Hemispheric laterality

Hemispheric laterality

The human brain is a bilaterally symmetrical structure which is for the most part richly interconnected by two main bridges of neurons called the corpus callosum and anterior commissure. These structures can be surgically sectioned in humans in an effort to control the spread of epileptic seizures. Although there is no apparent change in everyday behavior of these patients, dramatic differences in cognitive function can be demonstrated under specialized testing conditions. In normal humans these cerebral commissures are largely responsible for behavioral unity; the neural mechanism keeps the left side of the body up to date with the activities of the right side, and vice versa.

Changes in behavioral responses of persons whose cerebral commissures have been sectioned are almost undetectable. The person walks, talks, and behaves in a normal fashion. Dramatic effects are observed only under testing conditions which utilize stimuli that are lateralized exclusively to one hemisphere or the other. For example, if a picture of an apple is flashed in the right visual field, the person describes the object normally. However, if the same picture is flashed in the left visual field, in the early days of postoperative testing the person denies that the stimulus was presented at all. After many test sessions the person may have the impression that something was flashed, but is unable to say what. This disparity of recognition in the two sides of the visual field occurs because the information is projected to the right hemisphere, which is incapable of speech. Because the right hemisphere is now disconnected from the left, information arriving in the right hemisphere cannot be communicated by means of speech.

When tests are used which do not require a spoken response, numerous mental abilities are observed to be present in the “disconnected” right hemisphere. For example, even though the person is unable to describe a picture of an orange flashed to the left field, when the left hand searches through a field of objects placed out of view, it correctly retrieves the orange. If asked what the object is, the person would say he does not know. Here again the left hemisphere controls speech but cannot solve the problem. The right hemisphere solves the problem but cannot elicit speech.

Despite its linguistic superiority, the left hemisphere does not excel over the right in all tasks. Tests have demonstrated that in some specialized functions the right hemisphere is decidedly superior to the left. In the area of emotional reactions there appears to be equal reactivity in the two hemispheres.

Tests have been conducted on subjects in which the cerebral commissure had not been entirely sectioned (because it is now believed total commissure section is not necessary to stop the interhemispheric spread of some kinds of seizure activity). These persons showed dramatic breakdown in interhemispheric transfer. When the posterior part of the callosum is sectioned, visual aspects of the syndrome appear. When it is spared and more interior regions are cut, tactile and auditory communications are blocked, but not visual ones. It also appears that no fundamental reorganization of the interhemispheric transfer system takes place, since years after surgery these same deficits are present and are not compensated for in any way.

There appears to be a large variation in the lateralized talents of each half-brain. While the right hemisphere frequently appears to have some language talent, not all split-brain persons have language skills in the right hemisphere. Similarly, visual spatial skills, which are usually present exclusively in the right hemisphere, are frequently bilaterally represented and sometimes represented only in the left speech hemisphere. There is even some evidence that the commissure system itself varies in what is transferred where. See Brain, Psychology