split-brain

Also from Answers.com... Breast Cancer Screening and treatments every woman - and man - should know.

Of, relating to, or subjected to surgical separation of the hemispheres of the brain by severing the corpus callosum: split-brain operation to prevent epileptic seizures.

In the 1940s, it was discovered that surgical disconnection of the two cerebral hemispheres, by dividing the corpus callosum, the bridge of nerve fibres that connects them, effectively reduced seizures in patients with intractable epilepsy. When behavioural studies with these patients were first carried out, it appeared that sectioning the callosum led to no major breakdown in interhemispheric processing. A simple test, however, can reveal that there are in fact dramatic effects of this disconnection, known as the split-brain syndrome.

If such a patient's hands are obscured from his view and an object is placed in the right hand, he can name it easily. Conversely, if it is placed in the left hand, the patient is unable to identify it verbally. Given an array of items to choose from, however, the left hand is immediately able to pick out this same item. Since initiation of left-hand movement occurs in the right hemisphere, this indicates that the right hemisphere has knowledge about the object but is unable to name it.

Over the years, more sophisticated testing procedures have taken advantage of the contralateral organization of various perceptual systems, such as the visual system, to explore the specialized functions of the two hemispheres. These studies have resulted in remarkable insights into the specialized capacities of each disconnected hemisphere. They have also revealed how the two hemispheres work in concert in the normal brain to provide seamless integration of sensory, motor, and cognitive functions.

Split-brain patients behave in ways that were to a large extent predicted by classical neurology. For example, the most striking aspect of the split-brain syndrome is that the left hemisphere has access to speech and the right hemisphere does not. The left hemisphere's specialization for language was already long established, based on data from patients with unilateral brain damage. Although the effects of disconnection on language were therefore not unexpected, it was surprising to discover the extent to which the left hemisphere was specialized for problem-solving of all kinds. Indeed, not only could it, and it alone, solve a wide range of cognitive problems, it also possessed a special device that has been called the ‘interpreter’. In brief, this was revealed by presenting two pictures, one to each half brain. For example, a picture of a snow scene was presented (from the left field of vision) to the right hemisphere of a split-brain patient. The non-talking right hemisphere had four cards to choose from, one of which was a shovel. At the same time the left hemisphere was shown a chicken claw and its four choices included a picture of a chicken. Following presentation of these pictures the patient was allowed to choose between the eight possible pictures. The left hand, governed by the right hemisphere, chose the shovel and the right hand, governed by the left hemisphere, chose the chicken. When asked by the experimenter why he had responded in that manner, the speaking left hemisphere said, ‘Oh, the chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.’ The left hemisphere in fact did not know why the left hand chose the shovel. The left brain observed what the right hand was doing and came up with a theory that explained away an action.

The right hemisphere appears to lack the interpretive capacities of the left and consequently is markedly impoverished in problem solving. Nevertheless, the right brain has it own specializations. The right hemisphere is superior to the left in a wide range of perceptual skills, such as grouping of visual elements into a whole picture. Early demonstrations of this involved copying drawings with each hand separately. With their hands obscured from view, right-handed split-brain patients were able to produce reasonable copies with their left hands but not with their right. The drawings made by the right hand contained details of the original pictures but had little or no spatial coherence. More recent research has revealed the right hemisphere's superiority in making orientation judgments, remembering unknown, upright faces, and a host of other visual tasks.

Investigations into the realms of language and perception clearly reveal the functional differences between the two hemispheres. Studies on memory and attention, however, provide insights into the ways in which the two hemispheres work together in an intact brain. Memory research with split-brain patients suggests that the two hemispheres each provide a unique input into storing information and later retrieving it. The left hemisphere interpreter is thought to be continually generating theories to explain the information it is currently processing. As a result, this hemisphere is particularly suited to getting the ‘gist’ of an event. This interpretive function, however, means that the left hemisphere often makes errors in recalling details of an event. The more literal right hemisphere does not make inferences and generalizations about incoming information, so this hemisphere is much less likely to make factual errors. The two hemispheres therefore work together to provide a system which is capable of accurately recalling details while still allowing for elaboration and inferences about the world.

Attention is something else that involves interaction between the two hemispheres. Both hemispheres are able to orient reflexively to external stimuli, and this focusing of attention occurs independently in each hemisphere. Voluntary orienting, however, appears to involve a single shared resource. This is illustrated by experiments that require each hemisphere to be performing a task independently of the other; then, when one hemisphere has a difficult task, the performance of the other hemisphere on a separate task is impaired. If the task of the first hemisphere becomes relatively easy, however, the performance of the other hemisphere improves markedly. Thus although the two hemispheres co-operate in many aspects of neural functioning, in the realm of voluntary control of attention the two hemispheres appear to compete. Studies that demand this kind of hemispheric competition have revealed that control over voluntary attention seems to be preferentially lateralized to the left hemisphere.

In summary, studies with split-brain patients have provided invaluable insights into the specialized functions of the two hemispheres and the ways in which they interact to enable myriad perceptual and cognitive functions.

— Michael Gazzaniga, Paul M. Corballis

See also brain; language and the brain.

Of, relating to, or subjected to surgical separation of the hemispheres of the brain by severing the corpus callosum.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)