Split-brain
Split-brain or callosal syndrome is a type of disconnection syndrome when the corpus callosum connecting the two hemispheres of the brain is severed to some degree. It is an association of symptoms produced by disruption of, or interference with, the connection between the hemispheres of the brain. The surgical operation to produce this condition involves transection of the corpus callosum, and is usually a last resort to treat refractory epilepsy. Initially, partial callosotomies are performed; if this operation does not succeed, a complete callosotomy is performed to mitigate the risk of accidental physical injury by reducing the severity and violence of epileptic seizures. Before using callosotomies, epilepsy is instead treated through pharmaceutical means. After surgery, neuropsychological assessments are often performed.
After the right and left brain are separated, each hemisphere will have its own separate perception, concepts, and impulses to act. Having two "brains" in one body can create some problematic dilemmas. There was a case in which, when one split-brain patient would dress himself, sometimes he pulled his pants up with one hand and down with the other. He was also reported to have grabbed his wife with his left hand and shook her violently, at which point his right hand came to her aid and grabbed the aggressive left hand. However, such conflicts are very rare. If a conflict arises, one hemisphere usually overrides the other.
When split-brain patients are shown an image only in the left half of each eye's visual field, they cannot verbally name what they have seen. This is because the brain's experiences of the senses is contralateral. Communication between the two hemispheres is inhibited, so the patient cannot say out loud the name of that which the right side of the brain is seeing. A similar effect occurs if a split-brain patient touches an object with only the left hand while receiving no visual cues in the right visual field; the patient will be unable to name the object, as each cerebral hemisphere of the primary somatosensory cortex only contains a tactile representation of the opposite side of the body. If the speech-control center is on the right side of the brain, the same effect can be achieved by presenting the image or object to only the right visual field or hand.
The same effect occurs for visual pairs and reasoning. For example, a patient with split brain is shown a picture of a chicken foot and a snowy field in separate visual fields and asked to choose from a list of words the best association with the pictures. The patient would choose a chicken to associate with the chicken foot and a shovel to associate with the snow; however, when asked to reason why the patient chose the shovel, the response would relate to the chicken.
History
Early anatomists, Galen and Vesalius, recognized the corpus callosum and predicted that it connected the left and right hemispheres of the brain. In 1784, Félix Vicq-d’Azyr believed that it helped facilitate communication between the two hemispheres and that removing it would separate the brain into two independent regions. Then, in 1892, Joseph Jules Dejerine had a patient with damage to both the corpus callosum and visual cortex. The patient could write, but would not read, this condition is known as Dejerine syndrome. In 1908, Hugo Liepmann found that by lesioning the corpus callosum, the left sided brain could then result in apraxia and agraphia. These conditions impact the ability to carry out daily coordinated movements, such as writing.Walter Dandy was a neurosurgeon who performed the first partial corpus callostomies. He did this as a surgical approach for pineal tumors. In 1936, he reported three cases where he section the posterior two-thirds of the corpus callosum. He found that the procedure did not result in much blood loss and there were little-to-none postoperative deficits. He argued that his findings challenge previous claims about the functional importance of the brain region.
Before the 1960s, clinical observations of patients with left-hemisphere lesions supported the idea that language functions were localized primarily to that hemisphere. The lesions to this region resulted in difficulty with speech production, comprehension, and reading. Additionally, research performed by Roger Sperry and his colleagues later suggested that severing interhemispheric connections to treat severe epilepsy was revealed by functional capacities in the right hemisphere. Together they demonstrated that a disconnected right hemisphere could handle basic language tasks and was stronger in processing spatial information, music and emotional content. On the other hand, the isolated left hemisphere functioned for analytical reasoning and full language abilities. Sperry earned the 1981 Nobel Prize in Physiology or Medicine for his contributions in hemispheric specialization.
Sperry initiated split-brain research and his colleague, Michael Gazzangia, helped him summarize their findings. Their report in the 1967 Scientific American report titled, “The Split Brain in Man,” examined how the two hemispheres function independently of one another after a corpus callosotomy.This is a surgical procedure used to treat severe epilepsy by severing the corpus callosum. Ten patients had undergone the surgery at that time and four were participants in their research studies. The patients showed distinct cognitive patterns, yet their personalities and emotions were still intact. Sperry and Gazzangia analyzed these changes using tests involving visual input, tactile input, and combined visual and tactile processing.
Visual test
Participants fixated on the center of a light board while bulbs flashes in both visual fields. They were asked to report what they saw and they described only the lights presented to the right visual field. They were asked to point to the flashed lights and they could accurately indicate lights on both sides. Each hemisphere has received the visual information, but only the left hemisphere could verbally report it. These results demonstrated that verbal acknowledgement of a visual stimulus requires communication between the visual processing system and the left hemisphere region.Tactile test
Participants held an object in one hand without seeing it. When it was in the right hand, the left side of the brain could recognize it and say what it was. When it was in the left hand, the right side could tell what it was but couldn’t say it. The person also couldn’t pick the object out from similar ones. This shows the right side could feel the object but couldn’t use words to describe it.Combination of both tests
Images were presented to the right hemisphere only. Participants could not name or describe the pictures. They then explored the objects with the left hand and they correctly selected the object that matched the image. Participants could also choose items related to the picture if the exact object was not available. This further confirms that the right hemisphere can process the visual information but cannot access language systems.Later, Sperry and Gazzaniga expanded their own research and testing. This focused more on the right hemisphere's contributions to language, auditory processing, and emotional responses. Their work showed that each hemisphere has distinct cognitive functions. The left hemisphere functioned in language, writing, reading, and math. The right hemisphere functioned in spatial processing, facial recognition, and problem solving.
Sperry studied split-brain research until he passed away in 1994. Gazzaniga continued his studies. Later studies challenged some of their conclusions. In the 1980s, Jarre Levy argued that the hemispheres function in an integrated way and that no cognitive task relies on only one side of the brain. In 1998, a study by Hommet and Billiard reported that humans born without a corpus callosum showed information transfer between the two hemispheres. This suggests that subcortical pathways remain in these patients, but it is not clear if they remain in the split-brain patients. Research performed by Parsons, Gabrieli, Phelps, and Gazzaniga in 1998 indicated that interhemispheric communication is important for mentally simulating others’ movement. In 2001, Morin’s research on inner speech suggested that commissurotomy may produce two asymmetric forms of self-awareness. Meaning there is a more complete system in the left hemisphere and a more primitive system in the right hemisphere.