Hemispheric Specialization

Modified: 2020-06-10

The brain is divided into a left and a right half. Those halves are connected by a fibrous band of tissue called the corpus callosum. The corpus callosum serves as a pathway for information from one half of the brain to the other. Although the two halves are so physically similar that if you were asked to match brain halves of different people you could do so easily, the two halves are functionally different.

The left hemisphere is in charge of language functions and logical thought. Speech, song, and writing are all examples of left hemisphere functions. That means that there are cerebral areas in the left hemisphere that control speech. For example, Broca's area in the left temporal lobe is responsible for the motor control of speech. Similarly, there are areas that control singing and writing. Mel Tillis, the country singer who stutters, is a good example of a person who probably has a problem with the motor control of speech, but no problem with the motor control of song. That tells us that speech and song are controlled by different areas.

The right hemisphere is controlled by the left hemisphere, and the right is responsible for such things as the perception of rhythm, spatial-relation skills, and abstract or intuitive thought. The right hemisphere is less controlled by logic than the left is. A great deal of research has been conducted in the last 15 years on functional asymmetry or laterality. So much has been done, in fact, that the notion of left-brained versus right brained has passed into popular culture. You should probably beware of some of the accounts of laterality in the popular press, because this issue is more complex than those accounts may indicate.

There are some people who show the opposite pattern to that described above. Those people have their language functions on the right hemisphere and their left hemisphere is in charge of rhythm, spatial relation skills, and abstract or intuitive thought. Handedness only corresponds slightly to this reversed pattern. Among right handers, only 1% show the reversed pattern, whereas among left handers, 10% show the reversed pattern. While unusual, there are no abnormalities associated with individuals who show the reversed pattern.

An interesting correlate of the reversed pattern of laterality is how people hold pencils or pens. Both right and left handers who hold their pens in the "hooked" manner, that is, with their hands at nearly a right angle with their forearms so that their fingers point at them when they write, are very likely to have the reversed pattern. Again, those individuals are normal otherwise. They should be aware of their possible reversal, should they, for example, ever become candidates for brain surgery.

The diagnostic test for such individuals is called the Wada test. The Wada test consists of injections of sodium pentothal (a barbiturate, a.k.a. "truth serum") to the blood supply of each hemisphere. Suppose we suspect that someone has the reversed pattern. Then, we might inject them in the back of the neck on their left side, and ask them to read some text. If they continue to read, then we would repeat the procedure on their right side. They should be unable to read the text as the drug takes effect, and our diagnosis of reversed laterality would be confirmed.

One of the most fascinating stories in physiological research is that of the "split-brain." This line of research conducted by Myers, Sperry, and Gazzaniga, demonstrated the functional organization of the brain. It also won Roger Sperry a Nobel Prize. Epileptic men volunteered to be subjects. Their epilepsy was severe, and surgical severing of their corpus callosums was done in an attempt to limit the extent of their epileptic seizures.

After their recoveries, the men were normal except for the following characteristics. All wrote only with their right hands, and drew only with their left hands. When blindfolded, all could name an object in their right hands, but could not name the same objects in their left hands. All could identify objects in their right visual fields, but could not identify objects in their left visual fields. Care had to be taken to flash the objects for a fraction of a second, so they could not move their eyes.

Notice how you can explain the data from what you already know about physiology. All of the subjects had normal laterality, meaning their left hemispheres were their language hemispheres. So, when communication was cut, then writing, a language function, could only be accomplished with their right hands. Remember, the left side of the brain controls the right side of the body. Similarly, the right hemisphere is the one that controls drawing. These subjects could only draw with their left hands (regardless of whether they were right- or left-handed). Why? The answer, once again, is that the right side of the brain, which controls spatial relations, is connected to the left hand. In a normal, right-handed individual, the right side of the brain sends the drawing instructions to the left side of the brain, which, in turn, sends those instructions to the right hand. Finally, as text Figure 3-13 indicates, each eye by itself can stimulate both halves of the brain. However, when care is taken to stimulate the right visual field for an instant, while the subject is staring at a fixation point, that information goes to the left side of the brain only. So, these subjects could report visual information (words, pictures) that was flashed in the right visual field.

Agenesis is a rare condition in which a person is born without a corpus callosum. Do those people show the effects seen in the epileptics above? The answer is no. Apparently the brain, during development, makes other pathways for information transfer from left to right and vice-versa. Again, this condition provides evidence for the early plasticity of the brain.

The developing brain is plastic or changeable. So, brain damage at an early age, say 2 years or under, has much less effect on behavior than does brain damage at a later age. The reason is that much of the brain's development takes place in early childhood. So, early brain damage may be compensated for by other parts of the brain taking over the functions of the damaged part. However, in adulthood, the brain is set, and such takeovers are extremely rare. The rule of thumb is, the younger one is, the less effect will be seen from a given amount of brain damage.

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