The cerebral cortex of the human brain contains ~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. 26-1). 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 left-dominant perisylvian network for language, (2) a right-dominant parietofrontal network for spatial orientation, (3) an occipitotemporal network for face and object recognition, (4) a limbic network for explicit episodic memory, and (5) a prefrontal network for the executive control of cognition and comportment. Investigations based on functional imaging have also identified a default mode network, which becomes activated when the person is not engaged in a specific task requiring attention to external events. The clinical consequences of damage to this network are not yet fully defined.
Lateral (top) and medial (bottom) views of the cerebral hemispheres. The numbers refer to the Brodmann cyto-architectonic designations. Area 17 corresponds to the primary visual cortex, 41–42 to the primary auditory cortex, 1–3 to the primary somatosensory cortex, and 4 to the primary motor cortex. The rest of the cerebral cortex contains association areas. AG, angular gyrus; B, Broca’s area; CC, corpus callosum; CG, cingulate gyrus; DLPFC, dorsolateral prefrontal cortex; FEF, frontal eye fields (premotor cortex); FG, fusiform gyrus; IPL, inferior parietal lobule; ITG, inferior temporal gyrus; LG, lingual gyrus; MPFC, medial prefrontal cortex; MTG, middle temporal gyrus; OFC, orbitofrontal cortex; PHG, parahippocampal gyrus; PPC, posterior parietal cortex; PSC, peristriate cortex; SC, striate cortex; SMG, supramarginal gyrus; SPL, superior parietal lobule; STG, superior temporal gyrus; STS, superior temporal sulcus; TP, temporopolar cortex; W, Wernicke’s area.
THE LEFT PERISYLVIAN NETWORK FOR LANGUAGE AND APHASIAS
The production and comprehension of words and sentences is dependent on the integrity of a distributed network located along the perisylvian region of the language-dominant (usually left) hemisphere. One hub, situated in the inferior frontal gyrus, is known as Broca’s area. Damage to this region impairs fluency of verbal output and the grammatical structure of sentences. The location of a second hub, critical for language comprehension, is less clearly settled. Accounts of patients with focal cerebrovascular lesions identified Wernicke’s area, located at the parietotemporal junction, as a critical hub for word and sentence comprehension. Occlusive or embolic strokes ...