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Tingting Liu, Yi Jiang, Sheng He; Reduction of the crowding effect in spatially adjacent but cortically remote visual stimuli. Journal of Vision 2008;8(6):973. doi: 10.1167/8.6.973.
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When embedded in adjacent flanking distractors, a target becomes more difficult to perceive. The neural mechanism for this crowding effect remains unclear. Here we addressed the issue of whether cortical distance between target and distractor, independent of their distance in visual space, contributes to the strength of the crowding effect. Two spatially adjacent visual stimuli presented on either side of the vertical meridian are projected to contralateral cortical hemispheres. Thus, these two stimuli are initially processed in different hemispheres and are cortically distant from each other, at least in early visual areas, compared to two stimuli with the same spatial distance but presented to the same visual hemifield. In one experiment, we presented a peripheral target just next to the vertical meridian (left or right side) in the lower visual field. In an orientation discrimination task, observers performed significantly worse (more crowding) when more distractors were presented to the ipsilateral than to the contralateral visual field as the target stimulus. In another experiment, three texture patches were placed in a tangential configuration relative to the fixation point at the same eccentricity, and observers were asked to discriminate a small contrast difference inside the middle patch (target). Observers showed an abrupt increase in performance when one of the flankers crossed the vertical meridian. Together these results indicate that cortical distance between target and distractors, independent of their separation in visual space, is an important factor in determining crowding strength.
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