PERFORMANCE IN SIMPLE VISUAL-SEARCH AT THRESHOLD CONTRASTS

Purpose. Visual search for a target, orthogonal to distractors (nontar gets) in orientation, has been shown to be independent of the number o f distracters (set size). This finding has been thought to indicate th at the search occurs spatially in parallel and without capacity limita tions, The current study was designed to test whether an orientation d ifference of 90 degrees between the target and distractor gratings wou ld produce a set-size effect when performance was measured al contrast threshold, that is, whether the threshold contrast at which the targe t was detected among distracters increased as a function of the number of distracters. The second question studied was whether signal-positi on uncertainty could explain the possible set-size effect. Methods. Th e observer searched for a horizontal Gabor patch target in a two-inter val, forced-choice task. In the search condition, the target patch was among seven vertical distracter Gabors, all positioned along an isoec centric circle. The number of possible display locations monitored by the observer varied, and, before each block, he was informed which loc ations were relevant. In the single-element condition, tile target app eared alone, but the number of possible tat gel locations varied as ab ove. Results. In both conditions, the contrast thresholds almost doubl ed when the number of possible target locations increased From 1 to 8. Conclusions. Even though the orientation difference between target an d dish-actors was maximal, a set-size effeet was found. The effect cou ld be explained by positional uncertainty.