Unilateral neglect reflects a disturbance in the spatial distribution of directed attention. A review of unilateral neglect syndromes in monkeys and humans suggests that four cerebral regions provide an integrated network for the modulation of directed attention within extrapersonal space. Each component region has a unique functional role that reflects its profile of anatomical connectivity, and each gives rise to a different clinical type of unilateral neglect when damaged. A posterior parietal component provides an internal sensory map and perhaps also a mechanism for modifying the extent of synaptic space devoted to specific portions of the external world; a limbic component in the cingulate gyrus regulates the spatial distribution of motivational valence; a frontal component coordinates the motor programs for exploration, scanning, reaching, and fixating; and a reticular component provides the underlying level of arousal and vigilance. This hypothetical network requires at least three complementary and interacting representations of extrapersonal space: a sensory representation in posterior parietal cortex, a schema for distributing exploratory movements in frontal cortex, and a motivational map in the cingulate cortex. Lesions in only one component of this network yield partial unilateral neglect syndromes, while those that encompass all the components result in profound deficits that transcend the mass effect of the larger lesion. This network approach to the localization of complex functions offers an alternative to more extreme approaches, some of which stress an exclusive concentration of function within individual centers in the brain and others which advocate a more uniform (equipotential or holistic) distribution. In human beings, unilateral neglect syndromes are more frequent and severe after lesions in the right hemisphere. Also, right hemisphere mechanisms appear more effective in the execution of attentional tasks. Furthermore, the attentional functions of the right hemisphere span both hemispaces, while the left hemisphere seems to contain the neural apparatus mostly for contralateral attention. This evidence indicates that the right hemisphere of dextrals has a functional specialization for the distribution of directed attention within extrapersonal space.

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PET was used to image the neural system underlying visuospatial attention. Analysis of data at both the group and individual-subject level provided anatomical resolution superior to that described to date. Six right-handed male subjects were selected from a pilot behavioural study in which behavioural responses and eye movements were recorded. The attention tasks involved covert shifts of attention, where peripheral cues indicated the location of subsequent target stimuli to be discriminated. One attention condition emphasized reflexive aspects of spatial orientation, while the other required controlled shifts of attention. PET activations agreed closely with the cortical regions recently proposed to form the core of a neural network for spatial attention. The two attention tasks evoked largely overlapping patterns of neural activation, supporting the existence of a general neural system for visuospatial attention with regional functional specialization. Specifically, neocortical activations were observed in the right anterior cingulate gyrus (Brodmann area 24), in the intraparietal sulcus of right posterior parietal cortex, and in the mesial and lateral premotor cortices (Brodmann area 6).

The EPISTAR technique is a rapid, noninvasive means for creating qualitative maps of CBF.

SUMMARY Patients with unilateral hemispheric lesions were given visual target cancellation tasks. As expected, marked contralateral and less severe ipsilateral visual inattention were observed in patients with right-sided cerebral lesions whereas those with left-sided lesions showed only mild contralateral neglect. Stimulus material (shapes vs letters) and array (random vs structured) interacted in a complex manner to influence target detection only in patients with right-sided lesions. Furthermore, the search strategy of these patients tended to be erratic, particularly when the stimuli were in an unstructured array. A structured array prompted a more systematic and efficient search. It appears, therefore, that stimulus content and spatial array affect neglect behaviour in patients with right-sided lesions and that a lack of systematic visual exploration within the extrapersonal space is one factor that contributes to visual hemispatial inattention.The severity of visual hemispatial neglect manifested by patients with unilateral cerebral lesions can vary depending on the nature of the stimuli being examined.'-3 Many studies investigating stimulus parameters and neglect have concentrated on differences related to stimulus content, such as, verbal vs nonverbal symbols. Leicester et al' reported that neglect was more prominent in patients with right hemispheric lesions on match-to-sample tasks in which form discrimination was a factor while in patients with left hemispheric lesions tasks requiring letter discrimination were more likely to elicit neglect. Using target cancellation tasks, Heilman and Watson3 reported that patients with right cerebral lesions showed more hemispatial neglect for words than for slanted lines. This finding, directly opposed to that of Leicester et al, was cited in support of Kinsbourne's45 hypothesis that stimuli preferentially activating one cerebral hemisphere (that is, verbal-left vs spatial-right) would enhance orientation to the contralateral hemispace, reducing neglect in that field. lesions showed a similar degree of neglect with both verbal and nonverbal stimuli.In the present study we re-examined the effect of stimulus material on the severity of neglect by recording the number of target omissions and the time spent to detect single targets on visual cancellation tasks differing in stimulus material (letters vs shapes). Since visual hemispatial neglect may in part also be related to defective search strategies,7 we introduced the variable of stimulus array (random vs structured), a factor that has not been previously examined. Methods