in a split-brain? Note that intermediate results may be found. Perhaps some measures indicate unified consciousness while others do not. This would then provoke further interesting questions on the unity of consciousness. What are the crucial measures for unity of consciousness? If intermediate results are found, more unconventional possibilities should be entertained as well. For instance, although difficult to fathom, some philosophers have suggested that a split-brain does not contain one or two observers, but a non-whole number of conscious agents (Nagel, 1971 ; Perry, 2009 ), for instance one and a half first-person perspectives. If evidence for this position is found, then its implications would stretch beyond split-brain patients. It would suggest that our intuitions on the indivisibility of the experiential self may be mistaken. One way to think of this is as with the difference between conscious and unconscious processing. Perhaps this is not a dichotomous distinction, but a continuum between more or less conscious. Similarly, perhaps the existence of a first-person perspective is not dichotomous, but gradual as well. Another possibility is that a split-brain does contain a whole number of conscious agents, but that consciousness across these agents is only partially unified. That is, the agents share some conscious experiences and decisions, but not all (Lockwood, 1989 ; Schechter, 2014 ; Schechter, 2018 ; Schechter, 2013 ). Finally, another way to look at this is in terms of ‘dissociation’, as in depersonalization (Phillips et al., 2001 ; Sierra et al., 2002 ). Perhaps the number of agents is not altered, but the agent feels depersonalized in some situations, and therefore no longer feels that they control the actions, or even experience the information, that has just occurred in their brain.

New findings on the unity of consciousness in a split-brain could fundamentally impact currently dominant consciousness theories. Global Neuronal Workspace Theory (Dehaene & Naccache, 2001 ; Dehaene et al., 1998 ) asserts that consciousness arises when information that is processed in unconscious (or preconscious) modules is broadcast to a central ‘workspace’, primarily residing in frontal regions of the brain. Although not very explicit on the unity of consciousness in split-brain patients, Global Neuronal Workspace Theory seems to endorse the split consciousness idea, given that each hemisphere has its own prefrontal hub, enabling broadcasting of whatever information is processed in that hemisphere.

Integrated Information Theory (Tononi, 2005 ; Tononi, 2004 ) has specifically addressed the issue of split brain (for instance in Tononi, 2004 ). Integrated Information Theory asserts that ‘phi’, a measure of how integrated information is, determines the level of consciousness. The higher phi, the more conscious a system is. Moreover, local maxima in phi correspond to conscious agents. If in a system all subsystems are highly interconnected, then phi is highest for the system as a whole, and local maxima are absent. Thus, such a system produces only one conscious agent. However, if subsystems only exchange minimal amounts of information, then phi per subsystem is higher than phi for the system as whole. In such a case each subsystem creates its own conscious agent. In a split-brain, connectedness, that is integration of information, is much higher within than across hemispheres. Therefore, according to Integrated Information Theory consciousness should be split in a split-brain.

Recurrent Processing theory (Lamme & Roelfsema, 2000 ; Lamme, 2004 ; Lamme, 2010 ) argues for the independence of consciousness from attention, access, or report. This theory has addressed the issue of report specifically, making the case that consciousness and reportability, whether verbal or manual, should be viewed as entirely independent (Lamme, 2006 ; Tsuchiya, Wilke, Frässle, & Lamme, 2015 ). Crucially, this theory states that even in the normal mind, ‘islands’ of unattended yet conscious information reside (Lamme, 2006 ). In these cases, all the information, although functionally unintegrated, is nonetheless experienced by the same mind. Support for this view comes from findings in multiple object tracking (Pinto, Howe, Cohen, & Horowitz, 2010 ; Pinto, Scholte, & Lamme, 2012 ; Pylyshyn & Storm, 1988 ). Here, evidence indicates that when moving objects in two visual hemifields are tracked, attention is split (Howe, Cohen, Pinto, & Horowitz, 2010 ) and each hemisphere processes the relevant information in the contralateral visual field independently of the other (Alvarez & Cavanagh, 2005 ; Cavanagh & Alvarez, 2005 ; Drew, Mance, Horowitz, Wolfe, & Vogel, 2014 ). That is, the left hemisphere only tracks the right visual field and vice versa. Yet, although the visual information is not integrated across hemispheres, from a first person perspective, it seems clear that the subject experiences all moving objects across the entire visual field. Another example of the dissociation between consciousness and reportability is the so-called partial report paradigm (Pinto et al., 2017b ; Pinto, Sligte, Shapiro, & Lamme, 2013 ; Sligte, Scholte, & Lamme, 2008 ; Sperling, 1960 ). In these paradigms subjects seem to remember more than they can report. Thus, reportability and consciousness are dissociated. Perhaps in split-brain patients this dissociation is simply more pronounced. That is, consciousness remains unified, but reportability has become more dissociated, thereby inducing the appearance of two independent agents. In sum, according to the Recurrent Processing theory, integration of information is not needed for a unified mind, implying that the mind may remain unified when the brain is split. Thus, different theories of consciousness have different predictions on the unity of mind in split-brain patients, and await the results of further investigation into this intriguing phenomenon.

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This work was supported by an Advanced Investigator Grant by the European Research Council (ERC grant FAB4V (#339374) to EdH and a Templeton grant (ID# 61382, "Towards understanding a unified mind") to YP and VL.

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de Haan, E.H.F., Corballis, P.M., Hillyard, S.A. et al. Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness. Neuropsychol Rev 30 , 224–233 (2020). https://doi.org/10.1007/s11065-020-09439-3

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The split brain experiments

In the 19th century, research on people with certain brain injuries, made it possible to suspect that the "language center" in the brain was commonly situated in the left hemisphere. One had observed that people with lesions in two specific areas on the left hemisphere lost their ability to talk, for example.

The final evidence for this, however, came from the famous studies carried out in the 1960s by Roger Sperry and his colleagues. The results of these studies later led to Roger Sperry being awarded the Nobel Prize in Physiology or Medicine in 1981. Sperry received the prize for his discoveries concerning the functional specialization of the cerebral hemispheres. With the help of so called "split brain" patients, he carried out experiments (just like the one you can perform by yourself in the Split Brain Experiments Game), and for the first time in history, knowledge about the left and right hemispheres was revealed.

What does "split brain" mean?

In the 1960s, there was no other cure for people who suffered from a special kind of epilepsy than by cutting off the connection, corpus callosum , between the two hemispheres. Epilepsy is a kind of storm in the brain, which is caused by the excessive signaling of nerve cells, and in these patients, the brain storm was prevented from spreading to the other hemisphere when the corpus callosum was cut off. This made it possible for the patients to live a normal life after the operation, and it was only when carrying out these experiments one could notice their somewhat "odd behavior."

Each hemisphere is still able to learn after the split brain operation but one hemisphere has no idea about what the other hemisphere has experienced or learned. Today, new methods and technology in split brain operation make it possible to cut off only a tiny portion and not the whole of the corpus callosum of patients.

What came out of the split brain experiments?

The studies demonstrated that the left and right hemispheres are specialized in different tasks. The left side of the brain is normally specialized in taking care of the analytical and verbal tasks. The left side speaks much better than the right side, while the right half takes care of the space perception tasks and music, for example. The right hemisphere is involved when you are making a map or giving directions on how to get to your home from the bus station. The right hemisphere can only produce rudimentary words and phrases, but contributes emotional context to language. Without the help from the right hemisphere, you would be able to read the word "pig" for instance, but you wouldn't be able to imagine what it is.

"The great pleasure and feeling in my right brain is more than my left brain can find the words to tell you."

Roger Sperry

First published 30 October 2003

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Privacy Policy Terms of use The Nobel Prize Copyright © Nobel Prize Outreach AB 2024 Brain Development Childhood & Adolescence Diet & Lifestyle Emotions, Stress & Anxiety Learning & Memory Thinking & Awareness Alzheimer's & Dementia Childhood Disorders Immune System Disorders Mental Health Neurodegenerative Disorders Infectious Disease Neurological Disorders A-Z Body Systems Cells & Circuits Genes & Molecules The Arts & the Brain Law, Economics & Ethics Neuroscience in the News Supporting Research Tech & the Brain Animals in Research BRAIN Initiative Meet the Researcher Neuro-technologies Tools & Techniques Core Concepts For Educators Ask an Expert The Brain Facts Book Splitting the Human Brain In Two Published 31 Mar 2017 Reviewed 31 Mar 2017 Author Jessica P. Johnson Source BrainFacts/SfN As an experimental surgery to help patients with epilepsy, surgeons severed a piece of the brain connecting the left and right sides of the brain. The seizures stopped, but the side effects revealed key ways that parts of the brain communicate. To find out more, listen to this podcast from BrainFacts.org . Music by Josh Woodward , used under a creative commons license . About the Author Jessica P. Johnson Jessica is a freelance science writer and podcast producer with degrees in biology, microbiology, and science journalism. CONTENT PROVIDED BY BrainFacts/SfN SfN: I’m Jessica Johnson for BrainFacts.org. In the 1940s, neurosurgeons began a series of experimental surgeries to treat epilepsy in patients who failed to respond to medications. Ten patients volunteered to have the wide, flat bundle of neural fibers, called the corpus callosum, severed in their brains, effectively disconnecting the left and right hemispheres. The idea was to shut down this neural highway and stop a seizure’s uncontrolled electrical impulses from spreading from one hemisphere to the other. The procedure worked. Although patients still required some medication, the combination treatment reduced the spread of seizures, the severity of brain trauma, and the frequency of lost consciousness. Perhaps most encouraging was that these so-called split-brain patients suffered no apparent changes in personality, emotion, or intelligence. Michael Gazzaniga: The most remarkable thing is, you wouldn’t know a patient had the surgery unless you started doing some clever testing. One of the first patients could still play the piano afterward. You might think that would be impossible. This is Michael Gazzaniga at the University of California Santa Barbara. I’m Director of the Sage Center for the Study of Mind. SfN: Gazzaniga began some of this “clever testing” in the early 1960s under Roger Sperry, his PhD advisor at the time, who later won the 1981 Nobel Prize in Physiology and Medicine for his research on split brain patients. Subtle behavioral changes previously documented in split brain patients had led the researchers to suspect that the surgery may induce slight alterations in cognitive function. One split-brain patient, for example, reported having difficulty buttoning his shirt, because as one hand did the work of buttoning, the other hand would follow and unbutton – as if each hand was controlled by a different brain. So Gazzaniga and Sperry designed experiments to test each brain hemisphere separately to investigate the surgery’s impacts on cognition. Gazzaniga: We gave both the left brain and the right brain a simple problem. And the problem was we showed each a picture, and all each half brain had to do was point to a picture on the table in front of them that best matched the picture they saw. SfN: It’s important to remember here that things we see with our right eye, known as the right visual field, are sent to the brain’s left hemisphere to process. Similarly, anything we see in the left visual field is sent to the brain’s right hemisphere. Gazzaniga : We’re all wired up like that. SfN: In the experiment, the split brain patients accurately pointed to pictures of the objects they had seen. Both hemispheres performed the task equally well. But when the patients were asked to talk about what they saw, it was a completely different story. Gazzaniga : If you just ask a split brain patient: what did you see? All they would talk about is the picture to the right part of your visual field. SfN: The reason for this, Gazzaniga explains, is that the speech center of our brain is located in the left hemisphere, which again collects information only from the right visual field. So in the split brain patients, the only information the speech center receives – and can therefore talk about – comes from the right eye. Gazzaniga : If there was an apple up there, they would say, “I saw an apple,” even though at the same time we may have shown an orange to the right hemisphere. You cut the [corpus] callosum and the information can’t get over to the left speaking center, so the left brain doesn’t say anything about it. SfN: From this and other experiments, Gazzaniga and his colleagues concluded that in intact brains, the corpus callosum performs the vital task of passing information back and forth between the two hemispheres. Without this connection, the right hemisphere can’t send the visual information it collects to the left brain to verbalize. These observations helped reinforce the theory that different brain regions specialize in different cognitive processes and provided a possible explanation for how the outputs of these processes are shared with the rest of the brain. In a fascinating twist, when researchers later asked the split brain patients to verbally explain why they pointed to a picture of an object only processed by their right hemisphere, and was therefore unknown to the left hemisphere and its verbal center, their left hemispheres filled in the blanks and invented logical answers. Gazzaniga : The left brain doesn’t actually know why that left hand is pointing to this other picture. The whole latter half of the split brain research story is that there is this thing in our left hemisphere – this special capacity to make everything that’s coming out seem integrated with one story. And that thing that we have in our left hemisphere, we’ve dubbed it, just to give it a name, the “Interpreter.” It’s always interpreting our actions. We’re constantly explaining ourselves to ourselves. SfN: Gazzaniga and others continue to investigate the corpus callosum’s role as an information highway and the idea that, in its absence, an interpretive center located somewhere in the brain’s left hemisphere tries to integrate the outputs of the separate hemispheres into a single, coherent story. Gazzaniga’s decades of research have also led him and others to ponder the question: how does our brain anatomy determine who we are as individuals? Gazzaniga : You split the brain and you would think, “well, that should disrupt the mind.” You’re right in there with a jackhammer and all those networks that are supposed to give rise to mind. It’s pretty remarkable that if you disconnect the two hemispheres, all of a sudden you’ve got two minds. There are certainly times, while we’re doing these studies, where the two minds differently evaluate the same object. One can think positively about it, and the other side can be not happy with it. There are two systems seesawing on how they feel about it. SfN: Gazzaniga believes that the Interpreter is somehow responsible for mediating the sometimes conflicting thoughts and emotions produced by different brain regions in order to present a unified sense of self. Today, split brain surgery is very rare. But research on split-brain patients continues to inform our understanding of how the brain’s hemispheres function both independently and collaboratively. Thanks for listening. Check out more information about how the brain’s hemispheres cooperate to create a unified mind at BrainFacts.org . Also In Cells & Circuits Popular articles on BrainFacts.org Check out the latest news from the field. A beginner's guide to the brain and nervous system. Research & Discoveries See how discoveries in the lab have improved human health. SUPPORTING PARTNERS Privacy Policy Accessibility Policy Terms and Conditions Manage Cookies Some pages on this website provide links that require Adobe Reader to view. Open Course Split Brains: What Happens When You Sever the Corpus Callosum? In this video I cover research by Roger Sperry and Michael Gazzaniga on split brain patients who have had the corpus callosum severed. I explain some hemispheric specializations such as speech production and facial recognition as well as how visual information is separated into each hemisphere at the optic chiasm according to visual field. Don’t forget to subscribe to the channel to see future videos! Have questions or topics you’d like to see covered in a future video? Let me know by commenting or sending me an email! Need more explanation? Check out my full psychology guide: Master Introductory Psychology: http://amzn.to/2eTqm5s Severed Corpus Callosum from Scientific American Frontiers https://www.youtube.com/watch?v=lfGwsAdS9Dc Split Brain Behavioral Experiments https://www.youtube.com/watch?v=ZMLzP1VCANo Video Transcript : Hi, I’m Michael Corayer and this is Psych Exam Review. In this video we’re going to talk about split-brain patients. Now these are patients who have had their corpus callosum severed. So you might recall that the corpus callosum is this large band of nerve fibers. It connects the cerebral cortex of the left hemisphere to the cerebral cortex of the right hemisphere. So why would we ever want to sever this connection? Well for most of these patients this procedure is done because they have seizures. So they have seizures which are these bursts of neurons firing, uncontrollable bursts of neurons firing and this spreads so the neurons firing causes other neurons to fire and then that causes other neurons to fire. This ends up spreading back and forth between the two hemispheres. So the way to stop this is to sever the connection, the corpus callosum, between these two hemispheres and this helps to reduce the spreading of this neuron firing. The good news is that this procedure is very effective. If you have a patient who has very frequent seizures, very disruptive, severe, really affecting their ability to live a normal life then they might undergo this procedure and this will hopefully stop the seizures from occurring. Now there are some side effects of severing the corpus callosum. But the other good news is that the side effects are actually fairly small. If you met somebody who had undergone this procedure and had a split brain you probably wouldn’t know. So there wouldn’t be really obvious effects on their behavior that would stand out to you. But if you put them in certain situations and tested this you could see that they have these two separated hemispheres of their brain, unlike a person which an intact corpus callosum where the two hemispheres can communicate with one another freely. So how do we go about finding these differences in behavior that occur? It turns out there are some specializations of each hemisphere. So the left hemisphere has certain tasks that it’s good at that the right hemisphere doesn’t do and vice versa. The right hemisphere has some tasks that it’s specialized for that the left hemisphere doesn’t do. This is work that was done originally by Roger Sperry studying split brain patients and for this he was awarded the Nobel prize in 1981 and in fact that year it was a split Nobel prize because he shared it with David Hubel and Torsten Wiesel who we’re going to learn about in a future video because they did work on visual processing that’s also relevant for psychology. Ok, so Sperry did this initial work on these hemispheric specializations certain tasks that are specialized in either hemisphere. So what are these specializations and how do split brain patients reveal them? So the first specialization, the most important for these split brain studies, is the idea that language for most people occurs mostly in the left hemisphere and speech production occurs mostly because of activity in the left hemisphere. So this area in the left frontal lobe called Broca’s area coordinates the movements for speech as well as other language processing areas are mostly in the left hemisphere. OK so what this means is that if we send information to the left hemisphere we can talk about it. But if we send to the right hemisphere and we can’t get it over to the left hemisphere via the corpus callosum, then we can’t talk about it. So how could we demonstrate this with a split brain patient? Well, let’s say we gave them an object to hold. So if I were to place this object in their right hand, remember the idea of contralateral control, this is going to get sent from the right hand to the left hemisphere. So if I ask the person what are you holding, they’ll be able to tell me, if I blindfold them, place it in their hand then they say “it’s a key”, or “I think it’s a key”. They’re grasping it with their right hand, the left hemisphere tries to figure out what they’re grasping, then sends it to the speech production area, and they can tell me it’s a key. On the other hand, literally, I can place it in the left hand, they feel it, the right hemisphere is processing this information saying “I think it’s a key” the problem is the person can’t say this because the right hemisphere doesn’t control language production. So the person knows what it is but they can’t tell me. What is it? “I’m drawing a blank, I can’t can’t seem to say it”. So how do we know the right hemisphere even knows it? Maybe they really don’t know. Well, if I put a bunch of objects in front of them and I say, ok, using your left hand point the one that you were holding and they can immediately point to the key. So this shows that the right hemisphere does know it, it’s just not able to talk about it. OK so that’s one way we can reveal this specialization in the brain. Now we’ll get a little bit more complicated, we’re going to look at how this works with visual information. Not just touch sensation, which as I said before is purely contralateral, left side of the body goes to the right hemisphere. Vision is a little bit more complicated but not too bad. So I want to go over this and if you have any questions about this feel free to ask in the comments. But I’ll do my best to simplify this. OK so let’s say we’re looking down the top of somebody’s head. Here’s their left eye, here’s their right eye. We’ve got their left hemisphere here, we’re looking down so it’s going to overlap the eye there. But we’ll do our best, this won’t be the most anatomically correct brain diagram you’ve ever seen. But hopefully it will get the point across. Here’s the left eye, here’s the right eye, here’s the left hemisphere, here’s the right hemisphere. The important thing about the eyes is that they process information by visual field. This means they’re not contralateral the way the rest of the body is. It’s not the case that the left eye goes to the right hemisphere or the right eye goes to the left hemisphere. That’s not true. It’s a common mistake that students make, so banish that from your mind. What we’ll see is it’s divided by visual field. So if I focus on the center of the screen here everything over to the right is red and let’s say on the left side I have blue here. What’s gonna happen is that this blue side, the left side of the screen is gonna go, the light is going to travel straight across to this part of the retina on this eye. And same thing on this eye here. And the right side of the screen is going to travel across over here. Now it goes to the opposite side of the eye because the light is just traveling straight through the hole in the eye, the pupil, and just hitting that opposite side. But the more important thing is what happens after this. What happens is this information, the important point is really that the eye is getting two sides, a left visual field and a right visual field. What happens then is this information comes out here, comes out the optic nerve and in this case it’s the right visual field and it’s already in the left hemisphere so it’s going to actually stay in the left hemisphere. Alright so it’s gonna go initially to an area called the optic chiasm then it goes to the thalamus, then the thalamus sends it out to the appropriate area of the cortex, which in this case is the occipital lobe, to the primary visual cortex. Now what’s interesting is what happens over here on the other side. This right visual field needs to get over to the left hemisphere. So in this case it comes out the optic nerve and when it gets to the optic chiasm it actually crosses over to the other hemisphere and joins this information which is also about the right visual field. And then gets sent out to the occipital lobe for processing. And the opposite would be true over here. So this side, the left visual field is going to go down here out the optic nerve, it’s going to get to the optic chiasm but it’s already in the right hemisphere so it stays over here goes to the thalamus, then goes out to the occipital lobe for processing. And over here, this left visual field, is going to come out here and when it gets to the optic chiasm it’s going to cross over to the other side to the right hemisphere, then go to the thalamus, then go out to the occipital lobe for processing. So what this means is that everything in the right visual field goes to the left hemisphere and everything in the left visual field goes to the right hemisphere. So this is the right hemisphere over here, only seeing blue information and this is the left hemisphere which is getting the right visual field which is all red. Now if you think about this it makes sense that we would split by visual field instead of splitting by eye. If you were to lose an eye, if we think in evolutionary terms about how this is helpful, if you lose an eye it would be a real shame if that meant that that whole side of the brain was no longer getting any visual information. You’d be wasting all that brain space just because you lost an eye. In this situation we see that even if you lost and eye, information is still gonna get sent to both of the occipital lobes. So if we knocked out the right eye here, the left eye would still send some of its information to the right and some to the left based on visual field. OK let me just add a few labels in here. So this point here would be the optic chiasm, that’s where the information crosses over to get to the appropriate hemisphere and then here is the this is where it’s going into the thalamus after the optic chiasm, then from there it ends up in the occipital lobe in the primary visual cortex. This is an area called V1. The primary visual cortex. Ok, so let’s get back to our split brain patients. What happens when we split the corpus callosum, we sever the corpus callosum, we split the brain, we don’t cut the optic chiasm. So this process still occurs, this transfer from visual field over to the opposite hemisphere still occurs. So what does this have to do with split brain tasks? That means we can do this just like we did with the hand version. Except now I put the information on the screen and I have them focus on the center, that’s important, they have focus on the center. If they move their eyes around then they can send everything to both hemispheres. That’s how they’re normally going to do it in everyday life, that’s why you aren’t going to notice. But if we focus their vision, we flash these on the screen briefly so they don’t have time to move their eyes around, see what’s going on everywhere then anything to the right goes to the left hemisphere, anything to the left goes to the right hemisphere. If I flash these on the screen, this screwdriver is going to my left hemisphere over here so when you ask me “what did you see on the screen?” if I have a split brain, I’m going to say “I saw a screwdriver” because that’s all the left hemisphere saw, the right visual field. If however, you ask me to draw with my left hand what I saw, well the left side of the screen went to my right hemisphere which is going to tell my left hand to draw a key. Now the really interesting thing is when I ask the person. So they draw the key, then they look at it, and I say “why did you draw a key?”. Now I’m asking them to explain, using speech, so the left hemisphere saw a screwdriver then sees that the person drew a key. Why did this happen? It turns out the left hemisphere will make up an explanation. It’s something that the left hemisphere seems to be good at is coming up with logical explanations for behavior. So the person will look at the key and make up some story with the left hemisphere. Well, I saw screwdriver, but, last time I needed a screwdriver I couldn’t find one, and so I ended up using a key to turn a screw. They’ll make up some plausible explanation to why did they draw key when they saw a screwdriver. It’s also the case that there’s some things that the right hemisphere does better so one of these is face recognition. This, for most people, is going to predominantly happen in the right hemisphere. This means if we show things that are arranged into a face this certain detection of the pattern that makes up a face, rather than the individual parts is happening in the right hemisphere and in one of the videos that I’ve posted in the description you can see Michael Gazzaniga working with a split brain patient and he gives him this task where he shows these famous paintings by Arcimboldo that perhaps you’ve seen before, of faces made out of objects, made out of fruit, flowers, books, things like that. If he shows it to the right visual field, goes to the left hemisphere, the left hemispheres processes, sees the objects. What did you see? I saw books, flowers. But if he sends that same painting to the left visual field so it goes to the right hemisphere and the right hemisphere has this facial recognition area, it can see this pattern so the right hemisphere says I saw a face. Point with the left hand, what did you see? Right hemisphere saw a face. That stood out to the right hemisphere because it’s specialized to recognize faces. So that’s another specialization that can be revealed through these split bring patients. The key ideas here are to remember are, first of all, that it’s split by visual field, not by eye, keep that in mind, students often get tripped up by that. And the second point is it doesn’t really matter in daily life. We have these clever experiments revealing these differences between left hemisphere and right hemisphere but don’t take this too far. There’s a lot of books being sold you know talking about “left brainers” and “right brainers” and it’s to me, all very silly because most of us have our corpus callosum intact and everything is happening in all areas of our brain, the hemispheres are sharing information with each other. Yes, it’s true certain tasks are specialized to the left and right hemisphere but in terms of how that’s going to affect your behavior it doesn’t really matter. So you don’t sit there and say wow I looked at a bunch of faces so my right hemisphere is really tired. Or I’ve been producing a lot of speech so my left hemisphere is exhausted. It doesn’t really matter that way. I mean if you want to get good at a certain task you just practice doing it. You don’t need to think about whether it’s related to brain activity in the right or left hemisphere, I don’t really see the point of that. But I guess it helps to sell books so… That’s the other thing to keep in mind, don’t go too far in drawing conclusions from this. And even for people with split brains it’s true that both halves of their brain are very active all the time and coordinate in doing a number of activities and they’re moving their eyes around and sending information to both hemispheres so it really doesn’t matter too much about which hemisphere processes which information. OK so I hope you found this helpful. Check out the videos that I linked in the description, I think you’ll find them interesting, you can see some actual split brain patients doing some of these studies. If you found this helpful, please like the video and subscribe to the channel for more. Thanks for watching! Leave a Reply Cancel reply Your email address will not be published. Required fields are marked * Search Menu Sign in through your institution CNS Injury and Stroke Epilepsy and Sleep Movement Disorders Multiple Sclerosis/Neuroinflammation Neuro-oncology Neurodegeneration - Cellular & Molecular Neuromuscular Disease Neuropsychiatry Pain and Headache Advance articles Editor's Choice Author Guidelines Submission Site Why publish with this journal? Open Access About Brain Editorial Board Advertising and Corporate Services Journals Career Network Self-Archiving Policy Dispatch Dates Terms and Conditions Journals on Oxford Academic Books on Oxford Academic Article Contents Introduction, patients and methods, acknowledgements. < Previous Split brain: divided perception but undivided consciousness Article contents Figures & tables Supplementary Data Yair Pinto, David A. Neville, Marte Otten, Paul M. Corballis, Victor A. F. Lamme, Edward H. F de Haan, Nicoletta Foschi, Mara Fabri, Split brain: divided perception but undivided consciousness, Brain , Volume 140, Issue 5, May 2017, Pages 1231–1237, https://doi.org/10.1093/brain/aww358 Permissions Icon Permissions 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. A depiction of the traditional view of the split brain syndrome ( top ) versus what we actually found in two split-brain patients across a wide variety of tasks ( bottom ). The canonical idea of split-brain patients is that they cannot compare stimuli across visual half-fields ( left ), because visual processing is not integrated across hemispheres. This is what we found as well. However, another key element of the traditional view is that split-brain patients can only respond accurately to stimuli in the left visual field with their left hand and to stimuli in the right visual field with their right hand and verbally. This is not what we found. Across a wide variety of tasks, we observed that split-brain patients could reliably indicate presence, location, orientation and identity of stimuli throughout the entire visual field regardless of how they responded. Strikingly, although this clinical observation features in many textbooks ( Gazzaniga et al. , 1998 ; Gray, 2002 ) the reported data are never quantitative. For three reasons it is important to explicitly map out how often ‘blindness’ to the left visual field is indicated by verbal/right hand responses and unawareness to the right visual field is indicated by left hand responses. First, the number of split-brain patients is now rapidly decreasing, and it will soon be impossible to study this phenomenon. Second, there is some doubt about how clear-cut the textbook findings are. In one of the seminal publications on this topic, Sperry (1968) reports that split-brain patients seem blind to the left visual field when responding with the right hand and vice versa. However, in the last paragraph (p. 733), Sperry notes: ‘Although the general picture has continued to hold up in the main as described [… .] striking modifications and even outright exceptions can be found among the small group of patients examined to date’. Moreover, Levy et al. (1972) investigated perception of chimeric faces in five split-brain patients. Although not the focus of their research, they observed that all patients were better at matching a face to a sample when the face was presented in the left visual field, regardless of whether they responded with the left or the right hand (p. 65). Finally, note that there are multiple examples in the literature suggesting some kind of interhemispheric integration of information ( Corballis and Trudel, 1993 ; Corballis, 1995 ; Corballis and Corballis, 2001 ; Savazzi and Marzi, 2004 ; Savazzi et al. , 2007 ). This, like Sperry’s (1968) closing remark, casts doubt on the precise nature of the split-brain phenomenon. Third, the status of split-brain patients may have important consequences for current dominant theories of consciousness. Congruent with the canonical view of split-brain patients, both the Global Workspace theory ( Baars, 1988 , 2005 ; Dehaene and Naccache, 2001 ) and the Information Integration theory ( Tononi, 2004 , 2005 ; Tononi and Koch, 2015 ) imply that without massive interhemispheric communication two independent conscious systems appear. If the canonical view cannot be quantitatively replicated, and evidence for conscious unity in the split-brain syndrome is found, both theories may require substantial modifications. In our current studies we reproduced the classic finding that split-brain patients are unable to integrate visual information across the two visual half-fields. However, we also investigated systematically to what extent performance depends on where a stimulus appears. For various tasks and stimuli we studied whether there is a response type × visual field interaction: can split-brain patients only respond to stimuli in the left visual field with the left hand, and to stimuli in the right visual field with the right hand or verbally? Patients were tested across several years, during their routine neurological control visits. For Experiment 1 we tested Patients DDC and DDV, for Experiments 2–5 we tested Patient DDC. Both patients underwent a full callosotomy to relieve epileptic seizures. Crucially, for current purposes, in Patient DDC the complete corpus callosum and most of the anterior commissure was cut, and in Patient DDV the complete corpus callosum was removed. We selected Patient DDC for the extensive follow-up testing since his ‘split’ is the most severe. Note that other than the removal of the corpus callosum, both patients had no brain damage, and fell within the normal IQ range. See Supplementary material and Pizzini et al. (2010) and Corballis et al. (2010) for detailed descriptions of these patients. In all experiments the patient(s) responded with three response types (response conditions were blocked), verbal, right hand or left hand, except for Experiment 2A, where the patient only responded verbally; and Experiments 2C and 4A where only left and right hand responses and no verbal responses were given. The experimenter (who could not see the test stimuli) mouse-clicked on the response box or location indicated by the patient. In the case of verbal position indication, the mouse was moved by the experimenter (not having seen the stimulus) on the instructions of the patient until the desired position was obtained. In Experiment 1 both patients performed a combined detection/localization task. Either nothing appeared (50% of trials) or a red solid circle, on a grey background (see Supplementary material for all stimulus details), appeared for 120 ms anywhere in the visual field. Each trial the patient indicated whether a stimulus had appeared, and if so where. In Experiment 2A, Patient DDC indicated whether two rectangles had the same orientation. In Experiment 2B he reported if two simple shapes were the same, and in Experiment 2C he indicated if two pictures were equal. In all experiments the test stimuli appeared for 120 ms. The stimuli appeared (i) both in left visual field; (ii) both in right visual field; or (iii) they appeared around fixation with one stimulus in left visual field and on in right visual field. In Experiment 3A a picture was presented for 120 ms in the left or right visual field, after which Patient DDC selected the correct verbal label matching the picture. Experiment 3B was identical to 3A, but instead of selecting a verbal label, Patient DDC selected from two pictures which image he had just seen. In Experiment 4A either nothing appeared, or a simple shape (square, circle or triangle) appeared for 100 ms in the left or right visual field. Patient DDC indicated if something had appeared, and if so what. In Experiment 4B two rectangles were successively presented, the first of which appeared for 120 ms, in the left or right visual field. Patient DDC indicated whether both rectangles had the same orientation, and if not, how large the orientation difference was. In both experiments, after each trial, Patient DDC indicated confidence in his judgement (Experiment 4A on a scale from 1 to 4, Experiment 4B on a scale from 1 to 4). Experiment 5 was similar to Experiment 1, except after each trial Patient DDC indicated confidence in his presence and location judgement (on a scale from 1 to 5). Moreover, stimuli were bright green on a red background, or dim green on a red background. In the latter case stimuli and background were equiluminant (as determined by an objective measurement). An overview of the results of Experiment 1. Both split-brain patients, Patients DDC and DDV, accurately indicated presence and location (distance error is in degrees of visual angle) of stimuli appearing throughout the entire visual field, regardless of response type (verbally, left hand or right hand). These findings challenge the canonical view that split-brain patients can only respond correctly to the left visual field with the left hand and vice versa. In Experiment 1 ( Fig. 2 ), we explored to what extent Patients DDV and DDC can detect stimuli across the entire visual field using three response conditions: left hand, right hand, and verbally. Subjects were shown red circles in various locations of the visual field (50% of trials no stimulus was presented), and had to detect presence or absence either verbally or by indicating yes/no with either hand. Subsequently, for seen stimuli, they had to indicate the location of the stimulus. Both patients responded (nearly) perfectly in indicating presence of the stimulus (Patient DDV, hits: 100%, false alarms: 0%; Patient DDC, hits: 97.5%, false alarms: 7.7%), and were highly accurate in indicating location of the stimulus (average distance between pointed location and actual location: Patient DDV: 2.8°, Patient DDC: 4.5°). While presence and location performance was highly significantly above chance (all P < 0.001), the response type × visual half-field interaction did not approach significance in either patient or task (all P > 0.5). An overview of the results of Experiments 2 and 3. Patient DDC was not able to compare stimuli across visual half-fields, although he was able to do so within one visual half-field (Experiment 2A–C). Moreover, he was better at labelling stimuli in the right visual field (Experiment 3A) and better at matching stimuli in the left visual field (Experiment 3B). Crucially, although visual information remained unintegrated across visual half-fields, there was still no response type × visual field interaction. We found further evidence that visual information is not shared between hemispheres in Experiment 3 ( Fig. 3 ). Here we observed that Patient DDC was better at selecting the correct verbal label for an image when it had appeared in the right visual field than when it had appeared in the left visual field (Experiment 3A, left visual field: 73.4%, right visual field: 92.1%, left visual field versus right visual field: P < 0.001). Yet, he was better at matching a stimulus to sample for items in left visual field, replicating earlier split-brain findings ( Funnell et al. , 1999 ) (Experiment 3B, left visual field: 95.5%, right visual field: 73%, left visual field versus right visual field: P < 0.001). Note that, despite the seeming lack of transfer of visual information, we still observed no response type × visual field interaction in Experiments 2 and 3 (all P > 0.12). Thus, for instance, Patient DDC was better at matching to sample of stimuli in the left visual field even when he responded with the right hand. This suggests that processing of visual stimuli remains within each individual hemisphere, each with its own relative performances in various tasks, yet control over the report of the outcomes of this processing is undivided. Across three experiments Patient DDC performed better on high confidence than on low confidence trials, suggesting accurate metacognition. Moreover, also for high confidence trials we observed no response type × visual field interaction, suggesting that unity in responding was based on conscious perception, not on blindsight-like processes. First, Patient DDC was tested on two visual matching experiments (shape and orientation). Second, he performed a detection and localization task of simple stimuli as in the first experiment (with the addition that the stimuli were presented equiluminantly with the background or with a large luminance difference). Patient DDC performed nearly flawlessly in detecting objects in Experiment 5 (no false alarms and two misses in 167 trials). This ceiling effect precluded meaningful metacognitive assessment of this aspect of the task. However, in the other two experiments and the localization of objects in Experiment 5, performance did not show a ceiling or floor effect, allowing us to investigate metacognitive abilities in these cases. This revealed that in all three experiments Patient DDC’s performance was better on high than on low confidence trials. All trials: Experiment 4A, left visual field: 88.7%, right visual field: 43%; Experiment 4B, left visual field: 82.8% right visual field: 63.4%; Experiment 5, left visual field: accuracy: 100%, distance error: 3.27°, right visual field: accuracy: 98.3%, distance error: 2.33°. High confidence trials: Experiment 4A, left visual field: 100%, right visual field: 62.5%; Experiment 4B, left visual field: 95.9% right visual field: 84.6%; Experiment 5, left visual field: accuracy: 100%, distance error: 2.63°, right visual field: accuracy: 98.2%, distance error: 1.84°; all P < 0.005). Further, we found a robust Goodman and Kruskal’s γ correlation ( Goodman and Kruskal, 1954 ) between confidence and performance in all cases (Experiment 4A, γ = 0.527, P < 0.001; Experiment 4B, γ = 0.316, P = 0.003; Experiment 5, γ = −0.227, P = 0.02. There were no differences between γ correlations in left visual field and right visual field, all P > 0.09). Both analyses indicate that Patient DDC possessed accurate metacognition. Crucially, on high confidence trials we still found no response type × visual field interaction (all P > 0.63). This indicates that Patient DDC’s performance is not rooted in unconscious processes: his correct answers are based on conscious awareness and decisions. Note further that in the detection and localization task, luminance difference did not affect results (all P > 0.8), indicating that our findings are not due to overly strong stimulation, or stray-light leaking over to the other visual half-field. In addition to these five experiments we obtained phenomenal reports from both split-brain patients (see Supplementary material for an extensive description). Both patients indicated that they saw their entire visual field (so not just the visual field to the left or right of fixation). Further, they indicated that they felt, and were in control of their entire body. Finally, they reported that their conscious unity was unchanged since the operation (i.e. no other conscious agent seemed to be present in their brain/body). These phenomenal reports are congruent with earlier reports of split-brain patients, which documented that split-brain patients feel normal and behave normally in social situations ( Bogen et al. , 1965 ; Sperry, 1968 ). In conclusion, with two patients, and across a wide variety of tasks we have shown that severing the cortical connections between the two hemispheres does not seem to lead to two independent conscious agents within one brain. Instead, we observed that patients without a corpus callosum were able to respond accurately to stimuli appearing anywhere in the visual field, regardless of whether they responded verbally, with the left or the right hand—despite not being able to compare stimuli between visual half-fields, and despite finding separate levels of performance in each visual half-field for labelling or matching stimuli. This raises the intriguing possibility that even without massive communication between the cerebral hemispheres, and thus increased modularity, unity in consciousness and responding is largely preserved. This preserved unity of consciousness may be especially challenging for the two currently most dominant theories of consciousness, the Global Workspace theory ( Baars, 1988 , 2005 ; Dehaene and Naccache, 2001 ) and the Integration Information theory ( Tononi, 2004 , 2005 ; Tononi and Koch, 2015 ). A core assumption of the Global Workspace theory is that cortical broadcasting of selected information by the ‘global workspace’ leads to consciousness. Thus severing of the corpus callosum, which prevents broadcasting of information across hemispheres, seems to exclude the emergence of one global workspace for both hemispheres. Rather, it seems that without a corpus callosum either two independent global workspaces emerge, or only one hemisphere will have a global workspace, while the other does not. In either case, an integrated global workspace, and thus preserved conscious unity, seems to be difficult to fit into this framework. Also for Integration Information theory, conscious unity in the split-brain syndrome seems to be challenging. According to the Integration Information theory the richness of integration of information (called φ, defined by how much information is represented, and how integrated the information is) determines the level of consciousness. Moreover, only if the combined φ of two subsystems is larger than the φ per system, then the two subsystems combine to form one conscious entity. After removal of the corpus callosum, which all but eliminates communication between the cerebral hemispheres, integration of information is larger within each hemisphere than between hemispheres. Thus, according to the Integration Information theory, in the split-brain syndrome φ per hemisphere is larger than the combined φ, thus leading to two independent conscious systems rather than one conscious agent ( Tononi, 2005 ). It thus seems that the current results provide a challenge for the Global Workspace and the Integrated Information theory of consciousness. However, the current results may fit well with the local recurrent processing theory of consciousness ( Lamme and Roelfsema, 2000 ; Lamme, 2006 ; Block, 2007 ). This theory claims that local recurrent interactions between neural areas (for example between V1 and V5 in the visual system) are enough to create consciousness, even if these interactions are not part of a larger integrated network, and do not project their outcomes to a central processing unit. Thus, according to this theory, even in healthy subjects, relatively isolated processing in one hemisphere can lead to normal visual experiences. Therefore, the local recurrent processing theory suggests that consciousness in split-brain patients may be similar to consciousness in healthy subjects (and thus equally unified). How should these results be compared to our classic text-book knowledge of the split-brain phenomenon? It is unlikely that our results can be explained by the anterior and posterior commissure still being (somewhat) intact, as this was also the case for many of the previously tested patients, and this did not seem to play an important role then ( Gazzaniga et al. , 1985 ; Seymour et al. , 1994 ; Gazzaniga, 2005 ). Another possible explanation to consider is that the current findings were caused by cross-cueing (one hemisphere informing the other hemisphere with behavioural tricks, such as touching the left hand with the right hand). We deem this explanation implausible for four reasons. First, cross-cueing is thought to only allow the transfer of one bit of information ( Baynes et al. , 1995 ). Yet, both patients could localize stimuli throughout the entire visual field irrespective of response mode (Experiments 1 and 5), and localizing a stimulus requires more than one bit of information. Second, visual capabilities differed per hemifield (Experiment 3: better matching for stimuli in left visual field, better labelling of stimuli in right visual field) and comparison of stimuli over hemifields was not possible (Experiment 2). This suggests that transfer of visual information did not occur. Yet, in these same experiments response type did not affect performance, suggesting that unity in control was not driven by any form of transfer of visual information. Third, we explicitly set up the experiments to prevent cross-cueing (e.g. hands were not allowed to touch each other, or the other half of the body). Moreover, we did not observe any indications of cross-cueing occurring. Fourth, as cross-cueing is a slow process, ipsilateral responses driven by cross-cueing should be considerably slower than contralateral responses. Yet, in one experiment where Patient DDC indicated, as quickly as possible, the colour of a circle appearing shortly to the left or the right of fixation, average ipsilateral and contralateral responses were almost equally fast, and equally accurate (ipsilateral reaction times: 1229 ms, ipsilateral accuracy: 88.4%; contralateral reaction times: 1307 ms, contralateral accuracy: 97%; No significant difference between ipsilateral and contralateral reaction times: P = 0.13; or between ipsilateral and contralateral accuracy: P = 0.55, see Supplementary material for details). Finally, a possibility is that we observed the current results because we tested these patients well after their surgical removal of the corpus callosum (Patient DDC and Patient DDV were operated on at ages 19 and 22 years, and were tested 10–16 and 17–23 years after the operation, respectively). This would raise the interesting possibility that the original split brain phenomenon is transient, and that patients somehow develop mechanisms or even structural connections to re-integrate information across the hemispheres, particularly when operated at early adulthood. Even then, it remains the case that these patients’ minds have a curious property: somehow, their perception seems split, each hemisphere processing visual information independently, and at the best of their individual—yet different—abilities. When it comes to reporting this information to the outside world, however, the outcomes of the perceptual processes are unified in consciousness, verbalization and control of the body. This ‘split phenomenality’ combined with ‘unity of consciousness’ is difficult to grasp introspectively, and surely warrants further study, in a group of patients of which very few remain today. We thank Gabriella Venanzi for scheduling the patients’ exams. We also thank D.D.C. and D.D.V. and their families for their willingness to collaborate in these studies. This research was supported by a Marie Curie IEF grant (PIEF-GA-2011-300184) to Y.P., a Marie Curie IEF grant (PIEF-GA-2012-SOC-329134) to M.O. and by Advanced Investigator Grants by the European Research Council to V.A.F.L. and E.H.FdH. Supplementary material Supplementary material is available at Brain online. Baars BJ . A cognitive theory of consciousness . New York : Cambridge University Press ; 1988 . Google Scholar Google Preview Baars BJ . Global workspace theory of consciousness: toward a cognitive neuroscience of human experience . Prog Brain Res 2005 ; 150 : 45 – 53 . Baynes K , Wessinger CM , Fendrich R , Gazzaniga MS . The emergence of the capacity to name left visual field stimuli in a callosotomy patient: implications for functional plasticity . Neuropsychologia 1995 ; 33 : 1225 – 42 . Block N . 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Nature , 2012 ; 483 : 260 – 3 . consciousness related finding corpus callosum visual fields Supplementary data Month: Total Views: January 2017 557 February 2017 634 March 2017 218 April 2017 204 May 2017 414 June 2017 161 July 2017 115 August 2017 108 September 2017 143 October 2017 396 November 2017 198 December 2017 170 January 2018 392 February 2018 1,244 March 2018 1,272 April 2018 877 May 2018 2,026 June 2018 1,653 July 2018 1,378 August 2018 1,137 September 2018 1,522 October 2018 1,629 November 2018 2,055 December 2018 1,671 January 2019 1,247 February 2019 1,595 March 2019 1,898 April 2019 1,699 May 2019 1,615 June 2019 1,426 July 2019 1,576 August 2019 1,398 September 2019 1,966 October 2019 1,349 November 2019 2,389 December 2019 1,016 January 2020 844 February 2020 1,163 March 2020 845 April 2020 1,242 May 2020 674 June 2020 1,103 July 2020 771 August 2020 813 September 2020 2,541 October 2020 2,571 November 2020 1,407 December 2020 1,086 January 2021 945 February 2021 1,658 March 2021 1,584 April 2021 1,100 May 2021 1,094 June 2021 683 July 2021 515 August 2021 746 September 2021 1,931 October 2021 2,249 November 2021 1,383 December 2021 1,322 January 2022 1,125 February 2022 2,252 March 2022 1,408 April 2022 1,142 May 2022 1,205 June 2022 662 July 2022 672 August 2022 977 September 2022 1,875 October 2022 1,779 November 2022 1,315 December 2022 1,004 January 2023 1,162 February 2023 1,319 March 2023 929 April 2023 950 May 2023 851 June 2023 526 July 2023 573 August 2023 517 September 2023 1,429 October 2023 1,310 November 2023 953 December 2023 839 January 2024 945 February 2024 1,120 March 2024 1,149 April 2024 1,055 May 2024 979 June 2024 611 July 2024 518 Email alerts Citing articles via, looking for your next opportunity. 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Personality Split Brains The brain's processing of information affected by hemispheric transfer.. Posted November 6, 2012 | Reviewed by Ekua Hagan Split-brain surgery, or corpus calloscotomy, is a drastic way of alleviating epileptic seizures, the occurrence of sporadic electrical storms in the brain. The procedure involves severing the corpus callosum, the main bond between the brain’s left and right hemispheres. After a split-brain surgery, the two hemispheres do not exchange information as efficiently as before. This impairment can result in split-brain syndrome, a condition where the separation of the hemispheres affects behavior and agency. Michael Gazzaniga and Roger W. Sperry , the first to study split brains in humans, found that several patients who had undergone a complete calloscotomy suffered from split-brain syndrome. In patients with split-brain syndrome, the right hemisphere, which controls the left hand and foot, acts independently of the left hemisphere and the person’s ability to make rational decisions. This can give rise to a kind of split personality , in which the left hemisphere give orders that reflect the person’s rational goals , whereas the right hemisphere issues conflicting demands that reveal hidden desires. Gazzaniga and Sperry's split-brain research is now legendary. One of their child participants, Paul S., had a fully functional language center in both hemispheres. This allowed the researchers to question each side of the brain. When they asked the right side what their patient wanted to be when he grew up, he replied, "an automobile racer." When they posed the same question to the left, however, he responded, "a draftsman." Another patient pulled down his pants with the left hand and back up with the right in a continuing struggle. On a different occasion, this same patient's left hand made an attempt to strike the unsuspecting wife as the right hand grabbed the villainous limp to stop it. Split personality is a rare consequence of a split brain. In some cases, impaired interhemispheric communication leaves personality intact but still allows people to use the two hemispheres to complete independent intellectual tasks. An MRI scan of the savant Kim Peek, who inspired the fictional character Raymond Babbitt (played by Dustin Hoffman) in the movie Rain Man , revealed an absence of the corpus callosum, the anterior commissure and the hippocampal commissure, the three cables for information transfer between hemispheres. As a consequence of this complete split, Peek, who sadly died last year, was able to simultaneously read both pages of an open book and retain the information. He apparently had developed language areas in both hemispheres. Peek was a living encyclopedia. He spent every day with his dad in the library absorbing information. Among his most impressive feats was his ability to provide traveling directions between any two cities in the world. Today, hemisphere interaction can be studied using devices that measure the electric or magnetic fields surrounding the skull. Unlike split-brain surgery, these techniques are non-invasive. A team of researchers from UC Santa Barbara, led by Gazzaniga, recently tested information transfer using MEG. Language is processed in areas of the temporal lobe on the left side of the head. When you read with your left eye, the information first ends up in the right hemisphere and must be transferred to the left hemisphere via the corpus callosum to be processed. To test the efficiency of the hemispheric transfer, the researchers showed a randomized list of words and nonsense words to the left or right eye of a number of research participants. They then measured how effectively the subjects would be able to distinguish words from nonsense words. The study showed that subjects were significantly more efficient in determining the nature of the string of letters when the information was fed directly to the left hemisphere via the right eye. Apparently, the brain has difficulties processing information that has had to travel long distances. The researchers didn't compare both-eye exposure to single-eye exposure. At first glance, it may seem that it would be an advantage to get information from both eyes. However, one can also imagine that hemispheric transfer has a hampering effect on language processing. If this is true, you might want to wear a pirate eye patch covering your left eye when completing the verbal section of the GRE. At the very least, be careful not to shut your right eye while under time pressure. K. W. Doron, D. S. Bassett, M. S. Gazzaniga. Inaugural Article: Dynamic network structure of interhemispheric coordination . Proceedings of the National Academy of Sciences, 2012; DOI: 10.1073/pnas.1216402109 Berit Brogaard, D.M.Sci., Ph.D. , is a professor of philosophy and the Director of the Brogaard Lab for Multisensory Research at the University of Miami. Find a Therapist Find a Treatment Center Find a Psychiatrist Find a Support Group Find Online Therapy United States Brooklyn, NY Chicago, IL Houston, TX Los Angeles, CA New York, NY Portland, OR San Diego, CA San Francisco, CA Seattle, WA Washington, DC Asperger's Bipolar Disorder Chronic Pain Eating Disorders Passive Aggression Goal Setting Positive Psychology Stopping Smoking Low Sexual Desire Relationships Child Development Self Tests NEW Therapy Center Diagnosis Dictionary Types of Therapy Sticking up for yourself is no easy task. But there are concrete skills you can use to hone your assertiveness and advocate for yourself. Emotional Intelligence Gaslighting Affective Forecasting Neuroscience Roger Sperry’s Split Brain Experiments (1959–1968) - Image How to cite Articles rights and graphics. Copyright Arizona Board of Regents Licensed as Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported (CC BY-NC-SA 3.0)   Last modified Share this page. An Experiment with a Split-Brain Subject (the corpus callosum has been severed, a treatment for epilepsy), the left brain dominates for language, speech, and problem solving, the right brain dominates for visual-motor tasks, 1.  each hemisphere was presented a picture that related to one of four cards placed in front of the split-brain subject.  the right hemisphere saw the picture on the left (a snow scene), and the left hemisphere saw the picture on the right (a chicken foot).  both hemispheres could see all of the cards., 2.  the left and right hemispheres easily picked the card that related to the picture it saw.   the left hand pointed to the right hemisphere's choice, and the right hand pointed to the left hemisphere's choice., 3.  the patient was then asked why the left hand was pointing to the shovel.  only the left hemisphere can talk, and it did not know the answer because the decision to point to the shovel was made in the right hemisphere., 4.  immediately the left hemisphere made up a story about what it could see --- the chicken.  it said the right hemisphere chose the shovel to clean out a chicken shed., this reveals the left brain's interpreter in action.. Source:  Gazzaniga, Michael S., "The Split Brain Revisited," Scientific American , July 1998 Return to Honors PS1500 home page Last modified:  Monday, January 24, 2005 11:02 AM 101 COMMENTS Split-brain Split-brain or callosal syndrome is a type of disconnection syndrome when the corpus callosum connecting the two hemispheres of the brain is severed to some degree. It is an association of symptoms produced by disruption of, or interference with, the connection between the hemispheres of the brain. The surgical operation to produce this ... Split-Brain: What We Know Now and Why This is Important for In some patients all commissures were severed ("commissurotomy"), in others only the corpus callosum was cut ("callosotomy") and some patients fall somewhere in between these two boundaries. Now, the search term "split-brain" results in a total of 2848 publications in the database of the Web-of-Science and 29,300 hits on Google ... Roger Sperry's Split Brain Experiments (1959-1968) 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 ... Experiment Module: What Split Brains Tell Us About Language Communication between the two hemispheres of the brain is made possible by the bundles of axons, or commissures, that connect them. The largest of these bundles, known as the corpus callosum, consists of about 200 million axons running from one hemisphere to the other. In the 1950s, American neuroscientist Roger Sperry and his team discovered ... One Brain. Two Minds? Many Questions Abstract. For several decades, split-brain research has provided valuable insight into the fields of psychology and neuroscience. These studies have progressed our knowledge of hemispheric specialization, language processing, the role of the corpus callosum, cognition, and even human consciousness. Following a recent empirical paper by Pinto et ... The split brain: A tale of two halves The experiment he is running aims to separate the role of the corpus callosum in visual processing from that of deeper, 'subcortical' connections unaffected by the callosotomy. How An Epilepsy Treatment Shaped Our Understanding of ... 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 ... Split-brain syndrome split-brain syndrome, condition characterized by a cluster of neurological abnormalities arising from the partial or complete severing or lesioning of the corpus callosum, the bundle of nerves that connects the right and left hemispheres of the brain.. Although it is not fully understood whether the processing of specific tasks is dependent on both hemispheres of the brain, the two hemispheres ... Interaction in isolation: 50 years of insights from split-brain Introduction. Fifty years ago, one of the first studies that showed the neuropsychological consequences of sectioning the corpus callosum, that great bundle of fibres that connects the two cerebral hemispheres, was published in Brain ( Gazzaniga and Sperry, 1967 ). With the help of several patients who have undergone this procedure and ... The Roots Of Consciousness: We're Of 2 Minds 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 ... Split-Brain: What We Know Now and Why This is Important for ... The term "split-brain" refers to patients in whom the corpus callosum has been cut for the alleviation of medically intractable epilepsy. Since the earliest reports by van Wagenen and Herren and Akelaitis (1941, 1943) on the repercussions of a split-brain, two narratives have emerged.First and foremost is the functional description, pioneered by Gazzaniga, Sperry and colleagues (Gazzaniga ... 2.7: Split-Brain Measures-severing the corpus callosum In this surgery, the region that normally connects the two halves of the brain and supports communication between the hemispheres, known as the corpus callosum, is severed. As a result, the patient essentially becomes a person with two separate brains. Because the left and right hemispheres are separated, each hemisphere develops a mind of its ... The Split Brain Experiments The studies demonstrated that the left and right hemispheres are specialized in different tasks. The left side of the brain is normally specialized in taking care of the analytical and verbal tasks. The left side speaks much better than the right side, while the right half takes care of the space perception tasks and music, for example. PDF The Corpus Callosum What is a Split Brain? The Corpus Callosum Lecture ischarge of neurons through the corpus callosum and into the second hemisphere.To preve. t seizures in severe epileptics, the corpus callosum c. eaves the two hemispheres functionally separate.The History of the Split Brain"The. corpus callosum is sectioned longitudinally ...no symptoms follow its division. This simple experiment puts an end. Splitting the Human Brain In Two SfN: I'm Jessica Johnson for BrainFacts.org. In the 1940s, neurosurgeons began a series of experimental surgeries to treat epilepsy in patients who failed to respond to medications. Ten patients volunteered to have the wide, flat bundle of neural fibers, called the corpus callosum, severed in their brains, effectively disconnecting the left and right hemispheres. Split Brains: What Happens When You Sever the Corpus Callosum? This is an area called V1. The primary visual cortex. Ok, so let's get back to our split brain patients. What happens when we split the corpus callosum, we sever the corpus callosum, we split the brain, we don't cut the optic chiasm. So this process still occurs, this transfer from visual field over to the opposite hemisphere still occurs. Split brain: divided perception but undivided consciousness For Experiment 1 we tested Patients DDC and DDV, for Experiments 2-5 we tested Patient DDC. Both patients underwent a full callosotomy to relieve epileptic seizures. Crucially, for current purposes, in Patient DDC the complete corpus callosum and most of the anterior commissure was cut, and in Patient DDV the complete corpus callosum was removed. Split Brain In one experiment, the participant was shown two images on a screen, one on the right and one on the left. ... With a severed corpus callosum, each hemisphere of the brain continues to function ... Split-Brain Split Brain. A 'split brain,' usually the brain of a mammal, is one in which all direct, 'one-neuron' connections between the two forebrain cerebral cortices have been cut. The largest interhemispheric bridge, or commissure, is the corpus callosum (Bogen 1985, Innocenti 1986). The first split-brain operation was performed on a cat in ... Dual consciousness Dual consciousness (or Dual mind) is a hypothesis or concept in neuroscience.It is proposed that it is possible that a person may develop two separate conscious entities within their one brain after undergoing a corpus callosotomy.The idea first began circulating in the neuroscience community after some split-brain patients exhibited alien hand syndrome (AHS), which led some scientists to ... Split Brains Split-brain surgery, or corpus calloscotomy, is a drastic way of alleviating epileptic seizures, the occurrence of sporadic electrical storms in the brain. The procedure involves severing the ... Roger Sperry's Split Brain Experiments (1959-1968) Experiment Chemoaffinity Brain Function. Brain--Localization of functions Brain--Surgery Split-Brain Procedure Corpus callosum Experiments Split brain. ASU Center for Biology and Society. Support. Split Brain Experiment An Experiment with a Split-Brain Subject (Corpus Callosum severed, a treatment for epilepsy) Left brain dominates for language, speech, and problem solving: Right brain dominates for visual-motor tasks: 1. Each hemisphere was presented a picture that related to one of four pictures placed in front of the split-brain subject. Split Brain Experiment An Experiment with a Split-Brain Subject (The Corpus Callosum has been severed, a treatment for epilepsy) The left brain dominates for language, speech, and problem solving; The right brain dominates for visual-motor tasks; 1. Each hemisphere was presented a picture that related to one of four cards placed in front of the split-brain subject. Full article: Tropomyosin Receptor Kinase B Expressed in After tamoxifen, there is a 58% reduction of TrkB+CC1+/total CC1 cells in control mice and a 51% reduction in cuprizone mice. Scale bar = 20 μm. *Significantly different from Cre- corpus callosum, analyzed using paired Student's t test. *p < .05. N = 3 experiments. Each experiment includes four mice (Control +/- Cre and Cuprizone +/- Cre). essay homework paper phd project resume review speech study thesis