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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: 18 February 2015

Neuroscience: Halving it all

• Douwe Draaisma 1  

Nature volume  518 ,  pages 298–299 ( 2015 ) Cite this article

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Douwe Draaisma enjoys the autobiography of Michael Gazzaniga, who has studied split brains for half a century.

Tales from Both Sides of the Brain: A Life in Neuroscience

• Michael S. Gazzaniga

From the 1940s onwards, scores of people with intractable epilepsy were treated by surgically severing their corpus callosum, the nerve bundle that connects the left and right sides of the brain. In these 'split-brain' patients, each hemisphere operates independently. Michael Gazzaniga — known as the father of cognitive neuroscience — spent more than 50 years investigating these “splits”, as he calls them affectionately in his compelling autobiography, Tales From Both Sides of the Brain .

As a psychology student at Dartmouth College in Hanover, New Hampshire, Gazzaniga became interested in the way brain enables mind. In the summer of 1960, he positioned himself at just the right place: Roger Sperry's lab at the California Institute of Technology (Caltech) in Pasadena. Sperry had begun a research programme on split brains, based on studies with cats and monkeys. Gazzaniga and fellow pioneer Joseph Bogen extended this to people who had had the operation. Over the decades, as Gazzaniga relates, the programme branched out to explore perception, language, facial recognition, reasoning and many other cognitive processes. It produced a wealth of information on hemispheric specialization.

As the book unfolds, it becomes clear that split brains present a nested set of conundrums. The first is that roughly 200 million neural fibres have been cut, but nothing — apparently — happens. Memory, personality, cognition; everything is still intact.

To demonstrate that both hemispheres are operating separately requires shrewd experimental procedures, which Gazzaniga pioneered in the early 1960s. These revealed the second conundrum, that the left brain can see and feel things that the right brain does not, and vice versa, yet the patient experiences a single, unitary mind. Even downright discrepancies — the right brain seeing a picture of a naked person, leaving the left brain wondering about the blush — are explained away by the mind using cleverly improvised stories.

These stories point to yet a third conundrum. Why are humans, whether with an intact or a severed callosum, so left-sided? Split-brain experiments have pointed to the existence of a 'narrator' or 'interpreter', a faculty housed in the language hemisphere (almost always the left) that explains why we behave as we do.

Unlike Bogen, who proposed some now-discredited theories on 'left-brained' white city dwellers and 'right-brained' Hopi Indians in the 1970s, Gazzaniga always kept a sober perspective on hemispheric differences. Much of his later work served to debunk the popular idea of a rational, cold-hearted left brain ranged against an emotional, intuitive right brain.

In his autobiography, Gazzaniga often seems to be a man of two minds himself. His style is colloquial and unassuming (Caltech “was chock full of mighty smart cookies and most of them could run circles around me”). He is a self-confessed big-picture man, leaving mathematics and technicalities to others. He acknowledges that the course of a career, including his own, is often steered by luck and coincidence, rather than strategy. There is also a shocking nostalgia for the days before ethical committees on animal research, when cats were gathered “from the alley”.

This cheerfully detached tone, however, is absent when Gazzaniga deals with credit and priority. His experiment with Bogen's epilepsy patient W. J. in 1962 was the first to reveal that each hemisphere remains unaware of stimuli processed by the other. Bogen had suggested pre- and post-surgery experiments. “Thus begins a line of research that, twenty years later, almost to the day, will be awarded the Nobel Prize,” notes Gazzaniga. That 1981 prize (in Physiology or Medicine) was awarded to Sperry for his split-brain research — not to Sperry, Gazzaniga and Bogen. By then, Gazzaniga's relationship with Sperry had become tense, and Sperry refused to let him conduct further tests on Caltech patients.

Gazzaniga writes about Sperry with much admiration and little affection. He portrays him as a fierce competitor. Gazzaniga explains that at the pioneering stage of research, ideas become inextricably mixed, and that in science — as in families — people may come away from the same event with different memories. He clearly feels that the Nobel prize should have had more than one recipient.

Gazzaniga was at the heart of a pivotal research programme and struck up friendships with neuroscience and psychology luminaries, such as David Premack, George Miller, Leon Festinger, Endel Tulving and Steven Pinker (who wrote the book's introduction). Thus, his natural appetite to tell juicy behind-the-scenes stories is more than welcome. Historians in particular have always appreciated eighteenth-century philosopher Bernard Mandeville's dictum that private vices can be turned to public benefit.

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Invisibilia

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

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

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

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

The Other Self

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

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

Interview Highlights

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

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

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

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

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

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

What are "functions of the human condition"?

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

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

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

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

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

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

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Article Contents

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Cross-cueing mechanism and mirror neurons

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

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

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

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

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

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

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

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

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

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

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

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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

About the educational games The educational games are based on Nobel Prize awarded achievements and were produced between 2001 and 2012. Most games have not been updated since production (including potential scientific facts changes) and are provided here on an 'as is' basis by popular demand. Some of the games run in modern browsers without the need of any plugin (either as a new version or using Ruffle ), but many of the games still require Adobe Flash Player. Flash is an old technology that has reached end of life. These games will no longer work without a dedicated setup. If you are depending on these games in your profession, please advice your local IT support. We do not have the resources to provide support. We are working on supporting more games without Flash. Privacy Policy Terms of use The Nobel Prize Copyright © Nobel Prize Outreach AB 2024 An official website of the United States government The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site. The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely. Publications Account settings My Bibliography Collections Citation manager Save citation to file Email citation, add to collections. Create a new collection Add to an existing collection Add to My Bibliography Your saved search, create a file for external citation management software, your rss feed. Search in PubMed Search in NLM Catalog Add to Search One Brain. Two Minds? Many Questions Affiliation. 1 School of Psychology and Neuroscience, University of St. Andrews, St. Andrews, UK KY16 9JP. PMID: 30057510 PMCID: PMC6057762 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. Keywords: cognition; consciousness; corpus callosum; cross-cueing; hemispheric specialization; split-brain. PubMed Disclaimer Similar articles Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness. de Haan EHF, Corballis PM, Hillyard SA, Marzi CA, Seth A, Lamme VAF, Volz L, Fabri M, Schechter E, Bayne T, Corballis M, Pinto Y. de Haan EHF, et al. Neuropsychol Rev. 2020 Jun;30(2):224-233. doi: 10.1007/s11065-020-09439-3. Epub 2020 May 12. Neuropsychol Rev. 2020. PMID: 32399946 Free PMC article. Review. Consciousness after split-brain surgery: The recent challenge to the classical picture. Schechter E, Bayne T. Schechter E, et al. Neuropsychologia. 2021 Sep 17;160:107987. doi: 10.1016/j.neuropsychologia.2021.107987. Epub 2021 Aug 8. Neuropsychologia. 2021. PMID: 34371067 Split brain: divided perception but undivided consciousness. Pinto Y, Neville DA, Otten M, Corballis PM, Lamme VAF, de Haan EHF, Foschi N, Fabri M. Pinto Y, et al. Brain. 2017 May 1;140(5):1231-1237. doi: 10.1093/brain/aww358. Brain. 2017. PMID: 28122878 The Split-Brain Phenomenon Revisited: A Single Conscious Agent with Split Perception. Pinto Y, de Haan EHF, Lamme VAF. Pinto Y, et al. Trends Cogn Sci. 2017 Nov;21(11):835-851. doi: 10.1016/j.tics.2017.09.003. Epub 2017 Sep 25. Trends Cogn Sci. 2017. PMID: 28958646 Review. Complementary hemispheric specialization in monkeys. Hamilton CR, Vermeire BA. Hamilton CR, et al. Science. 1988 Dec 23;242(4886):1691-4. doi: 10.1126/science.3201258. Science. 1988. PMID: 3201258 Akelaitis AJ. Psychobiological studies following section of the corpus callosum. A preliminary report. Am J Psychiatry. 1941;97:1147–1157. Corballis MC, Corballis PM, Berlucchi G, Marzi CM. Perceptual unity in the split brain: the role of subcortical connections. Brain. 2018;141(6):e46. doi: 10.1093/brain/awy085. - DOI - PubMed Gazzaniga MS, Sperry RW. Language after section of the cerebral commissures. Brain. 1967;90:131–148. - PubMed Pinto Y, Lamme VAF, de Haan EHF. Cross-cueing cannot explain unified control in split-brain patients. Brain. 2017b;140:e68. - PubMed Pinto Y, Neville DA, Otten M, Corballis PM, Lamme VAF, de Haan EHF, Foschi N, Fabri M. Split brain: divided perception but undivided consciousness. Brain. 2017a;140:1231–1237. - PubMed LinkOut - more resources Full text sources. Europe PubMed Central PubMed Central Citation Manager NCBI Literature Resources MeSH PMC Bookshelf Disclaimer The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). Unauthorized use of these marks is strictly prohibited. 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 that curiously enough, severing the corpus callosum in the brain of a cat or monkey had no notable effects on the animal’s behaviour. Only some special experimental protocols revealed that these animals were actually sometimes behaving as if they had two brains. This absence of major deficits in animals with a severed corpus callosum gave neurosurgeons the idea of performing this operation on certain patients whose frequent, severe epileptic attacks were ruining their lives. In some of these patients, the epileptic focus was located in only one hemisphere, so this operation could successfully prevent the attacks from propagating to the other hemisphere. Having had this operation, these “split-brain” individuals could go back to enjoying their lives; as with the animals in Sperry’s experiments, their day-to-day behaviour was practically unaffected by the separation of their brains into two halves.   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. The subjects could easily repeat numbers or words or describe images projected in their right visual field, because the left hemisphere, which received and processed this information, is the dominant hemisphere for language. Similarly, when asked to close their eyes and feel an object with their right hand, they could describe the object readily. But when the visual stimuli were projected in the subjects’ left visual field or when they were asked to feel objects with their left hand, their performance was quite different: they could not describe the stimuli or objects concerned. In fact, for the visual stimuli, they even said that they hadn’t seen anything at all! Though the right hemisphere does have some serious gaps in its language-processing abilities, it is not completely devoid of them. It can read and understand numbers, letters, and short statements, so long as the individual does not have to demonstrate this understanding verbally. For example, if the name of an object is projected so that a subject with a severed corpus callosum sees it with the right hemisphere only, he will say that he doesn’t see anything, because the severed connection has in fact prevented his left hemisphere, which is dominant for language, from doing so. But if the experimenter then asks the subject to use his left hand to choose a card with a drawing of the object whose name he saw, or to identify this object by feeling it with his left hand, he will have no problem in performing the task. Thus the right hemisphere cannot express itself in complex sentences, but it clearly can recognize words. In another experiment, a photo of a naked man was presented to the right hemisphere of a female split-brain patient. When asked about the nature of the photo, she began to laugh and explained that she didn’t know why she was laughing, but that maybe it was because of the machine that was projecting the images. Certain experiments that Gazzaniga conducted with split-brain patients also led him to develop the concept of the “left-hemisphere interpreter”. In one of these classic experiments, the split-brain patient had to point with his two hands at pictures of two objects corresponding to two images that he had seen on the divided screen (one with each of his two separated hemispheres). In the test shown here, the patient’s left hand is pointing at the card with a picture of a snow shovel, because the right hemisphere, which controls this hand, has seen the projected image of a winter scene. Meanwhile, his right hand is pointing at the card with a picture of a chicken, because his left hemisphere has seen the image of a chicken’s foot. But when the patient is asked to explain why his left hand is pointing at the shovel, his talking hemisphere—the left one—has no access to the information seen by the right, and so instead interprets his behaviour by responding that the reason is that you use a shovel to clean out the chicken house! Experiments like this show just how ready the brain is to provide language-based explanations for behaviour. Gazzaniga’s experiments thus helped to demonstrate the lateralization of language as well as other functional differences between the left and right hemispheres. Who’s in Charge—Us? Split Brain Experiments: Are You One Person or Two? by The Oxford Scholastica Team | 16 Aug, 2023 | Blog Articles , Psychology Articles Would you believe me if I told you that the two halves of your brain know different things about the world: that they have different beliefs, abilities and even personalities? Many top psychology books discuss the widely held misconception that logical people are ‘left brained’ and artsy people are ‘right brained’. This isn’t quite the case. However, it is true that the left and right halves of the brain are specialised for carrying out different tasks. For instance, while the left brain is more aligned with linguistic tasks, the right brain excels in functions like spatial mapping . And, when separated, they seem to act like two very distinct individuals. Discover More Thanks for signing up. Table of Contents History of split-brain research In the sixties, patients with severe epilepsy were treated with a controversial procedure. The corpus callosum, which joins the two hemispheres (halves) of the brain and allows them to communicate, was severed. This worked: it cured their epilepsy. And, surprisingly, this did not seem to have an obvious effect on the patients’ lives. Split-brain experiment pioneers: R oger Sperry and Michael Gazzaniga Roger Sperry and Michael Gazzaniga carried out many experiments with these split-brain patients. They presented stimuli to each hemisphere separately and at the same time. They achieved this by showing things to only their left visual field (LVF) or right visual field (RVF). A stimulus presented to the LVF is processed by the right hemisphere, and stimulus presented to the RVF is processed by the left hemisphere. The left-right visual field tests In one of these famous psychology experiments , patients were simply asked to say words presented to them. When a word was processed by the left hemisphere, they could say what it was. However, when it was processed by the right hemisphere, they stated that there was nothing. This is because the right hemisphere cannot process language and so was not even able to recognise that the word was there! This was the first sign that we rely on the left hemisphere for our consciousness: without language, the right seemed to lack this ability. Discovering the left brain interpreter Then, in 1978, Michael Gazzaniga and Joseph DeLoux, discovered a phenomenon that they named the ‘Left Brain Interpreter’. In that same experimental setup, they showed images to each hemisphere simultaneously. They then asked the patients to select the corresponding pictures from a selection, using their left and right hands (the left half of the brain controls the right half of the body and vice versa). For example, patients were presented with a snow shovel to the right hemisphere and a chicken head to the left hemisphere. They were able to correctly select both the pictures of a chicken head with their right hand and snow shovel with their left hand. The interesting discovery came when patients were asked why they had made those selections. Just as before, they were unable to say they had seen the snow shovel with their right hemisphere – yet they still picked it. One patient explained this choice of pictures by saying: “The chicken claw goes with the chicken head, and you need a snow shovel to clean out the chicken shed”. This answer seems strange to you and me because our hemispheres can communicate. But, this person has no knowledge of the shovel in their left hemisphere! So, the left invented an explanation for the actions of the right hemisphere. Implications and anomalies of split brain experiments From this, it seems that the left hemisphere is responsible for our conscious interpretation of the world. But the right still has its own interpretation. Does this mean that they are each separate ‘people’? Alien hand syndrome and dual consciousness One interesting phenomenon suggests this is Alien Hand Syndrome. This was a symptom in some patients with the split-brain surgery. It is a rare disorder which can also affect patients with other types of brain damage, such as stroke patients. People with this syndrome have a hand which they cannot control – and which seems to have a mind of its own. This hand interferes with what their other hand is trying to do. For example, if the hand in control is unbuttoning a shirt, patients reported the other hand redoing the buttons. This may be a sign that each hemisphere has its own consciousness. Impact of hemisphere loss So, if the hemispheres are two different people, could someone survive with only one hemisphere? The answer to this is both yes and no. As we have seen, the hemispheres have different abilities and even intentions. Because of this, losing a hemisphere in adulthood (after these abilities have developed) causes significant problems. For example, even a small amount damage to areas in the left hemisphere can lead to difficulties in understanding or using language. Adult vs child brain adaptability But, if a hemisphere is lost at a young age, then it actually has very few noticeable effects. In these cases, the other hemisphere is often able to take on the functions of the missing one. Patient EB had his left hemisphere removed at the age of two. When older, he was capable of almost normal language (with some subtle grammatical errors). Of course, it is difficult to study this as very few people have this much brain damage. Delving deeper into psychology and consciousness And so – the question remains: are there actually two people in our brain? Can each hemisphere have its own consciousness? If such topics intrigue you, then you’re not alone. Take your passion for understanding the human mind to the next level with Oxford Scholastica Academy’s Neuroscience Summer Courses for teens. Dive into the world of psychology, explore exciting theories, and maybe even find answers to these perplexing questions. Join us this summer and embark on an intellectual journey like no other! Ready to get a head start on your future? Recommended articles Best Universities to Study Medicine in the World A degree in Medicine spans many years, so it’s important to make a good choice when committing yourself to your studies. This guide is designed to help you figure out where you'd like to study and practise medicine. For those interested in getting a head start, the... What Is A Year Abroad? One of the great opportunities offered to UK university students is taking a year abroad. But what does this involve? Who can do it? What are some of the pros and cons? In our year abroad guide, we’ll explain some of the things to bear in mind when considering this... The Ultimate Guide To Summer Internships Are you eager to make the most of your summer break and jumpstart your career? There are so many productive things students can do in the summer or with their school holidays, and an internship is one of the most valuable! A summer internship could be the perfect... 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. Institutional Login Loading content We were unable to load the content Article Summary Bibliography Split brains Marks, Charles content locked version 2 content unlocked version 1 Severing the direct neural connections between the two cerebral hemispheres produces a ‘split brain’. Does it also multiply minds? The most extensive tests of the psychological results of this operation were conducted by Roger Sperry and his colleagues. He concluded that split-brain patients have ‘Two separate spheres of conscious awareness, two separate conscious entities or minds, running in parallel in the same cranium, each with its own sensations, cognitive processes, learning processes, memories and so on’. Sperry’s view faces both conservative and radical challenges. The conservative challenge is that the results of the tests do not imply that split-brain patients have two minds and are two persons. The radical challenge is that the operation does not multiply minds but, instead, reveals a startling fact: human beings with intact commissures already have two spheres of consciousness, house two minds, and are two persons. For the purposes of this entry, a split-brain patient is one who has had a complete forebrain commissurotomy. In this operation (which has been replaced by less radical procedures), the corpus callosum and the other neural links (‘commissures’) between the two cerebral hemispheres were completely severed. Patients underwent the operation for the relief of otherwise uncontrollable epilepsy, and it was considered a therapeutic success. Epileptic attacks disappeared, became less frequent or were confined to one hemisphere. Once the patients recovered from the operation, they were able to resume their normal lives; people who knew these patients before the operation would not notice any dramatic changes in their personality, intellect or everyday behaviour. Observation under controlled conditions discloses a different picture. When input is limited to one cerebral hemisphere and response demanded of it, the behaviour of split-brain patients is decidedly abnormal, as the following simple example illustrates. ‘Key ring’ is flashed on a screen for a tenth of a second so that ‘key’ appears in the left visual field and ‘ring’ in the right. If split-brain subjects are asked to say what they saw, they respond that they saw ‘ring’ and show no sign of seeing ‘key’. But, if they are asked, instead, to retrieve with their left hands the object named on the screen from an array of items concealed from sight, they will pick out a key while rejecting a ring. Asked to point with the left hand to the object named on the screen, they point to a key or a picture of a key and not to a ring or a picture of a ring. If they are allowed to use both hands to pick out the object named from an array of items hidden from sight, their left and right hands work independently, the right settling on a ring while rejecting a key and the left doing the opposite. Someone seems to have seen ‘key’. Someone else seems to have seen ‘ring’. No one seems to have seen ‘key ring’. With suitable controls, input from the other sensory modalities, except taste, can also be confined exclusively to one hemisphere. When a response depends upon it, split-brain patients behave in similar abnormal ways. The standard explanation of such behaviour is roughly as follows. The structure of the visual system assures that the left half of the field of vision is conveyed to the right hemisphere and vice versa. Normally, information about the contralateral visual field is supplied to each hemisphere by neural communication across the commissures and by subsequent eye movement. Since the commissures of split-brain patients are severed and the short exposure time serves as a control for eye movement, their right hemispheres see only the word ‘key’ and their left only the word ‘ring’. In most people, speech production is localized in the left hemisphere; and so the oral response to the question reports only what the left hemisphere saw: the word ‘ring’. The left hand is primarily controlled by the right hemisphere; so it retrieves the object the right hemisphere saw named – a key – and points to a key or a picture of a key. (Notice that this explanation presupposes speech comprehension in the mute right hemisphere.) Similarly, the right hand is primarily controlled by the left hemisphere, thus accounting for the independent search of items concealed from sight. The failure to elicit any response suggesting that ‘key ring’ was seen is that the contents of the visual field available to each hemisphere are not the same and, because of the severing of the commissures and the experimental controls, not communicated to the opposite hemisphere. Behaviour of the sort illustrated in the ‘key ring’ example and its explanation fuel a natural, tantalizing line of inference. The behaviour shows that the split-brain patient sometimes has a disunified consciousness. No one has doubted that the behaviour associated with the left hemisphere in the ‘key ring’ example indicates that the subject consciously saw ‘ring’. The behaviour associated with the right hemisphere seems to be equally clear and prototypical evidence that the subject has a conscious experience of seeing ‘key’; in fact, it is difficult to see how this can be denied short of a general scepticism about the consciousness of human beings who can comprehend, but not produce, language. So, in the example, the patient has simultaneous conscious experiences of seeing ‘key’ and of seeing ‘ring’, but none of seeing both. This disunity of consciousness is a standing condition. The cause of the disunity of consciousness, behaviourally evident in the ‘key ring’ example, is the severing of the neural connections between the two cerebral hemispheres. These remain severed and their neural functions unduplicated whatever the behaviour of split-brain patients. In the absence of controls to prevent it, the separate spheres of consciousness associated with the left and right hemispheres have highly similar contents. This overlap of content and other factors explain why the split-brain patient’s everyday behaviour does not dramatically display two separate spheres of consciousness. Split-brain patients have two minds and are two persons. Despite the significant differences between the two hemispheres, each sustains a range and complexity of psychological functions, including self-awareness, that is characteristically human. Examinations of hemispherectomy patients and their near functional kin – patients with severe strokes in a single hemisphere – confirm the observations of split-brain patients. Neither hemisphere has any better access to the conscious contents of the other than we do to those of other people; each has as direct access to its own experiences as we do to ours. So split-brain patients have two minds. If an embodied mind of characteristic human complexity is a person, then the split-brain patient is two persons since the patient embodies two of them. If split-brain patients have two minds and are two persons, so do human beings with intact commissures. Even with commissures intact, each hemisphere receives a separate neural representation of ‘key’ and ‘ring’ in conditions like the ‘key ring’ example. Why, then, do we not see the behaviour of a split-brain patient? The usual answer is that neural communication between the hemispheres ensures that the right hemisphere is made aware that the left is seeing ‘ring’ and, perhaps, brought to have such an experience itself and vice versa for the right hemisphere. But, then, we have a duplicate of the split-brain case. Communication between the two hemispheres provides a behavioural mask of two independent streams of consciousness – two minds – and two persons just as the duplication of content in everyday circumstances does in the split-brain patients. So, starting from incontestable neurological and behavioural facts about split-brain patients, one apparently arrives at the paradoxical conclusion that we are small collectives of two minds and two persons (see Mind, bundle theory of ; Personal identity ). The line of inference just sketched can be used to define philosophical positions on split brains. For a variety of reasons, conservative challenges to Sperry (see above) hold that it should stop short of its third step. Eccles ( 1970 ) once claimed that there are no conscious mental phenomena known to be associated with the nonverbal right hemisphere and, later, that whatever conscious processes might be associated with it are subhuman in character. Others have argued that the disunity in consciousness split-brain patients sometimes exhibit is not a standing condition and does not imply two minds; yet others that, although split-brain patients have two minds, they constitute a single person because a single control structure governs both. Sperry ( 1968 ) has consistently held the middle ground, endorsing the line of inference as far as its third step. He has refused to join Bogen ( 1985 ) and Puccetti ( 1973 ) in taking the radical fourth step because, he claims, intact cerebral commissures are the physical basis of unity of consciousness and mind. Many other positions have been taken besides those mentioned. Thomas Nagel’s is particularly striking. He claims that there is no answer to the question of how many minds or persons split-brain patients contain, and that this shows that our ordinary concept of the unity of a person ‘may resist the sort of coordination with the understanding of humans as physical systems, that would be necessary to anything describable as an understanding of the physical basis of mind’ ( 1971 ). The wide diversity of opinion has several sources. The data are unexpected and sometimes messy. Any attempt to deal with them faces a special version of the mind–body problem. One must decide, on some principled grounds, the relation of various anatomic, neurological and behavioural data to mentalistic descriptions; and the proper account of the mentalistic notions of prime concern – consciousness, mind and person – is, to put it mildly, controversial. We also still lack a detailed understanding of how brain structures are responsible for the psychological distinctions involved, for example, what specific role the corpus callosum plays, what a control structure is, and how to count centres of consciousness. Besides raising the philosophical issues discussed above, research on split brains has provided much insight into problems of deep physiological and psychological interest, for example, hemispheric specialization, which are not immediately tied to them. It has also provided a launching pad for a variety of ‘thought experiments’ in philosophical discussions of personal identity. Related Articles Mind, bundle theory of By Candlish, Stewart Consciousness By Lormand, Eric Personal identity By Schechtman, Marya Share full article Advertisement Supported by Unresponsive Brain-Damaged Patients May Have Some Awareness Many patients thought to be in vegetative or minimally conscious states may be capable of thought, researchers reported. By Carl Zimmer When people suffer severe brain damage — as a result of car crashes, for example, or falls or aneurysms — they may slip into a coma for weeks, their eyes closed, their bodies unresponsive. Some recover, but others enter a mysterious state: eyes open, yet without clear signs of consciousness. Hundreds of thousands of such patients in the United States alone are diagnosed in a vegetative state or as minimally conscious. They may survive for decades without regaining a connection to the outside world. These patients pose an agonizing mystery both for their families and for the medical professionals who care for them. Even if they can’t communicate, might they still be aware? A large study published on Wednesday suggests that a quarter of them are. Teams of neurologists at six research centers asked 241 unresponsive patients to spend several minutes at a time doing complex cognitive tasks, such as imagining themselves playing tennis. Twenty-five percent of them responded with the same patterns of brain activity seen in healthy people, suggesting that they were able to think and were at least somewhat aware. Dr. Nicholas Schiff, a neurologist at Weill Cornell Medicine and an author of the study, said the study shows that up to 100,000 patients in the United States alone might have some level of consciousness despite their devastating injuries. The results should lead to more sophisticated exams of people with so-called disorders of consciousness, and to more research into how these patients might communicate with the outside world, he said: “It’s not OK to know this and to do nothing.” We are having trouble retrieving the article content. Please enable JavaScript in your browser settings. Thank you for your patience while we verify access. If you are in Reader mode please exit and  log into  your Times account, or  subscribe  for all of The Times. Thank you for your patience while we verify access. Already a subscriber?  Log in . Want all of The Times?  Subscribe . 105 IMAGES What Are Split Brain Experiments? The split brain: A tale of two halves Severed Corpus Callosum PPT A review of early split-brain experiments Split Brain Experiment on emaze COMMENTS Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness 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. The split brain: A tale of two halves Metrics. 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 ... 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 ... Split brain: divided perception but undivided consciousness 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. Roger Sperry's Split Brain Experiments (1959-1968) The split-brain enabled animals to memorize double the information. Later, Sperry tested the same idea in humans with their corpus callosum severed as treatment for epilepsy, a seizure disorder. He found that the hemispheres in human brains had different functions. The left hemisphere interpreted language but not the right. Sperry shared the ... 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 ... 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 ... Forty-five years of split-brain research and still going strong Elizabeth Jefferies. Brain Structure and Function (2022) Forty-five years ago, Roger Sperry, Joseph Bogen and I embarked on what are now known as the modern split-brain studies. These experiments ... Split-brain: What we know now and why this is important for 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. ... Neuroscience: Halving it all Split-brain experiments have pointed to the existence of a 'narrator' or 'interpreter', a faculty housed in the language hemisphere (almost always the left) that explains why we behave as we do ... The Two Halves Of The Brain See The World In Very Different Ways ... 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 ... PDF Split-Brain: What We Know Now and Why This is Important for ... Introduction. The term split-brain refers to patients in whom the corpus. " ". callosum has been cut for the alleviation of medically intrac-table epilepsy. Since the earliest reports by van Wagenen and Herren (1940) and Akelaitis (1941, 1943) on the repercus-sions of a split-brain, two narratives have emerged. Interaction in isolation: 50 years of insights from split-brain The results of this very simple experiment led to numerous questions and more testing of the split-brain patients, resulting in more intriguing answers and inferences which are well summarized by the notion of the 'left hemisphere interpreter' (Fig. 4; for a detailed account see Gazzaniga and LeDoux, 1978; Gazzaniga, 2000). Split-Brain, Split-Mind Decades of research on split-brain patients have shown that splitting the corpus callosum splits the conscious mind as well. The corpus callosum enables interhemispheric communication and consists of approximately 200-250 million axons that project from the cerebral cortex of one hemisphere to the other (Aboitiz et al., 1992, Nolte, 2009).In some cases of intractable epilepsy, the corpus ... Discovering the split mind Discovering the split mind. Seeing is believing but the results seemed hard to fathom. In 1962, scientists observed an epilepsy patient named W.J. as he attempted to complete a seemingly simple task: manipulate a set of painted blocks to match a specific pattern. Strangely, he could only execute the task with his left hand, not his right. The Split Brain Experiments The split brain experiments Background. 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. 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 ... Experiment Module: What Split Brains Tell Us About Language In one of these classic experiments, the split-brain patient had to point with his two hands at pictures of two objects corresponding to two images that he had seen on the divided screen (one with each of his two separated hemispheres). In the test shown here, the patient's left hand is pointing at the card with a picture of a snow shovel ... The Split Brain Revisited split-brain monkeys, the animals retain the ability to The Split Brain Revisited Testing for Synthesis A bility to synthesize information is lost after split-brain sur gery, as this experiment shows. One hemisphere of a patient was flashed a card with the word "bow"; the other hemisphere saw "arrow." Be- PDF The Split Brain Revisited split-brain monkeys, the animals retain the ability to The Split Brain Revisited Testing for Synthesis A bility to synthesize information is lost after split-brain surgery, as this experiment shows. One hemispher e of a patient was flashed a card with the word "bow"; the other hemisphere saw "arrow." Be- Split Brain Experiments: Are You One Person or Two? Split-brain experiment pioneers: R oger Sperry and Michael Gazzaniga. Roger Sperry and Michael Gazzaniga carried out many experiments with these split-brain patients. They presented stimuli to each hemisphere separately and at the same time. They achieved this by showing things to only their left visual field (LVF) or right visual field (RVF). 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 brains He concluded that split-brain patients have 'Two separate spheres of conscious awareness, two separate conscious entities or minds, running in parallel in the same cranium, each with its own sensations, cognitive processes, learning processes, memories and so on'. Sperry's view faces both conservative and radical challenges. Unresponsive Brain-Damaged Patients May Have Some Awareness Six groups of experts, including Dr. Owen's and Dr. Schiff's teams, began collaborating on a survey in 2008. To accelerate it, they figured out how to record brain activity in patients with an ... essay homework paper phd project resume review speech study thesis