split brain effects a wide array of behavioral effects consequent upon the severing of the cerebral commisures, and generally interpreted as indicating asymmetry in cerebral functions. The human brain has considerable left–right functional differentiation, or asymmetry, that affects behavior. The most obvious example is handedness. By the 1860s Bouillaud, Dax, and Broca had observed that the effects of unilateral damage indicated that the left hemisphere was preferentially involved in language. Since the 1960s, this commitment to functional asymmetry has been reinforced by studies of patients in whom communication between the hemispheres has been surgically disrupted. Split brain effects depend on severing the cerebral commisures, and especially the corpus callosum, which are neural structures mediating communication between the cerebral hemispheres. Commisurotomies have been performed since the 1940s to control severe epilepsy. This is intended to leave both hemispheres intact and functioning independently. Beginning in the 1960s, J. E. Bogen, M. S. Gazzaniga, and R. W. Sperry conducted an array of psychological tests to evaluate the distinctive abilities of the different hemispheres. Ascertaining the degree of cerebral asymmetry depends on a carefully controlled experimental design in which access of the disassociated hemispheres to peripheral cues is limited. The result has been a wide array of striking results. For example, patients are unable to match an object such as a key felt in one hand with a similar object felt in the other; patients are unable to name an object held in the left hand, though they can name an object held in the right.
Researchers have concluded that these results confirm a clear lateralization of speech, writing, and calculation in the left hemisphere (for righthanded patients), leaving the right hemisphere largely unable to respond in speech or writing, and typically unable to perform even simple calculations. It is often concluded that the left hemisphere is specialized for verbal and analytic modes of thinking, while the right hemisphere is specialized for more spatial and synthetic modes of thinking. The precise character and extent of these differences in normal subjects are less clear. R.C.R.