One of the most important therapeutic advances in epilepsy and neurosurgery is the section of the corpus callosum. This structure connects the two hemispheres and, among other things, allows information from one side to the other. It also allows the electrical activation of epilepsy to spread, so that its severing and separation of the two hemispheres prevents epileptic seizures from going any further.
What happens when we cut the brain in half? It has been described how the disconnection between the two cerebral hemispheres leads to difficulties and changes in the performance of tasks that require the integration of information. When this happens, it is as if one part of the brain knows the information and the other part does not, as if we have a double brain. Can we then speak of a double consciousness?
The divided brain
When the researchers tested the visual functions of patients undergoing callosotomy, they discovered a curious phenomenon. Apparently, when we present them with an object in their right visual field, they are able to recognize and point at it both verbally and by raising their right hand. However, when the object to be recognized is in the left field, while the patient says he sees absolutely no object, his left hand points to it.
This apparent contradiction is resolved quickly if we know that the control of the body is crossed.: The right hemisphere controls the left side of the body, while the left hemisphere controls the right side. So when the object is presented in the right field, the left hemisphere responds by raising the right hand and verbally, because the 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 tongue 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 reduced and less and less because today we have better alternatives to the callosotomy to treat epilepsy. This creates replicability issues that are difficult to record. On the other hand, there are doubts as to whether the classic cases described in the literature are really as representative as they are, because in the already small sample of callosotomized patients there are exceptions which are not not in accordance with what is predicted according to theory.
Theories on 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 offers the theatrical metaphor for understanding consciousness. All of these processes and phenomena of which we are aware are those which are illuminated by the focus of attention, in the same way that in a work the spotlights illuminate the parts of the scene which are relevant to the action. In the shadows all kinds of processes take place which, by not being focused, do not reach consciousness. Thus, consciousness is a unitary process and the cutting of the brain in half should give rise to either a double consciousness or a consciousness focused on one hemisphere of the two.
The IIT proposes that it is the sum of the integration of information that builds consciousness. The more information is integrated, the higher the level of awareness. In a unitary brain, all information converges at the same point forming a single consciousness. In a divided brain in which information on 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 hemispherical information.
Are two consciousnesses really formed?
The researchers tested the stillness of the classic divided brain theory through the corpus callosum section. So they recruited two people who had been treated for the injury therapeutically and conducted five visual recognition experiments.
Contrary to what is described in the textbooks, the participants were perfectly able to indicate where the visual stimulus was located, if it appeared, anywhere in the visual field, by hand and verbally. In one experiment, it was found that one of two participants was better able to name the stimulus used (an animal) when presented in the appropriate visual hemicampus, due to the location of the tongue. Although the visual information appeared to be disintegrated, the site 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 does not rigidly happen. In fact, it has not yet been shown to be affected in most patients. The truth is, the evidence with these two patients in five tasks that challenge basic assumptions is not only in conflict with old clinical cases, but also with the theories of consciousness outlined above.
The GWD and IIT both predict that after the corpus callosum is severed and the flow of information from side to side is interrupted, two distinct consciousnesses will form. The truth is, none of these patients showed any signs of double consciousness and explained that they felt they had a very well integrated single consciousness. These data fit well with another theory of consciousness: that of recurrent local treatment. This theory predicts that the simple interaction and exchange between two different areas of the brain is already sufficient to bring information to consciousness. Thus, it is not necessary to have two hemispheres connected to bring separate information to the same consciousness by means of a callosotomy.
Other possible explanations
Results are not final and should be taken with tweezers. It is possible to propose alternative explanations which integrate what is described in typical cases and which was found in this study. For example, it should be noted that the patients taken as subjects were callosotomized more than 15 years ago. It may be that after the operation the information is indeed disintegrated, but over time the brain has found a way to unite the double consciousness and re-form one.
However, it is fascinating that these patients with a split perception are able to put information together and represent it in a single consciousness, giving a unified response. This is a phenomenon that will certainly have to be answered one day if we want to have a truly explanatory theory of consciousness.