Do Two Consciousnesses Appear When The Brain Is Divided?
One of the most important therapeutic advances in epilepsy and neurosurgery is the section of the corpus callosum. This structure unites 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 epileptic seizures from escalating.
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?
When researchers tested the visual functions of patients undergoing callosotomy, they found a curious phenomenon. Apparently, when we present 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 resolved if we know that control over the body is crossed : The 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 language 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 limited and is becoming less because today we have better alternatives to callosotomy to treat epilepsy. This generates replicability problems that are difficult to overcome. On the other hand, there are doubts about whether the classic cases described in the literature are really as representative as they claim, since within the already small sample of callosotomized patients there are exceptions that do not comply with what is predicted according to theory.
Theories about consciousness
The two most relevant theories for understanding the split-brain phenomenon are Bernard Baars’ Global Workspace Theory (GWT) and Information Integration Theory (IIT).
The GWT proposes the metaphor of theater to understand consciousness All those processes and phenomena of which we are aware are those that are illuminated by the attentional focus, in the same way that in a play the spotlights shed light on those parts of the stage that are relevant to the action. All kinds of processes occur in the shadows that, when not focused, do not reach consciousness. Thus, consciousness is a unitary process and the section of the brain in two should give rise to either a double consciousness or a consciousness focused on only one of the two hemispheres.
The IIT proposes that it is the sum of informational integration that builds consciousness. The more information integrated, the higher level of consciousness. In a unitary brain, all information converges at the same point, forming a single consciousness. In a divided 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 different consciousnesses, each with its own hemispheric information.
Researchers tested the immobility of the classical theory of the divided brain through the section of the corpus callosum To do this, they recruited two individuals who had undergone this lesion therapeutically and carried out five visual recognition experiments.
Contrary to what was described in the textbooks, participants were perfectly able to indicate where the visual stimulus was located, if it appeared, anywhere in the visual field, both by pointing with their hand and verbally. In one experiment it was found that one of the two participants was better able to name the stimulus that was used (an animal) when it was presented in the right visual hemifield, due to the location of the language. Although the visual information did appear to be disintegrated, the location of stimulus presentation was not found to be associated with a specific type of response.
Conflict with classical theories
These data, although far from being conclusive due to the small sample, show that what was predicted by classical theory is not rigidly fulfilled. In fact, it has yet to be demonstrated that this is true in the majority of patients. The truth is that the evidence with these two patients in five tasks that challenge basic assumptions not only conflicts with the old clinical cases, but also with the theories of consciousness described above.
Both GWD and IIT predict that after sectioning the corpus callosum and interrupting the flow of information from one side to the other, two separate consciousnesses will form. The truth is that none of these patients showed signs of double consciousness and explained feeling that they had a single, very well integrated consciousness. These data do fit well with another theory of consciousness: that of recurrent local processing. This theory predicts that the mere interaction and exchange between two different areas of the brain is enough to bring information to consciousness. Thus, two connected hemispheres are not necessary to bring separate information to the same consciousness through callosotomy.
Results are not final and should be taken with a grain of salt It is possible to offer alternative explanations that integrate what is described in the typical cases and what was found in this study. For example, it must be taken into account that the patients taken as subjects were callosotomized more than 15 years ago. It could be that after the operation the information is effectively disintegrated, but over time the brain has found a way to unite the double consciousness and re-form a single one.
Even so, it is fascinating that these patients with a perception divided in two are able to put together the information and represent it in a single consciousness, giving a unified response. This is a phenomenon that will undoubtedly have to be answered one day if we want to have a truly explanatory theory of consciousness.
