The cerebral cortex of the human brain contains approximately 20 billion neurons spread over an area of 2.5 m2. The primary sensory areas provide an obligatory portal for the entry of sensory information into cortical circuitry, and the primary motor areas provide a final common pathway for coordinating complex motor acts. 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 behavior. A systematic testing of these mental functions is necessary for the effective clinical assessment of the association cortex and its diseases.
Lateral (top) and medial (bottom) views of the cerebral hemispheres. The numbers refer to the Brodmann cytoarchitectonic 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.
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. The network approach to higher cerebral function has at least four implications of clinical relevance: (1) A single domain such as language or memory can be disrupted by damage to any one of several areas as long as those areas belong to the same network, (2) damage confined to a single area can give rise to multiple deficits involving the functions of all the networks that intersect in that region, (3) damage to a network component may give rise to minimal or transient deficits if other parts of the network undergo compensatory reorganization, and (4) individual anatomic sites within a network display a relative (but not absolute) specialization for different behavioral aspects of the relevant function. Five anatomically defined large-scale networks are most relevant to clinical practice: (1) a perisylvian network for language, (2) a parietofrontal network for spatial cognition, (3) an occipitotemporal network for face and object recognition, (4) a limbic network for retentive memory, and (5) a prefrontal network for cognitive and behavioral control.
Language allows the communication and elaboration of thoughts and experiences by linking them to arbitrary symbols known as words. The neural substrate of language is composed of a distributed network centered in the perisylvian region of the left hemisphere. The posterior pole of this network is located at the temporoparietal junction and includes a region known as Wernicke's area. An essential function of Wernicke's area is to transform sensory inputs into their neural word representations so that they can establish the distributed associations that give a word its meaning. The anterior pole of the language network is located in the inferior frontal gyrus and includes a region known as Broca's area. An essential function of this area is to transform neural word representations into their articulatory sequences so that the words can be uttered in the form of spoken language. The sequencing function of Broca's area also appears to involve the ordering of words into sentences that contain a meaning-appropriate syntax (grammar). Wernicke's and Broca's areas are interconnected with each other and with additional perisylvian, temporal, prefrontal, and posterior parietal regions, making up a neural network that subserves the various aspects of language function. Damage to any one of these components or to their interconnections can give rise to language disturbances (aphasia). Aphasia should be diagnosed only when there are deficits in the formal aspects of language, such as naming, word choice, comprehension, spelling, and syntax. Dysarthria and mutism do not by themselves lead to a diagnosis of aphasia. The language network shows a left hemisphere dominance pattern in the vast majority of the population. In approximately 90% of right-handers and 60% of left-handers, aphasia occurs only after lesions of the left hemisphere. In some individuals no hemispheric dominance for language can be discerned, and in some others (including a small minority of right handers) there is a right hemisphere dominance for language. A language disturbance that occurs after a right hemisphere lesion in a right hander is called crossed aphasia.
The clinical examination of language should include the assessment of naming, spontaneous speech, comprehension, repetition, reading, and writing. A deficit of naming (anomia) is the single most common finding in aphasic patients. When asked to name a common object (pencil or wristwatch), the patient may fail to come up with the appropriate word, may provide a circumlocutious description of the object ("the thing for writing"), or may come up with the wrong word (paraphasia). If the patient offers an incorrect but related word ("pen" for "pencil"), the naming error is known as a semantic paraphasia; if the word approximates the correct answer but is phonetically inaccurate ("plentil" for "pencil"), it is known as a phonemic paraphasia. Asking the patient to name body parts, geometric shapes, and component parts of objects (lapel of coat, cap of pen) can elicit mild forms of anomia in patients who otherwise can name common objects. In most anomias, the patient cannot retrieve the appropriate name when shown ...