split-brain syndrome, condition characterized by a cluster of neurological abnormalities arising from the partial or complete severing or lesioning of the corpus callosum, the bundle of nerves that connects the right and left hemispheres of the brain.

Although it is not fully understood whether the processing of specific tasks is dependent on both hemispheres of the brain, the two hemispheres appear to each have some control over certain tasks. The left hemisphere, for example, is generally responsible for analytical tasks, such as calculating and reading. In many individuals, it is also the dominant centre for speech and language (though the right hemisphere is involved in language processing to a minor extent). In general, the right hemisphere is more efficient at dealing with spatial tasks, such as navigating a maze or reading a map, than the left hemisphere. The two hemispheres, however, routinely communicate with one another through the corpus callosum. This connection further serves as the conduit through which certain sensory signals are transmitted from one side of the body to the contralateral (opposite) side of the brain and through which motor control is effected in the reverse direction (i.e., the right hemisphere controls the left side of the body, and vice versa).

Among the first to characterize split-brain syndrome was American neurobiologist Roger Wolcott Sperry, who in the 1960s studied human split-brain subjects and contributed to the discovery that the left and right hemispheres of the brain carry out specialized duties. For this work, Sperry received a share of the 1981 Nobel Prize for Physiology or Medicine.

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Causes of split-brain syndrome

The primary cause of split-brain syndrome is intentional severing of the corpus callosum, partially or completely, through a surgical procedure known as corpus callosotomy. Rarely performed in the 21st century (having been replaced largely by drug treatments and other procedures), this operation is reserved as a last measure of treatment for extreme and uncontrollable forms of epilepsy in which violent seizures spread from one side of the brain to the other. By preventing the propagation of seizure activity across the hemispheres, corpus callosotomy can greatly improve the patient’s quality of life. However, following the operation, patients develop acute hemispheric disconnection symptoms that last for days or weeks and chronic symptoms that often are permanent.

Less-common causes of split-brain syndrome include stroke, infectious lesion, tumour, or ruptured artery. Many of these events result in varying degrees of spontaneous damage to the corpus callosum. The syndrome can also be caused by multiple sclerosis and in rare instances by agenesis of the corpus callosum, in which the connection fails to develop or develops incompletely. (Lesions in the corpus callosum also occur in patients with Marchiafava-Bignami disease, a rare alcoholism-related condition, but the more global brain damage associated with this disease leads to stupor, seizures, and coma, rather than the features typical of split-brain syndrome.)

Symptoms of split-brain syndrome

Many patients with split-brain syndrome retain intact memory and social skills. Split-brain patients also maintain motor skills that were learned before the onset of their condition and require both sides of the body; examples include walking, swimming, and biking. They can also learn new tasks that involve either parallel or mirrored movements of their fingers or hands. They cannot, however, learn to perform new tasks that require interdependent movement of each hand, such as learning to play the piano, where both hands must work together to produce the desired music. Eye movements also remain coordinated.

Since information cannot be directly shared between the two hemispheres, split-brain patients display unusual behaviours, particularly concerning speech and object recognition. For instance, when blindfolded a split-brain patient may not be able to name a familiar object that is held in the left hand, because information for the sense of touch is relayed from the left side of the body to the right hemisphere, which typically has a weak language centre. Without an intact corpus callosum, a person cannot access verbal information in the left hemisphere as long as the object remains in the left hand. For the same reason, the patient may have difficulty using the left hand to execute verbal commands; the inability to respond to commands with motor activity is a form of apraxia. To compensate for deficiencies in touch recognition by the left hand and left-hand apraxia, the patient (still blindfolded) may hold the object in the right hand, which relays information to the left hemisphere, providing access to the patient’s dominant verbal bank and enabling him to speak the name of the object. Upon hearing the name of a given object, the patient may also use the left hand to retrieve it; this presumably is because auditory information is processed by both hemispheres. The diffuse nature by which sounds and smells are processed across the brain appears to underlie other problems experienced by split-brain patients. For example, patients are unable to name odours presented to the right nostril, though the left hand can point out the source. Some symptoms of chronic disconnection can improve with time.

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