Figure 1317

Areas found clinically to be involved in the comprehension (Wernicke's area) and motor (Broca's area) aspects of language. Blue lines indicate divisions of the cortex into frontal, parietal, temporal, and occipital lobes.

Even if the left hemisphere is traumatized in older children, functional language ability may be reestablished to some extent in the right hemisphere. Language develops in these children, however, at the expense of usual right hemisphere functions, such as spatial localization. By puberty, the transfer of language functions to the right hemisphere is less successful, and often language skills are lost permanently.

Although language skills emerge spontaneously in all normal children in all societies, there is a critical period during childhood when exposure to language is necessary for these skills to develop, just as the ability to see depends upon effective visual input early in life. The critical period is thought to end at puberty or earlier. The dramatic change at puberty in the possibility of learning language, or the ease of learning a second language, occurs as the brain attains its structural, biochemical, and functional maturity at that time.

As part of these basic language skills, the left hemisphere contains the "rules" for general grammatical principles; thus, it is much more skilled at changing verb tenses and constructing possessives than is the right hemisphere. It is also dominant for events that occur in sequences over time such as those seen in language usage as, for example, speaking the first part of a word before the last. In addition, the left hemisphere seems constantly to form theories about how the world works, to find relationships between events encountered in the world, and to assess where one stands in relation to the world. The left hemisphere has been called "the interpreter."

The right hemisphere, on the other hand, more typically handles sensory information in basic ways, such as the perception of faces and other three-dimensional objects.

Memories are handled differently in the two hemispheres, too, with verbal memories more apt to be associated with the left hemisphere, and nonverbal memories (for example, visual patterns or nonverbal memories that convey emotions) with the right. Even the emotional responses of the two hemispheres seem to be different; for example, the left hemisphere has the greater ability to understand the emotional states of oneself or others. When electroconvulsive therapy is administered in the treatment of depression, however, better effects are often obtained when the electrodes are placed over the right hemisphere. The two sides of the brain also differ in their sensitivity to psy-choactive drugs. Differences between the two hemispheres are often masked by integration via the corpus callosum and other pathways that connect the two sides of the brain.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

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