Search

Examines the modalities of the left and right hemisphere of the human brain. Significance of the corpus callosum to the hemispheric functions of the brain; Implications of segregation in sensory signals for hemispheric brain functions; Factor which affects the coherence between neural responses in the right and left hemisphere.

Unilateral brain-damaged patients and normal control subjects were asked to remember visual hierarchical stimuli consisting of larger forms constructed from smaller forms. The right-hemisphere damaged patients made more errors in remembering the larger forms relative to the smaller forms, whereas the left-hemisphere damaged patients made more errors in remembering the smaller forms relative to the larger forms. These findings are discussed as they relate to hemispheric specialization for visuospatial processing.

Patients with hemispatial neglect fail to respond to stimuli on one side of space. We assessed to what extent the complexity and number of visual stimuli on both sides determine the severity of neglect. Patients with neglect were required to find specified targets in a cluttered visual field. Two sets of stimuli were used. One set produced effortless and parallel search performance in normal controls; the other set was more complex and produced serial search performance in normal controls. Both sets of stimuli resulted in a serial performance pattern in the patients. Their baseline search performance on both sides was similar when all stimulus items were restricted to one side. A pronounced difference between the two sides was evident when stimuli appeared on both sides. Search for targets on the intact side of space was unaffected by distractors on the neglected side, whereas search for targets on the neglected side was slowed disproportionately by distractors on the intact side. The slowing on the neglected side was more severe during the more demanding search task and when more items were present on the intact side. The results indicate that neglect is associated with an inability to move attention from objects on the intact side to items on the neglected side.

Preattentive processes such as perceptual grouping are thought to be important in the initial guidance of visual attention and may also operate in unilateral neglect by contributing to the definition of a task-appropriate reference frame. We explored this question with a visual search task in which patients with unilateral visual neglect (5 with right-, 2 with left-hemisphere damage) searched a diamond-shaped matrix for a conjunction target that shared one feature with each of two distractor elements. Additional grouping stimuli appeared as flanks either on the left, right, or both sides of the central matrix, and significantly changed performance in the search task. As expected, when flanks appeared only on the ipsilesional side a decrement in search performance was observed, but the further addition of contralesional flanks actually reduced the decrement and returned performance to near baseline levels. These data suggest that flanking stimuli on the neglected contralesional side of visual space can influence the reference frame by grouping with task-relevant stimuli, and that this preattentive influence can be preserved in patients with unilateral visual neglect.