Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 9)

The Parietofrontal Network for Spatial Orientation: Neglect and Related ConditionsHemispatial NeglectAdaptive orientation to significant events within the extrapersonal space is subserved by a large-scale network containing three major cortical components. The cingulate cortex provides access to a limbic-motivational mapping of the extrapersonal space, the posterior parietal cortex to a sensorimotor representation of salient extrapersonal events, and the frontal eye fields to motor strategies for attentional behaviors (Fig. 27-2). Subcortical components of this network include the striatum and the thalamus. Contralesional hemispatial neglect represents one outcome of damage to any of the cortical or subcortical components of this network. The...
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Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 9) Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 9) The Parietofrontal Network for Spatial Orientation: Neglect andRelated Conditions Hemispatial Neglect Adaptive orientation to significant events within the extrapersonal space issubserved by a large-scale network containing three major cortical components.The cingulate cortex provides access to a limbic-motivational mapping of theextrapersonal space, the posterior parietal cortex to a sensorimotor representationof salient extrapersonal events, and the frontal eye fields to motor strategies forattentional behaviors (Fig. 27-2). Subcortical components of this network includethe striatum and the thalamus. Contralesional hemispatial neglect represents oneoutcome of damage to any of the cortical or subcortical components of thisnetwork. The traditional view that hemispatial neglect always denotes a parietallobe lesion is inaccurate. In keeping with this anatomic organization, the clinicalmanifestations of neglect display three behavioral components: sensory events (ortheir mental representations) within the neglected hemispace have a lesser impacton overall awareness; there is a paucity of exploratory and orienting acts directedtoward the neglected hemispace; and the patient behaves as if the neglectedhemispace was motivationally devalued. Figure 27-2 Functional magnetic resonance imaging of language and spatialattention in neurologically intact subjects. The dark areas show regions of task-related significant activation. (Top) The subjects were asked to determine if twowords were synonymous. This language task led to the simultaneous activation ofthe two epicenters of the language network, Brocas area (B) and Wernickes area(W). The activations are exclusively in the left hemisphere. (Bottom) The subjectswere asked to shift spatial attention to a peripheral target. This task led to thesimultaneous activation of the three epicenters of the attentional network, theposterior parietal cortex (P), the frontal eye fields (F), and the cingulate gyrus(CG). The activations are predominantly in the right hemisphere. (Courtesy ofDarren Gitelman, MD; with permission.)According to one model of spatialcognition, the right hemisphere directs attention within the entire extrapersonalspace, whereas the left hemisphere directs attention mostly within the contralateralright hemispace. Consequently, unilateral left hemisphere lesions do not give riseto much contralesional neglect since the global attentional mechanisms of the righthemisphere can compensate for the loss of the contralaterally directed attentionalfunctions of the left hemisphere. Unilateral right hemisphere lesions, however,give rise to severe contralesional left hemispatial neglect because the unaffectedleft hemisphere does not contain ipsilateral attentional mechanisms. This model isconsistent with clinical experience, which shows that contralesional neglect ismore common, severe, and lasting after damage to the right hemisphere than afterdamage to the left hemisphere. Severe neglect for the right hemispace is rare, evenin left handers with left hemisphere lesions. Patients with severe neglect may fail to dress, shave, or groom the left sideof the body; may fail to eat food placed on the left side of the tray; and may fail toread the left half of sentences. When the examiner draws a large circle [12–15 cm(5–6 in.) in diameter] and asks the patient to place the numbers 1–12 as if thecircle represented the face of a clock, there is a tendency to crowd the numbers onthe right side and leave the left side empty. When asked to copy a simple linedrawing, the patient fails to copy detail on the left; and when asked to write, thereis a tendency to leave an unusually wide margin on the left. Two bedside tests that are useful in assessing neglect are simultaneousbilateral stimulation and visual target cancellation. In the former, the examinerprovides either unilateral or simultaneous bilateral stimulation in the visual,auditory, and tactile modalities. Following right hemisphere injury, patients whohave no difficulty detecting unilateral stimuli on either side experience thebilaterally presented stimulus as coming only from the right. This phenomenon isknown as extinction and is a manifestation of the sensory-representational aspectof hemispatial neglect. In the target detection task, targets (e.g., As) areinterspersed with foils (e.g., other letters of the alphabet) on a 21.5 x 28.0 cm (8.5x 11 in.) sheet of paper and the patient is asked to circle all the targets. A failure todetect targets on the left is a manifestation of the exploratory deficit in ...