Hemispheric patterns in visual search

Right-dominant contextual cueing for global configuration cues, but not local position cues

Contextual cueing can depend on global configuration or local item position. We investigated the role of these two kinds of cues in the lateralization of contextual cueing effects. Cueing by item position was tested by recombining two previously learned displays, keeping the individual item locations intact, but destroying the global configuration. In contrast, cueing by configuration was investigated by rotating learned displays, thereby keeping the configuration intact but changing all item positions. We observed faster search for targets in the left display half, both for repeated and new displays, along with more first fixation locations on the left. Both position and configuration cues led to faster search, but the search time reduction compared to new displays due to position cues was comparable in the left and right display half. In contrast, configural cues led to increased search time reduction for right half targets. We conclude that only configural cues enabled memory-guided search for targets across the whole search display, whereas position cueing guided search only to targets in the vicinity of the fixation. The right-biased configural cueing effect is a consequence of the initial leftward search bias and does not indicate hemispheric dominance for configural cueing.

[Congenital hemiparesis: complementary neuropsychological evaluation with computer]

We present the neuropsychological assessment with computer aid of six cerebral palsy children. Three children had right hemiparesis and three, left hemiparesis. The tomographic examination showed parietal cavities (porencephalic cyst in 4 children, ischemic injury in 1 case and subarachnoid cyst in 1 case). We have proposed to assess the visuo-spatial function since we suspected the children could have disturbance of this function. We did not detect this disturbance. On the other hand, the children had astereognosia and the right hemiparetic children preferred to execute signs on the right part of the computer visor. We discuss and propose explanations for both findings.

Hemispheric local/global processing revisited

Hemispheric differences for local and global processing were assessed with square and rectangular stimulus shapes which contained either squares or rectangles within them. Experiment 1 manipulated the overall size and area of the stimulus items, while experiment 2 kept stimulus area constant and manipulated the number of shapes inside the stimulus. In different conditions subjects judged whether the inside shapes (local processing) or whether the outside shapes (global processing) were squares or rectangles. Reaction time measures revealed that hemispheric differences were unrelated to the local or global processing requirements as well as the spatial frequency of the stimulus texture. A left hemisphere advantage generally was obtained across stimulus and processing conditions, with the pattern of task effects dependent on the specific stimuli employed. The results suggest that the overall stimulus shape rather than processing mode or specific spatial frequency is a primary determinant of hemispheric differences for visual information.

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.

The effect of analytic versus holistic encoding instructions on hemispheric superiority

A modification of the tachistoscopic letter detection task employed by Neisser (1963) was utilized to examine hemispheric differences in employing analytic and holistic processing strategies. Stimulus arrays designed to elicit either serial or parallel processing sets were presented to the right hemisphere-left visual field (RH-LVF) or to the left hemisphere-right visual field (LH-RVF). Subjects were explicitly directed to perform an analytic or holistic encoding process on both types of stimulus arrays. The serial array produced longer reaction times than the parallel array for both hemispheres. A RH-LVF reaction time advantage was found across both stimulus sets and instructions. In addition, an overall reaction time advantage was found for holistic instructions relative to analytic instructions, but this superiority was not effected by hemisphere or contextual stimuli.

Hemispheric differences for feature detection

The role of hemispheric processing for visual features was explored by tachistoscopically presenting subjects with stimulus displays composed of the letter I, displays in which a single T was embedded in an array of I's, or displays in which the letter O was embedded in an array of I's. The number of array elements was also manipulated (4, 16, or 36) to assess the effects of display size on featural detection for each visual field. Subjects verbally indicated whether array elements were all the same, or whether a T or O was present. Hemispheric error rates varied as a function of the type of letter to be detected, with left-hemisphere presentations producing superior performance for the detection of T's, right-hemisphere presentations yielding superior performance for the detection of O's, and Same displays demonstrating hemispheric patterns consistent with these results. The findings suggest that the analysis of perceptual features during visual stimulus processing may determine hemispheric outcomes in a variety of task situations.