Another page of the album has a slightly faded printout of a paper from The Journal of Neuroscience glued into it: the first published report involving data gleaned from Vicki, in which researchers describe how she, like P.
When pressed to share the most difficult aspect of her life in science, the perpetually upbeat Vicki says that it would have to be an apparatus called the dual Purkinje eye tracker.
This medieval-looking device requires the wearer to bite down on a bar to help keep the head still so that researchers can present an image to just the left or right field of view. It is quite possible that Vicki has spent more of her waking hours biting down on one of those bars than anyone else on the planet. Soon, it is time to get back to work. Turk uses some two-sided tape to affix a pair of three-dimensional glasses onto the front of Vicki's thin, gold-rimmed bifocals.
The experiment he is running aims to separate the role of the corpus callosum in visual processing from that of deeper, 'subcortical' connections unaffected by the callosotomy. Focusing on the centre of the screen, Vicki is told to watch as the picture slowly switches between a house and different faces — and to press the button every time she sees the image change.
Adjusting her seat, she looks down the bridge of her nose at the screen and tells Turk that she's ready to begin. Other researchers are studying the role of subcortical communication in the coordinated movements of the hands.
Split-brain patients have little difficulty with 'bimanual' tasks, and Vicki and at least one other patient are able to drive a car. In , a team led by Liz Franz at the University of Otago in New Zealand asked split-brain patients to carry out both familiar and new bimanual tasks. A patient who was an experienced fisherman, they found, could pantomime tying a fishing line, but not the unfamiliar task of threading a needle.
Franz concluded that well-practised bimanual skills are coordinated at the subcortical level, so split-brain people are able to smoothly choreograph both hands 5.
Miller and Gazzaniga have also started to study the right hemisphere's role in moral reasoning. It is the kind of higher-level function for which the left hemisphere was assumed to be king.
But in the past few years, imaging studies have shown that the right hemisphere is heavily involved in the processing of others' emotions, intentions and beliefs — what many scientists have come to understand as the 'theory of mind' 6. To Miller, the field of enquiry perfectly illustrates the value of split-brain studies because answers can't be found by way of imaging tools alone. In work that began in , the researchers presented two split-brain patients with a series of stories, each of which involved either accidental or intentional harm.
The aim was to find out whether the patients felt that someone who intends to poison his boss but fails because he mistakes sugar for rat poison, is on equal moral ground with someone who accidentally kills his boss by mistaking rat poison for sugar 7. Most people conclude that the former is more morally reprehensible. The researchers read the stories aloud, which meant that the input was directed to the left hemisphere, and asked for verbal responses, so that the left hemisphere, guided by the interpreter mechanism, would also create and deliver the response.
So could the split-brain patients make a conventional moral judgement using just that side of the brain? The patients reasoned that both scenarios were morally equal. The results suggest that both sides of the cortex are necessary for this type of reasoning task.
But this finding presents an additional puzzle, because relatives and friends of split-brain patients do not notice unusual reasoning or theory-of-mind deficits. Miller's team speculates that, in everyday life, other reasoning mechanisms may compensate for disconnection effects that are exposed in the lab. It's an idea that he plans to test in the future. As the opportunities for split-brain research dwindle, Gazzaniga is busy trying to digitize the archive of recordings of tests with cohort members, some of which date back more than 50 years.
Other split-brain patients may become available — there is a small cluster in Italy, for instance. But with competition from imaging research and many of the biggest discoveries about the split brain behind him, Gazzaniga admits that the glory days of this field of science are probably gone.
And maybe it's not — as long as there are scientists pushing to tackle new questions about lateralized brain function, connectivity and communication, and as long as Vicki and her fellow cohort members are still around and still willing participants in science. Her involvement over the years, Vicki says, was never really about her.
Gazzaniga, M. Natl Acad. USA 48 , — Brain , — Article Google Scholar. Science , — Sidtis, J. Franz, E. Young, L. NeuroImage 40 , — Miller, M. Neuropsychologia 48 , — Download references. You can also search for this author in PubMed Google Scholar. They found that when consciousness was reduced, local irregularities were still detected - for instance after three high auditory tones a low tone evoked a P3.
However, global irregularities - several times a low tone followed three high tones, then on the critical trial three high tones were followed by another high tone - did not evoke a P3 when consciousness was reduced.
Crucially, when consciousness was unimpaired both local and global irregularities evoked a P3 response. Right hemisphere consciousness may also be studied in other patient groups where interhemispheric communication is hampered. One particularly interesting group are post-hemispherotomy patients Lew, These patients have been surgically treated to disconnect an entire hemisphere usually for intractable epilepsy , but unlike hemispherectomy patients the disconnected hemisphere remains in place in the cranium and remains vascularized.
Clearly, the central question, whether each hemisphere supports an independent conscious agent, is not settled yet. Novel paradigms in this respect could lead to progress. For instance, a pivotal question is whether each hemisphere makes its own decisions independent of the other hemisphere.
If each hemisphere produces its own autonomous conscious agent then this should be the case. That is, if two agents are asked to freely choose a random number, then the odds that they consistently pick the same number are small. And vice versa, if each hemisphere makes its own conscious decisions, independent of the other hemisphere, then this seems to rule out unity of mind. Note that each hemisphere making its own decisions is different from information processing occurring independently per hemisphere.
Unconscious information processing is almost certainly split across hemispheres in a split-brain. However, this does not prove that consciousness is split or unified.
Even in a healthy brain, where consciousness is unified, many unconscious processes run independently, and in parallel. One way to tackle the central question is by having the hemispheres respond to questions in parallel.
Overt behavior most likely does not allow for this, due to bilateral motor control processes sketched earlier. However, perhaps parallel responding is possible if the hemispheres produce covert responses. For instance, the patient could be asked to pick one of four options and indicate their choice by carrying out certain content-specific mental imagery tasks. This imagery can then be decoded in parallel from each hemisphere using neuroimaging techniques see Owen et al. If each hemisphere harbors an autonomous conscious agent, then it is highly unlikely that the two hemispheres will consistently make the same choices.
Thus, if the choices are uncorrelated across hemispheres, then this may critically challenge the unified mind view. Another way to tackle the question of unified consciousness in the split-brain is to employ ERPs as markers of concurrent conscious processing in the left and right hemispheres.
This suggests some type of integration of conscious processing. Studies employing ERPs may indicate whether conscious processing is unified, while unconscious processing is split, which would be suggestive of unified consciousness.
In summary, the pivotal issue in split-brain research is whether dividing the brain divides consciousness. That is, do we find evidence for the existence of one, or two conscious agents in a split-brain? Note that intermediate results may be found. Perhaps some measures indicate unified consciousness while others do not. This would then provoke further interesting questions on the unity of consciousness. What are the crucial measures for unity of consciousness? If intermediate results are found, more unconventional possibilities should be entertained as well.
For instance, although difficult to fathom, some philosophers have suggested that a split-brain does not contain one or two observers, but a non-whole number of conscious agents Nagel, ; Perry, , for instance one and a half first-person perspectives. If evidence for this position is found, then its implications would stretch beyond split-brain patients. It would suggest that our intuitions on the indivisibility of the experiential self may be mistaken.
One way to think of this is as with the difference between conscious and unconscious processing. Perhaps this is not a dichotomous distinction, but a continuum between more or less conscious. Similarly, perhaps the existence of a first-person perspective is not dichotomous, but gradual as well.
Another possibility is that a split-brain does contain a whole number of conscious agents, but that consciousness across these agents is only partially unified. That is, the agents share some conscious experiences and decisions, but not all Lockwood, ; Schechter, ; Schechter, ; Schechter, Perhaps the number of agents is not altered, but the agent feels depersonalized in some situations, and therefore no longer feels that they control the actions, or even experience the information, that has just occurred in their brain.
New findings on the unity of consciousness in a split-brain could fundamentally impact currently dominant consciousness theories. Although not very explicit on the unity of consciousness in split-brain patients, Global Neuronal Workspace Theory seems to endorse the split consciousness idea, given that each hemisphere has its own prefrontal hub, enabling broadcasting of whatever information is processed in that hemisphere.
Integrated Information Theory Tononi, ; Tononi, has specifically addressed the issue of split brain for instance in Tononi, The higher phi, the more conscious a system is. Moreover, local maxima in phi correspond to conscious agents. If in a system all subsystems are highly interconnected, then phi is highest for the system as a whole, and local maxima are absent.
Thus, such a system produces only one conscious agent. However, if subsystems only exchange minimal amounts of information, then phi per subsystem is higher than phi for the system as whole.
In such a case each subsystem creates its own conscious agent. In a split-brain, connectedness, that is integration of information, is much higher within than across hemispheres. Therefore, according to Integrated Information Theory consciousness should be split in a split-brain. In these cases, all the information, although functionally unintegrated, is nonetheless experienced by the same mind. That is, the left hemisphere only tracks the right visual field and vice versa.
Yet, although the visual information is not integrated across hemispheres, from a first person perspective, it seems clear that the subject experiences all moving objects across the entire visual field. Another example of the dissociation between consciousness and reportability is the so-called partial report paradigm Pinto et al.
In these paradigms subjects seem to remember more than they can report. Thus, reportability and consciousness are dissociated. Perhaps in split-brain patients this dissociation is simply more pronounced. That is, consciousness remains unified, but reportability has become more dissociated, thereby inducing the appearance of two independent agents.
In sum, according to the Recurrent Processing theory, integration of information is not needed for a unified mind, implying that the mind may remain unified when the brain is split. Thus, different theories of consciousness have different predictions on the unity of mind in split-brain patients, and await the results of further investigation into this intriguing phenomenon.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. National Center for Biotechnology Information , U. Neuropsychology Review. Neuropsychol Rev. Published online May Edward H. Corballis , 2 Steven A. Hillyard , 3 Carlo A. Marzi , 4 Anil Seth , 5 Victor A. Paul M. Steven A. Carlo A. Victor A. Author information Article notes Copyright and License information Disclaimer.
Corresponding author. Received Nov 5; Accepted Apr The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
This article has been cited by other articles in PMC. Open in a separate window. Interpretations There are three, not-mutually exclusive, hypotheses concerning the mechanisms involved in, seemingly, preserved unity in the split-brain. What do We Need to Know?
Conclusions In summary, the pivotal issue in split-brain research is whether dividing the brain divides consciousness. References Akelaitis AJ. Studies on the corpus callosum: II. The higher visual functions in each homonymous field following complete section of the corpus callosum. Archives of Neurology and Psychiatry. Studies on the corpus callosum: VII. Study of language functions tactile and visual lexia and graphia unilaterally following section of the corpus callosum.
Independent resources for attentional tracking in the left and right visual hemifields. Psychological Science. Divided visuo-spatial attention systems with total and anterior callosotomy. The unity of consciousness and the split-brain syndrome. The Journal of Philosophy. The unity of consciousness. The split-brain enabled animals to memorize double the information.
Later, Sperry tested the same idea in humans with their corpus callosum severed as treatment for epilepsy, a seizure disorder. He found that the hemispheres in human brains had different functions. The left hemisphere interpreted language but not the right.
Sperry shared the Nobel Prize in Physiology or Medicine in for his split-brain research. Sperry also studied other aspects of brain function and connections in mammals and humans , beyond split-brains, in s and s. In , he developed the chemoaffinity hypothesis, which held that the axons, the long fiber-like process of brain cells, connected to their target organs with special chemical markers.
This explained how complex nervous systems could develop from a set of individual nerves. Sperry then also studied brain patterns in frogs, cats, monkeys, and human volunteers. Sperry performed much of his research on the split-brain at California Institute of Technology , or Caltech, in Pasadena, California, where he moved in Sperry began his research on split-brain in late s to determine the function of the corpus callosum.
He noted that humans with a severed corpus callosum did not show any significant difference in function from humans with intact corpus callosum , even though their hemispheres could not communicate due to the severing of the corpus callosum. Sperry postulated that there should be major consequences from cutting the brain structure, as the corpus callosum connected the two hemispheres of the brain, was large, and must have an important function.
Sperry began designing experiments to document the effects of a severed corpus callosum. At the time, he knew that each hemisphere of the brain is responsible for movement and vision on the opposite side of the body, so the right hemisphere was responsible for the left eye and vice versa.
Therefore, Sperry designed experiments in which he could carefully monitor what each eye saw and therefore what information is was going to each hemisphere. Sperry experimented with cats, monkeys, and humans. His experiments started with split-brain cats. He closed one of their eyes and presented them with two different blocks, one of which had food under it.
After that, he switched the eye patch to the other eye of the cat and put the food under the other block. The cat memorized those events separately and could not distinguish between the blocks with both eyes open.
Next, Sperry performed a similar experiment in monkeys, but made them use both eyes at the same time, which was possible due to special projectors and light filters. The split-brain monkeys memorized two mutually exclusive scenarios in the same time a normal monkey memorized one. Sperry concluded that with a severed corpus callosum , the hemispheres cannot communicate and each one acts as the only brain.
The corpus callosum is a band of nerve fibers located deep in the brain that connects the two halves of the brain. It helps the hemispheres share information, but it also contributes to the spread of seizure impulses from one side of the brain to the other. A corpus callosotomy is an operation that cuts the corpus callosum, interrupting the spread of seizures from hemisphere to hemisphere. Seizures generally do not completely stop after this procedure they continue on the side of the brain in which they originate.
However, the seizures usually become less severe, as they cannot spread to the opposite side of the brain. A corpus callosotomy, sometimes called split-brain surgery, may be performed in patients with the most extreme and uncontrollable forms of epilepsy, when frequent seizures affect both sides of the brain.
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