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The cerebral cortex of the human brain contains approximately 20 billion neurons spread over an area of 2.5 m2. The primary sensory and motor areas constitute 10% of the cerebral cortex. The rest is subsumed by modality-selective, heteromodal, paralimbic, and limbic areas collectively known as the association cortex (Fig. 22-1). The association cortex mediates the integrative processes that subserve cognition, emotion, and comportment. A systematic testing of these mental functions is necessary for the effective clinical assessment of the association cortex and its diseases. According to current thinking, there are no centers for “hearing words,” “perceiving space,” or “storing memories.” Cognitive and behavioral functions (domains) are coordinated by intersecting large-scale neural networks that contain interconnected cortical and subcortical components. Five anatomically defined large-scale networks are most relevant to clinical practice: (1) a perisylvian network for language, (2) a parietofrontal network for spatial orientation, (3) an occipitotemporal network for face and object recognition, (4) a limbic network for retentive memory, and (5) a prefrontal network for the executive control of cognition and comportment.
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THE LEFT PERISYLVIAN NETWORK FOR APHASIAS
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The areas that are critical for language make up a distributed network located along the perisylvian region of the left hemisphere. One hub, located in the inferior frontal gyrus, is known as Broca’s area. Damage to this region impairs phonology, fluency, and the grammatical structure of sentences. The location of a second hub, known as Wernicke’s area, is less clearly settled but is traditionally thought to include the posterior parts of the temporal lobe. Cerebrovascular accidents that damage this area interfere with the ability to understand spoken or written sentences as well as the ability to express thoughts through meaningful words and statements. These two hubs are interconnected with each other and with surrounding parts of the frontal, parietal, and temporal lobes. Damage to this network gives rise to language impairments known as aphasia. Aphasia should be diagnosed only when there are deficits in ...