It is proposed that it is possible that a person may develop two separate conscious entities within their one brain after undergoing a corpus callosotomy.
The idea first began circulating in the neuroscience community after some split-brain patients exhibited alien hand syndrome (AHS), which led some scientists to believe that there must be two separate consciousnesses within the brain's left and right hemispheres in competition with one another once the corpus callosum is severed.
For example, communication across the corpus callosum allows information from both the left and right visual fields to be interpreted by the brain in a way that makes sense to comprehend the person's actual experience (e.g., visual inputs from both eyes are interpreted by the brain to make sense of the experience that you are looking at a computer that is directly in front of you).
[citation needed] Split-brain patients have been subjects for numerous psychological experiments that sought to discover what occurs in the brain after the primary interhemispheric pathways have been disrupted.
Notable researchers in the field include Roger Sperry, one of the first to publish ideas involving a dual consciousness; and his famous graduate student, Michael Gazzaniga.
Their results found a pattern among patients: severing the entire corpus callosum stops the interhemispheric transfer of perceptual, sensory, motor, and other forms of information.
[2] The purpose of the procedure was to alleviate the effects of epilepsy when other forms of treatment (medications) had failed to stop the violent convulsions associated with the disorder.
[3] Epileptic seizures occur because of abnormal electrical discharges that spread across areas of the brain.
[4] William Van Wagenen proposed the idea of severing the corpus callosum to eliminate transcortical electrical signals across the brain's hemispheres.
This procedure removes a section of the skull, leaving the brain exposed and accessible to the surgeon.
The dura mater is pulled back so the deeper areas of the brain, including the corpus callosum, can be seen.
[8] The corpus callosotomy and commissurotomy have been successful in reducing, and in some cases, eliminating epileptic seizures.
Upon her death, an autopsy was performed which concluded that the event may have been caused by several strokes in her right hemisphere and corpus callosum.
In the 1940s, reports surfaced of patients who had undergone corpus callosotomies that were experiencing uncontrollable hand movements following surgery.
[11] The first is the frontal variant, which is characterized by the nondominant hand grabbing items and manipulating objects.
The second is the callosal variant, which is the most common and is characterized by the uncontrollable and conflicting movement of a right-handed patient’s left hand.
[12] The third type is the posterior variant, which is characterized by the affected hand rising in the air and making non-purposeful movements.
[14][15] Switching one's attention from one task to another can also lessen the amount of control that they can allocate to their affected hand.
[citation needed] In 1978, Michael Gazzaniga and Joseph DeLoux[16][17] discovered a unique phenomenon among split-brain patients who were asked to perform a simultaneous concept task.
Both the winter house and the shovel were being projected to the patient from his LVF, so his right hemisphere received and processed the information; this input is completely independent from what is going on in the RVF, which involves the chicken's claw and head (the information being processed in the left hemisphere).
[citation needed] Antti Revonsuo explained a procedure that was similar in nature to the Sperry–Gazzaniga design.
After observing the struggles of the execution of activities involving the left and right arms and legs, Joseph was led to believe that the two hemispheres each possessed their own consciousness.
[23] The person is never consciously aware of these alternative possibilities the brain has considered before they pick it up with the right hand; they just do it.
It would make the appearance that there is a dual consciousness competing for dominance over the other for control of the brain, but it is not the case.
The disappearance of alien hand syndrome in some split-brain patients is not evidence of one consciousness "defeating" the other and taking complete control of the brain.
Eventually the split patient's brain may find adaptive routes to compensate for the lost interhemispheric communication, such as alternative pathways involving subcortical structures that perform subcortical interhemispheric inhibition to regain a sense of normalcy between the two hemispheres.