Do two consciousnesses appear when the brain is divided?
When the cerebral hemispheres are separated, two consciousnesses appear, but only under certain conditions.
One of the most important therapeutic advances in epilepsy and neurosurgery is the sectioning of the corpus callosum. This structure links the two hemispheres and, among other things, allows information from one side to pass to the other. It also allows the electrical activation of epilepsy to spread, so that its section and separation of the two hemispheres prevents further seizures.
What happens when we cut the brain in two? It has been described how the disconnection between the two cerebral hemispheres causes difficulties and changes in the execution of tasks that require the integration of information. When this happens, we act as if one part of the brain knew the information and the other did not, as if we had a double brain. Can we speak, then, of a double consciousness?
The split brain
When researchers tested the visual functions of patients undergoing callosotomy, they found a curious phenomenon. Apparently, when presented with an object in their right visual field, they are able to recognize it and point to it both verbally and by raising their right hand. However, when the object to be recognized is in the left field, while the patient claims to see absolutely no object, his left hand points to it.
This apparent contradiction is quickly solved if we know that control over the body is crossedThe right hemisphere controls the left side of the body, while the left hemisphere controls the right side. Thus, when the object is presented in the right field, the left hemisphere responds by raising the right hand and verbally, since speech is on the left side. On the other hand, when the object is in the left field, the right hemisphere responds by raising the left hand, but cannot express it verbally because speech is housed in the other hemisphere.
However, this view of the split-brain phenomenon is not as conclusive as we would like. The evidence in favor of this phenomenon is small and dwindling because we now have better alternatives to callosotomy for treating epilepsy. This generates replicability problems that are difficult to overcome. On the other hand, there are doubts as to whether the classic cases described in the literature are really as representative as they claim to be, since within the already small sample of callosotomized patients there are exceptions that do not comply with what is predicted according to the theory.
Theories of consciousness
The two most relevant theories for understanding the split brain phenomenon are Bernard Baars' Global Workspace Theory (GWT) and Integration Information Theory (IIT).
The GWT proposes the metaphor of the theater to understand consciousness.. All those processes and phenomena of which we are conscious are those that are illuminated by the attentional focus, just as in a play the spotlights shed light on those parts of the stage that are relevant to the action. In the shadows all kinds of processes occur which, not being focused, do not reach consciousness. Thus, consciousness is a unitary process and the section of the brain in two should give rise either to a double consciousness or to a consciousness focused in only one hemisphere of the two.
IIT proposes that it is the sum of informational integration that builds consciousness. The more integrated information, the higher the level of consciousness. In a unitary brain, all information converges at the same point forming a single consciousness. In a split brain in which information from one side does not reach the other, two different points of convergence of information should form, leading to the formation of two distinct consciousnesses, each with its own hemispheric information.
Do two consciousnesses really form?
Researchers tested the immovability of the classical theory of the brain divided through the section of the corpus callosum.. For this, they recruited two individuals who had undergone such a lesion therapeutically and conducted five visual recognition experiments.
Contrary to textbook descriptions, the participants were perfectly capable of indicating where the visual stimulus was located, if it appeared at all, anywhere in the visual field, both by hand pointing and verbally. In one experiment, it was found that one of the two participants was better able to name the stimulus being used (an animal) when it was presented in the right visual hemifield, due to language localization. Although the visual information did appear to be disintegrated, the location of stimulus presentation was not found to be associated with a particular type of response.
Conflict with classical theories
These data, although far from conclusive due to the small sample size, show that what is predicted by classical theory is not rigidly true. In fact, it has yet to be demonstrated that it is true in the majority of patients. What is certain is that the evidence with these two patients in five tasks that challenge the basic assumptions conflicts not only with the older clinical cases, but also with the theories of consciousness described above.
Both GWD and IIT predict that after sectioning of the corpus callosum and disruption of the flow of information from one side to the other, two separate consciousnesses will form. In fact, none of these patients showed signs of dual consciousness and reported feeling that they had a very well integrated single consciousness. These data do fit well with another theory of consciousness: that of recurrent local processing. This theory predicts that the interaction and exchange between two different areas of the brain alone is sufficient to bring information to consciousness. Thus, there is no need for two connected hemispheres to bring separate information into the same consciousness by callosotomy.
Other possible explanations
The results are not final and should be taken with a pinch of salt.. It is possible to offer alternative explanations that integrate what is described in the typical cases and what is found in this study. For example, it should be noted that the patients taken as subjects were callosotomized more than 15 years ago. It could be that after the operation the information is indeed disintegrated, but over time the brain has found a way to merge the dual consciousness back into a single consciousness.
Even so, it is fascinating that these patients with a split perception are able to put the information together and represent it in a single consciousness, giving a unified response. This is a phenomenon that will undoubtedly have to be answered someday if we are to have a truly explanatory theory of consciousness.
(Updated at Apr 12 / 2024)