Hemispheric memory for random forms revisited

Hemispheric differences in visual search of simple line arrays

The effects of perceptual organization on hemispheric visual-information processing were assessed with stimulus arrays composed of short lines arranged in columns. A visual-search task was employed in which subjects judged whether all the lines were vertical (same) or whether a single horizontal line was present (different). Stimulus-display organization was manipulated in two experiments by variation of line density, linear organization, and array size. In general, left-visual-field/right-hemisphere presentations demonstrated more rapid and accurate responses when the display was perceived as a whole. Right-visual-field/left-hemisphere superiorities were observed when the display organization coerced assessment of individual array elements because the physical qualities of the stimulus did not effect a gestalt whole. Response times increased somewhat with increases in array size, although these effects interacted with other stimulus variables. Error rates tended to follow the reaction-time patterns. The results suggest that laterality differences in visual search are governed by stimulus properties which contribute to, or inhibit, the perception of a display as a gestalt. The implications of these findings for theoretical interpretations of hemispheric specialization are discussed.

Recognition of random shapes in brain-damaged patients

In two experiments the hypothesis was tested that left hemisphere-damaged patients and especially those with aphasia are impaired in the recognition of meaningless random shapes because they fail to attribute a meaning to the shapes. In a multiple choice recognition task, left hemisphere-damaged patients with aphasia and left and right hemisphere-damaged patients without aphasia were shown complex random shapes together with either a pictorial cue (experiment I and II) or a dotted drawing of its outline on which more or less outstanding parts were specially marked (experiment I). In experiment I no difference between conditions or groups emerged. In experiment II aphasics and left hemisphere-damaged patients without aphasia were generally inferior to right hemisphere-damaged controls and performed significantly better when a pictorial cue was given than when it was absent, however only when the conditions were given in a certain order.

Stimulus effects in unilateral neglect?

Unilaterally brain-damaged subjects and normal controls were administered parallel verbal and nonverbal forms of a visual cancellation task. The attentional theory of unilateral neglect, which predicts less manifest neglect on tasks that preferentially engage the damaged hemisphere, was not supported. Subjects who exhibited neglect did so to a comparable degree on both verbal and nonverbal tasks. The data are consistent with a model of right hemisphere dominance for attention across the visual field.

Hemispheric patterns in visual search

A visual search paradigm was employed to examine hemispheric serial and parallel processing. Stimulus arrays containing 4, 9, or 16 elements were tachistoscopically presented to the right visual field-left hemisphere (RVF-LH) or left visual field-right hemisphere (LVF-RH). Subjects judged whether all of the elements within an array were physically the same (all X's) or whether one (O) was different from the rest. Left hemisphere presentations were processed more quickly and accurately than LVF-RH presentations for all stimulus conditions. As the number of array elements increased, more errors and longer response times were obtained for different stimulus items whereas fewer errors and somewhat shorter response times were obtained for same stimulus items. These and previous results suggest that the left hemisphere obtains an advantage for visual search because of that hemisphere's superiority for fine-grained feature analysis rather than because of a fundamental hemispheric serial/parallel processing dichotomy.