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 (corpus callosotomy) 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 interesting dilemmas. When one split-brain patient dressed himself, he sometimes pulled his pants up with one hand (that side of his brain wanted to get dressed) and down with the other (this side did not). He also reported to have grabbed his wife with his left hand and shaken 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 vocally name what they have seen. This is because the image seen in the left visual field is sent only to the right side of the brain (see optic tract), and most people's speech-control center is on the left side of the brain. Communication between the two sides 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 (e.g. "the shovel is for cleaning out the chicken coop").
History
Early anatomists, such as Galen (129 – ) and Vesalius (1514–1564 CE), identified the corpus callosum. They generally described its function as a structure holding together the two halves of the brain. In 1784, Félix Vicq-d'Azyr described the corpus callosum as allowing communication between the two halves of the brain. He proposed that eliminating the corpus callosum would divide the brain into two independent parts. In 1892, Joseph Jules Dejerine reported symptoms in a person who had destruction of part of the corpus callosum (along with damage to the visual cortex: inability to read while retaining the ability to write, now referred to as pure alexia or as Dejerine syndrome. In 1908, Hugo Liepmann observed left-sided apraxia (a motor disorder of motor planning to perform tasks or movements) and agraphia (loss of the ability to communicate through writing) following a lesion in the corpus callosum.
According to Vaddiparti et al. (2021), the first surgical cuts to the corpus callosum, partial corpus callosotomy, were made by neurosurgeon Walter Dandy in order to access and to remove tumors in the pineal gland. In 1936, Dandy described three cases in which he cut the corpus callosum from its posterior (towards the back of the head) across about two thirds of its width. He described these cuts as "bloodless" and that "no symptoms follow[ed] [the] .. division" of the corpus callosum. He concluded that his operations "dispose ... of the extravagant claims to function of the corpus callosum" (p. 40).