Cerebral Dominance and Language

The two cerebral hemispheres appear to be nearly symmetrical, but each has anatomical, chemical, and functional specializations. We have already mentioned that the left hemisphere deals with the somatosensory (Chapter 9) and motor (Chapter 12) functions of the right side of the body, and vice versa. In addition, in 90 percent of the population the left hemisphere is specialized to produce language—the conceptualization of what one wants to say or write, the neural control of the act of speaking or writing, and recent verbal memory. This is even true of the sign language used by deaf people.

Language is a complex code that includes the acts of listening, reading, and speaking. The major centers for language function are in the left hemisphere in temporal, parietal, and frontal cortex (the so-called peri-sylvian area, Figure 13-15a) next to the sylvian fissure, which separates the temporal lobe from the frontal and parietal lobes. Other language areas also exist in the cerebral cortex, each dealing with a separate aspect of language. For example, distinct areas are specialized for hearing, seeing, speaking, and generating words (Figure 13-15b). There are even distinct brain networks for different categories of things, such as "animals" and "tools." The cerebellum is important in speaking and writing, which involve coordinated muscle con tractions. Males and females use different brain areas for language processing, probably reflecting different strategies (Figure 13-16).

Neural specialization is demonstrated by the aphasias, any language defect resulting from brain damage. For example, in most people, damage to the left cerebral hemisphere, but not to the right, interferes with the capacity for language manipulation, and damage to different areas of the left cerebral hemisphere affects language use differently.

Damage to the temporal region known as Wernicke's area (Figure 13-17) generally results in aphasias that are more closely related to comprehension—the individuals have difficulty understanding spoken or written language even though their hearing and vision are unimpaired, and although they may have fluent speech, their speech is incomprehensible. In contrast, damage to Broca's area, the language area in the frontal cortex responsible for the articulation of speech, can cause expressive aphasias—the individuals have difficulty carrying out the coordinated respiratory and oral movements necessary for language even though they can move their lips and tongue. They understand spoken language and know what they want to say but have trouble forming words and putting them into grammatical order.

The potential for development of language-specific mechanisms in the left hemisphere is present at birth, but the assignment of language functions to specific brain areas is fairly flexible in the early years of life.

Frontal lobe

Perisylvian area

Parietal lobe

Perisylvian Area

FIGURE 13-15

(a) The perisylvian area, the site of many language functions, is indicated in orange. Blue lines indicate divisions of the cortex into frontal, parietal, temporal, and occipital lobes. (b) PET scans reveal changes in blood flow during various language-based activities. %

Part b courtesy of Dr. Marcus E. Raichle.

FIGURE 13-15

(a) The perisylvian area, the site of many language functions, is indicated in orange. Blue lines indicate divisions of the cortex into frontal, parietal, temporal, and occipital lobes. (b) PET scans reveal changes in blood flow during various language-based activities. %

Part b courtesy of Dr. Marcus E. Raichle.

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

Consciousness and Behavior CHAPTER THIRTEEN

Consciousness and Behavior CHAPTER THIRTEEN

Nmr Brain Man Woman Language

FIGURE 13-16

Images of the active areas of the brain of a male (left) and a female (right) during a language task. Note that both sides of a woman's brain are used in processing language but a man's brain is more compartmentalized.

Shaywitz, et al., 1995 NMR Research/Yale Medical School.

FIGURE 13-16

Images of the active areas of the brain of a male (left) and a female (right) during a language task. Note that both sides of a woman's brain are used in processing language but a man's brain is more compartmentalized.

Shaywitz, et al., 1995 NMR Research/Yale Medical School.

Thus, for example, damage to the perisylvian area of the left hemisphere during infancy or early childhood causes temporary, minor language impairment, but similar damage acquired during adulthood typically causes permanent, devastating language deficits.

Broca's area

Broca's area

Wernicke's area

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