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Chapters 4 and 6 mapped the primary motor cortex (precentral gyrus of the frontal lobe), primary somatosensory cortex (postcentral gyrus of the parietal lobe), and the primary visual cortex (calcarine cortex of the posterior occipital lobe) onto the cerebral hemispheres. The primary auditory cortex is housed in the superior temporal gyrus of the temporal lobe. Knowing the locations of the motor cortex and these three primary sensory cortices allows for a logical deduction of the functions of the rest of the cortical surface as is discussed below.
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The hemisphere contralateral to the side of handedness is considered the dominant hemisphere (e.g., the left hemisphere in a right-handed patient), and the hemisphere ipsilateral to the side of handedness is considered the nondominant hemisphere (e.g., the right hemisphere in a right-handed patient). Most patients are right-handed, so their left hemisphere is the dominant hemisphere. Language dysfunction is most commonly due to lesions in the dominant (usually left) hemisphere, whereas neglect (see “Attention” below) is most commonly due to lesions in the nondominant (usually right) hemisphere (causing left-sided neglect).
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Parietal Lobes: Spatial Attention and Praxis
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The parietal lobe regions bounded by the somatosensory cortex anteriorly and the visual cortex posteriorly are ideally situated to combine visual and spatial information, playing roles in awareness of the body in space, spatial reasoning, and mathematical processing (Fig. 7–1). The projection from the occipital lobe superiorly to the parietal lobe (the dorsal stream) is referred to as the “where” pathway: Visual information is processed here to determine where things are in space with respect to the body. Lesions here can cause neglect: The patient is unaware of one half of the world. Neglect is more common with lesions in the nondominant parietal lobe, which is most commonly the right parietal lobe causing left-sided neglect. Examination findings in patients with neglect may include extinction to double simultaneous stimulation (see Ch. 4), lack of awareness of deficits (anosognosia; e.g., not acknowledging that a paretic limb is weak despite inability to move it), and in severe cases, inability to recognize the neglected body parts as one’s own.
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Lesions in the angular gyrus of the dominant (usually left) parietal lobe can cause Gerstmann’s syndrome: left-right confusion, inability to count (acalculia), inability to name the fingers (finger agnosia), and inability to write (agraphia).
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Parietal lesions can also cause difficulty performing a complex learned action (apraxia). This can be demonstrated by ...