Right visual half . field fs

Right ear

Simple language comprehension

Spatial concepts

Left visual half field

Split brain

■ Figure 8.12 Different functions of the right and left cerebral hemispheres. These differences were revealed by experiments with people whose corpus callosum—the tract connecting the two hemispheres—was surgically split.

Clinical Investigation Clues

Remember that Frank had paralysis of the right side of his body and suffered speech impairment. What is the most likely explanation for the paralysis on the right side of his body?

How does this relate to his speech impairment?

These findings have led to the concept of cerebral dominance, which is analogous to the concept of handedness—people generally have greater motor competence with one hand than with the other. Since most people are right-handed, and the right hand is also controlled by the left hemisphere, the left hemisphere was naturally considered to be the dominant hemisphere in most people. Further experiments have shown, however, that the right hemisphere is specialized along different, less obvious lines—rather than one hemisphere being dominant and the other subordinate, the two hemispheres appear to have complementary functions. The term cerebral lateralization, or specialization of function in one hemisphere or the other, is thus now preferred to the term cerebral dominance, although both terms are currently used.

Experiments have shown that the right hemisphere does have limited verbal ability; more noteworthy is the observation that the right hemisphere is most adept at visuospatial tasks. The right hemisphere, for example, can recognize faces better than the left, but it cannot describe facial appearances as well as the left. Acting through its control of the left hand, the right hemisphere is better than the left (controlling the right hand) at arranging blocks or drawing cubes. Patients with damage to the right hemisphere, as might be predicted from the results of split-brain research, have difficulty finding their way around a house and reading maps.

Perhaps as a result of the role of the right hemisphere in the comprehension of patterns and part-whole relationships, the ability to compose music, but not to critically understand it, appears to depend on the right hemisphere. Interestingly, damage to the left hemisphere may cause severe speech problems while leaving the ability to sing unaffected.

The lateralization of functions just described—with the left hemisphere specialized for language and analytical ability, and the right hemisphere specialized for visuospatial ability—is true for 97% of all people. It is true for all right-handers (who account for 90% of all people) and for 70% of all left-handers. The remaining left-handers are split about equally into those who have language-analytical ability in the right hemisphere and those in whom this ability is present in both hemispheres.

It is interesting to speculate that the creative ability of a person may be related to the interaction of information between the right and left hemispheres. The finding of one study—that the number of left-handers among college art students is disproportionately higher than the number of left-handers in the general population—suggests that this interaction may be greater in left-handed people. The observation that Leonardo da Vinci and Michelangelo were both left-handed is interesting in this regard, but obviously does not constitute scientific proof of any hypothesis.

The Central Nervous System 199

Further research on the lateralization of function of the cerebral hemispheres may reveal much more about brain function and the creative process.

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