1 hours ago · Several investigations (Corballis, Corballis, Fabri, Paggi, & Manzoni, 2005; Pinto et al., 2017a; Trevarthen & Sperry, 1973) have demonstrated convincingly that split-brain patients can accurately report the presence and location of stimuli for any position in the whole visual field, with either hand, and even verbally. Accurate detection and localization appears to be … >> Go To The Portal
However, Pinto et al. found that their patients were able to report the presence of stimuli on either the left or right visual field using either hand, as well as verbally. In other words, the split brain patients performed perfectly normally on this task
Both split-brain patients, Patients DDC and DDV, accurately indicated presence and location (distance error is in degrees of visual angle) of stimuli appearing throughout the entire visual field, regardless of response type (verbally, left hand or right hand).
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 extensive studies with two split-brain patients we replicate the standard finding that stimuli cannot be compared across visual half-fields, indicating that each hemisphere processes information independently of the other.
Split brain: Divided perception but undivided consciousness. Brain, 140(5), 1231–1237. [PubMed] Pinto, Y., Vandenbroucke, A. R., Otten, M., Sligte, I. G., Seth, A. K., & Lamme, V. A. F. (2017b).
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.
Communication between the two sides is inhibited, so the patient cannot say out loud the name of that which the right side of the brain is seeing.
Split-Brain, Split-Mind But it does—the two disconnected hemispheres of a split-brain patient appear to possess all of the defining attributes of consciousness (Damasio and Meyer, 2009), such as wakefulness, emotion, attention, and purposeful behavior (Gazzaniga, 2000; LeDoux et al., 1977).
Notice that while patients are typically unable to name stimuli presented to the left visual field, they can draw them—with their left hand—with a high degree of accuracy. This demonstrates a dissociation between a patient's ability to recognize and verbally name an object.
In this study, the post-surgery abilities of two other split-brain cases are compared to V.J.'s. Those patients are right-handed. Each is able to write words displayed to the left side of the brain, but not words displayed to the right.
So, a split brain patient couldn't type a separate email with each hemisphere. But could he or she even use both hands to type an email being viewed by both eyes on a standard screen? Again, probably not.
For instance, when blindfolded a split-brain patient may not be able to name a familiar object that is held in the left hand, because information for the sense of touch is relayed from the left side of the body to the right hemisphere, which typically has a weak language centre.
Numbers, words, and pictures visually presented in the right visual field (and thus the left hemisphere of the brain) can be repeated or described with no difficulty because the left hemisphere is usually dominant for language.
Instead, the researchers behind the study, led by UvA psychologist Yair Pinto, have found strong evidence showing that despite being characterised by little to no communication between the right and left brain hemispheres, split brain does not cause two independent conscious perceivers in one brain.
A new paper challenges a decades-old theory in neuroscience: Split brain: divided perception but undivided consciousness. According to the famous work of Roger Sperry and Michael Gazzaniga, “split brain” patients seem to experience a split in consciousness: the left and the right side of their brain can independently become aware of, and respond, ...
A stimulus in the right visual field can be reported with the right hand alone and also verbally; this is because language is located in the left hemisphere of the brain, and the left hemisphere receives visual input from the right visual field.
The authors say that this “surprising finding” suggests that “severing the cortical connections between the two hemispheres does not seem to lead to two independent conscious agents within one brain”. Similarly, when asked about their subjective experiences (although perhaps these should be taken with a pinch of salt),
However, Pinto et al. found that their patients were able to report the presence of stimuli on either the left or right visual field using either hand, as well as verbally. In other words, the split brain patients performed perfectly normally on this task
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.
According to the Integration Information theory the richness of integration of information (called φ, defined by how much information is represented, and how integrated the information is) determines the level of consciousness . Moreover, only if the combined φ of two subsystems is larger than the φ per system, then the two subsystems combine to form one conscious entity. After removal of the corpus callosum, which all but eliminates communication between the cerebral hemispheres, integration of information is larger within each hemisphere than between hemispheres. Thus, according to the Integration Information theory, in the split-brain syndrome φ per hemisphere is larger than the combined φ, thus leading to two independent conscious systems rather than one conscious agent ( Tononi, 2005 ).
The corpus callosum is the main route for communication between both cerebral hemispheres ( Innocenti, 1986; Gazzaniga, 2000; Wahl et al., 2007 ). In ‘split-brain’ patients, the corpus callosum has been surgically cut to alleviate intractable, severe epilepsy. One of the Nobel Prize-winning discoveries in neuroscience is that severing the corpus callosum leads to a curious phenomenon ( Fig. 1 ): when an object is presented in the right visual field, the patient responds correctly verbally and with his/her right hand. However, when an object is presented in the left visual field the patient verbally states that he/she saw nothing, and identifies the object accurately with the left hand only ( Gazzaniga et al., 1962; Gazzaniga, 1967; Sperry, 1968, 1984; Wolman, 2012 ). This is concordant with the human anatomy; the right hemisphere receives visual input from the left visual field and controls the left hand, and vice versa ( Penfield and Boldrey, 1937; Cowey, 1979; Sakata and Taira, 1994 ). Moreover, the left hemisphere is generally the site of language processing ( Ojemann et al., 1989; Cantalupo and Hopkins, 2001; Vigneau et al., 2006 ). Thus, severing the corpus callosum seems to cause each hemisphere to gain its own consciousness ( Sperry, 1984 ). The left hemisphere is only aware of the right visual half-field and expresses this through its control of the right hand and verbal capacities, while the right hemisphere is only aware of the left visual field, which it expresses through its control of the left hand.
Either nothing appeared (50% of trials) or a red solid circle, on a grey background (see Supplementary material for all stimulus details), appeared for 120 ms anywhere in the visual field. Each trial the patient indicated whether a stimulus had appeared, and if so where.
An overview of the results of Experiments 2 and 3. Patient DDC was not able to compare stimuli across visual half-fields, although he was able to do so within one visual half-field (Experiment 2A–C). Moreover, he was better at labelling stimuli in the right visual field (Experiment 3A) and better at matching stimuli in the left visual field (Experiment 3B). Crucially, although visual information remained unintegrated across visual half-fields, there was still no response type × visual field interaction.
Strikingly, although this clinical observation features in many textbooks ( Gazzaniga et al., 1998; Gray, 2002) the reported data are never quantitative. For three reasons it is important to explicitly map out how often ‘blindness’ to the left visual field is indicated by verbal/right hand responses and unawareness to the right visual field is indicated by left hand responses. First, the number of split-brain patients is now rapidly decreasing, and it will soon be impossible to study this phenomenon. Second, there is some doubt about how clear-cut the textbook findings are. In one of the seminal publications on this topic, Sperry (1968) reports that split-brain patients seem blind to the left visual field when responding with the right hand and vice versa. However, in the last paragraph (p. 733), Sperry notes: ‘Although the general picture has continued to hold up in the main as described .] striking modifications and even outright exceptions can be found among the small group of patients examined to date’. Moreover, Levy et al. (1972) investigated perception of chimeric faces in five split-brain patients. Although not the focus of their research, they observed that all patients were better at matching a face to a sample when the face was presented in the left visual field, regardless of whether they responded with the left or the right hand (p. 65). Finally, note that there are multiple examples in the literature suggesting some kind of interhemispheric integration of information ( Corballis and Trudel, 1993; Corballis, 1995; Corballis and Corballis, 2001; Savazzi and Marzi, 2004; Savazzi et al., 2007 ). This, like Sperry’s (1968) closing remark, casts doubt on the precise nature of the split-brain phenomenon.
We found further evidence that visual information is not shared between hemispheres in Experiment 3 ( Fig. 3 ). Here we observed that Patient DDC was better at selecting the correct verbal label for an image when it had appeared in the right visual field than when it had appeared in the left visual field (Experiment 3A, left visual field: 73.4%, right visual field: 92.1%, left visual field versus right visual field: P < 0.001). Yet, he was better at matching a stimulus to sample for items in left visual field, replicating earlier split-brain findings ( Funnell et al., 1999) (Experiment 3B, left visual field: 95.5%, right visual field: 73%, left visual field versus right visual field: P < 0.001). Note that, despite the seeming lack of transfer of visual information, we still observed no response type × visual field interaction in Experiments 2 and 3 (all P > 0.12). Thus, for instance, Patient DDC was better at matching to sample of stimuli in the left visual field even when he responded with the right hand. This suggests that processing of visual stimuli remains within each individual hemisphere, each with its own relative performances in various tasks, yet control over the report of the outcomes of this processing is undivided.
One of several qualities that make split-brain patients so astonishing is that they seem utterly unaware of their special status. The loss of the ability to transfer information from the left hemisphere to the right hemisphere and vice-versa seems to have no impact on their overall psychological state.
Split-brain patients constitute a small subpopulation of epileptic patients who have received the surgical resection of the callosal fibers in an attempt to reduce the spread of epileptic foci between the cerebral hemispheres. The study of callosotomy patients allowed neuropsychologists to investigate the effects of the hemispheric disconnection, shedding more light on the perceptual and cognitive abilities of each hemisphere in isolation. This view that callosotomy completely isolates the hemispheres has now been revised, in favor of the idea of a dynamic functional reorganization of the two sides of the brain; however, the evidence collected from split-brain patients is still a milestone in the neurosciences. The right-hemispheric superiority found in the healthy population concerning face perception has been further supported with split-brains, and it has been shown that the right disconnected hemisphere appears superior to the left hemisphere in recognizing and processing faces with similar characteristics as the observers’ (e.g., gender, identity, etc.). Even more controversial is the field of hemispheric asymmetries for processing facial emotion, some evidence suggesting a right-hemispheric superiority for all emotions, some others showing a complementary hemispheric asymmetry depending on the positive or negative emotional valence. Although the practice of callosotomy is mostly abandoned today in favor of pharmacological alternatives, further studies on the remaining split-brain patients could help advance our understanding of hemispheric specialization for social stimuli.
The study of callosotomy patients allowed neuropsychologists to investigate the effects of the hemispheric disconnection, shedding more light on the perceptual and cognitive abilities of each hemisphere in isolation.
The findings had an important and enduring impact on brain research in countless areas. Interest in hemispheric specialization in particular was sparked by these studies and paved the way for countless discoveries. Right hemisphere specialization for visuospatial functions and facial processing was confirmed with these patients. The further unraveling of right-hemisphere cognition, the “mute” hemisphere, was a major goal in Sperry’s laboratory, and much factual knowledge was learned that was not known previously. However, the linking of art and creativity with the right hemisphere was a nonempirically based inference made not by Sperry’s lab but rather by others wishing to “assign” functional hemisphericity. The general assumption was that “art” is anchored in spatial cognition, that it is a nonverbal activity requiring imagery and thus must be controlled by the right, nonlanguage hemisphere. To this day, robust evidence that the right specializes in art expression or art perception is yet to be shown, if for no other reason than that art is not a single, unitary form of expression or cognition. The conjectured right hemisphere–art link turned into a popular story that filtered back into science, shaped future research of brain and art, and overlooked other avenues for insights. This chapter traces and explores this background.
Another possible explanation for the preserved consciousness of split-brain patients stems from Giulio Tononi’s integrated information theory (IIT) of consciousness, which posits that consciousness is proportional to the capacity of a system to integrate information , such that greater information integration produces richer conscious experiences ( Tononi and Edelman, 1998; Tononi, 2004, 2008; Oizumi et al., 2014 ). Integrated information, denoted by phi, is determined by a system’s degree of differentiation (a measure of possible conscious experiences) and integration (the degree to which information across modules is unified). According to the theory, the human brain has a large phi—and therefore a rich conscious experience—because it consists of many specialized modules (high differentiation) that can form countless functional assemblies (high integration). The brain’s capacity to integrate information decreases after commissurotomy because interhemispheric connections are lost (less integration) and each hemisphere has fewer specialized modules and possible conscious states than an intact brain (less differentiation). Therefore, according to IIT, the consciousness of each hemisphere should correspondingly decrease. In a computation model of split-brain patients’ capacity for information integration, Tononi (2004) estimates that phi equals 72 in the united brain but only 61 in each disconnected hemisphere. The model predicts that phi decreases after commissurotomy, but not drastically so, because many cognitive functions are present in both cerebral hemispheres. The model is consistent with research on split-brain patients, with one exception: as we have seen, both hemispheres appear to be conscious following commissurotomy, but the conscious experience of the left hemisphere is superior to that of the right. In relation to the integrated information model, these findings may suggest that the disconnected left hemisphere has a greater capacity to integrate information—and therefore a greater phi—than the right hemisphere.
In line with these theories, each hemisphere of a split-brain patient may remain conscious following commissurotomy because the connections between the cortex and thalamus in each hemisphere remain intact . In a related vein, each hemisphere may remain conscious following surgery because of spared cortico-cortical connections. In support of this view, recent fMRI ( Monti et al., 2013) and EEG ( Boly et al., 2012) studies have shown that loss of consciousness is associated with a decrease in cortico-cortical connectivity, even when thalamocortical connectivity remains unchanged. Spared subcortical and cortico-cortical connections may enable bilateral functional connectivity after split-brain surgery ( Uddin et al., 2008 ). In a recent study, O’ Reilly et al. (2013) measured rhesus monkeys’ resting state functional connectivity with fMRI before and after callosotomy. They found that corpus callosum sectioning abolished nearly all interhemispheric functional connectivity, but these effects were mitigated if the anterior commissure was spared. In sum, these theories suggest that both hemispheres of a split-brain patient are conscious because subcortical and intra-hemispheric cortico-cortical connections remain intact following commissurotomy.
The brain’s capacity to integrate information decreases after commissurotomy because interhemispheric connections are lost (less integration) and each hemisphere has fewer specialized modules and possible conscious states than an intact brain (less differentiation).
An example of this being shown is in the most famous split brain study of all time Sperry, 1968. In this study each participant, who had had their corpus callosum removed, was shown the same picture in each eye and when shown in the second visual field they had no recollection of having seen it already.
When the right and left side of the brain are un able to communicate then it can cause an almost split personality and actions performed by either hemisphere can be performed without the other hemisphere being aware of it.
This usually involves cutting through the middle of the brain, known as the corpus callosum, and separating the two halves of the hemispheres effectively leaving the patient with two brains.
The Left Hemisphere. It is currently thought that the left side of your brain, the dominant side, contains the area which controls your speech and language in a section known as Broca’s area.
In some rare cases when you have a developed language centre on both the left and right side of the brain then you are able to read two pages of a book at the same time and retain both sets of information.
This shows that the left side of the brain, which controls the right visual field, contains the information to be able to describe an object when seen visually.
Advantages of Having a Split Brain. Often this operation is seen as a bad thing and it does have both positive and negative effects but on thing that you can do better after the operation is performing two tasks at once. A study performed by Rogers et al, 2004 found that when you have two halves of a brain then it increases your ability ...