Knowledge of the brain regions involved in language has been gained primarily by the study of aphasias—speech and language disorders caused by damage to the brain through head injury or stroke. In most people, the language areas of the brain are primarily located in the left hemisphere of the cerebral cortex, as previously described. Even in the nineteenth century, two areas of the cortex— Broca's area and Wernicke's area (fig. 8.13)—were recognized as areas of particular importance in the production of aphasias.
Broca's aphasia is the result of damage to Broca's area, located in the left inferior frontal gyrus and surrounding areas. Common symptoms include weakness in the right arm and the right side of the face. People with Broca's aphasia are reluctant to speak, and when they try, their speech is slow and poorly articulated. Their comprehension of speech is unimpaired, however. People with this aphasia can understand a sentence but have difficulty repeating it. It should be noted that this is not simply due to a problem in motor control, since the neural control over the musculature of the tongue, lips, larynx, and so on is unaffected.
Wernicke's aphasia is caused by damage to Wernicke's area, located in the superior temporal gyrus of the left hemisphere (in most people). This results in speech that is rapid and fluid but without meaning. People with Wernicke's aphasia produce speech that has been described as a "word salad." The words used may be real words that are chaotically mixed together, or they may be made-up words. Language comprehension is destroyed; people with Wernicke's aphasia cannot understand either spoken or written language.
It appears that the concept of words originates in Wer-nicke's area. Thus, in order to understand words that are read, information from the visual cortex (in the occipital lobe) must project to Wernicke's area. Similarly, in order to understand spoken words, the auditory cortex (in the temporal lobe) must send information to Wernicke's area.
To speak intelligibly, the concept of words originating in Wernike's area must be communicated to Broca's area; this is accomplished by a fiber tract called the arcuate fasciculus. Broca's area, in turn, sends fibers to the motor cortex (precentral gyrus), which directly controls the musculature of speech. Damage to the arcuate fasciculus produces conduction aphasia, which is fluent but nonsensical speech as in Wernicke's aphasia, even though both Broca's and Wernicke's areas are intact.
The angular gyrus, located at the junction of the parietal, temporal, and occipital lobes, is believed to be a center for the integration of auditory, visual, and somatesthetic information. Damage to the angular gyrus produces aphasias, which suggests that this area projects to Wernicke's area. Some patients with damage to the left angular gyrus can speak and understand spoken language but cannot read or write. Other patients can write a sentence but cannot read it, presumably because of damage to the projections from the occipital lobe (involved in vision) to the angular gyrus.
Clinical Investigation Clues
Remember that Frank had difficulty speaking, but his speech was coherent.
Which type of aphasia did Frank most likely have? Which part of the brain sustained the damage?
Motor cortex (precentral gyrus)
Motor speech area (Broca's area)
■ Figure 8.13 Brain areas involved in the control of speech. Damage to these areas produces speech deficits, known as aphasias. Wernicke's area, required for language
Wernicke's comprehension, receives information from many areas of the brain, including the auditory cortex (for heard words), the visual cortex (for read words), and other brain areas. In order for a person to be able to speak intelligibly, Wernicke's area must send messages to Broca's area, which controls the motor aspects of speech by way of its input to the motor cortex.
Bft Recovery of language ability, by transfer to the right hemisphere after damage to the left hemisphere, is ^ very good in children but decreases after adolescence. Recovery is reported to be faster in left-handed people, possibly because language ability is more evenly divided between the two hemispheres in left-handed people. Some recovery usually occurs after damage to Broca's area, but damage to Wernicke's area produces more severe and permanent aphasias.
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