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 epileptic seizures from going any further.
What happens when we cut the brain in two? It has been described how the disconnection between the two brain hemispheres causes difficulties and changes in the execution of tasks that require the integration of information. When this happens, it acts as if one part of the brain knows the information and the other does not, as if we had a double brain. Can we talk, then, about a double consciousness?
The Divided Brain
When the researchers tested the visual functions of patients undergoing callosotomy, they found a curious phenomenon. It seems that when an object is presented to them 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 part of the body, while the left hemisphere controls the right part. Thus, when the object is presented in the right field, the left hemisphere responds by raising the right hand and verbally, as 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 the language is in the other hemisphere.
However, this view of the split-brain phenomenon is not as conclusive as we would like. The evidence in favour of this phenomenon is limited and is becoming less so as we now have better alternatives to callosotomy for treating epilepsy. This creates replication 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 the theory.
Theories of consciousness
The two most relevant theories for understanding the phenomenon of the divided brain are Bernard Baars’ Global Workspace Theory (GWT) and the Integration Information Theory (IIT).
The GWT proposes the metaphor of the theater to understand consciousness . All those processes and phenomena of which we are aware are those that are illuminated by the spotlight, just as in a play the spotlights throw light on those parts of the stage that are relevant to the action. In the shadows occur all kinds of processes that, not being focused, do not come to 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 on only one hemisphere of the two.
The IIT proposes that it is the sum of informational integration that builds consciousness. The more integrated information, the higher the level of awareness. In a unitary brain, all the information converges at the same point forming a single consciousness. In a divided brain where information from one side does not reach the other, two different points of information convergence should be formed, leading to the formation of two different consciousnesses, each with its own hemispheric information.
Are two consciousnesses really formed?
Researchers tested the immobility of the classical theory of the brain divided through the section of the corpus callosum . For this purpose they recruited two individuals who had been therapeutically treated for this lesion and carried out five visual recognition experiments.
Contrary to textbook descriptions, participants were perfectly capable of indicating where the visual stimulus was, if at all, in any part of the visual field, either by hand pointing or verbally. In some experiments it was found that one of the two participants was more capable of naming the stimulus used (an animal) when it appeared in the right visual field, due to the location of the language. Although the visual information did seem to be disintegrated, it was not found that the place of presentation of the stimulus was associated with a specific type of response.
Conflict with classical theories
These data, although far from conclusive due to the small sample, show that what is predicted by the classical theory is not fulfilled in a rigid way. In fact, it has yet to be demonstrated that it is fulfilled in the majority of patients. What is certain is that the evidence with these two patients in five tasks that challenge the 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 the sectioning of the corpus callosum and the interruption of the information flow from one side to the other, two separate consciousnesses will form. The truth is that neither of these patients showed signs of double consciousness and explained that they felt they had a single, well-integrated consciousness. These data do fit well with another of the theories of consciousness: that of recurrent local processing. This theory predicts that the single interaction and exchange between two different areas of the brain is already sufficient to bring the information to consciousness. Thus, it does not take two connected hemispheres to bring separate information to the same consciousness through callosotomy.
Other possible explanations
The results are not final and should be taken with tweezers . It is possible to offer alternative explanations that integrate what was described in the typical cases and what was found in this study. For example, it should 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 with time the brain has found a way to combine the double consciousness and form one again.
Still, it is fascinating that these patients with a split perception are able to bring 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 one day if we are to have a truly explanatory theory of consciousness.