Performance in simple visual search at threshold contrasts

Visual search for a target, orthogonal to distractors (nontargets) in orientation, has been shown to be independent of the number of distractors (set size). This finding has been thought to indicate that the search occurs spatially in parallel and without capacity limitations. The current study was designed to test whether an orientation difference of 90 degrees between the target and distractor gratings would produce a set-size effect when performance was measured at contrast threshold, that is, whether the threshold contrast at which the target was detected among distractors increased as a function of the number of distractors. The second question studied was whether signal-position uncertainty could explain the possible set-size effect.

The observer searched for a horizontal Gabor patch target in a two-interval, forced-choice task. In the search condition, the target patch was among seven vertical distractor Gabors, all positioned along an isoeccentric circle. The number of possible display locations monitored by the observer varied, and, before each block, he was informed which locations were relevant. In the single-element condition, the target appeared alone, but the number of possible target locations varied as above.

In both conditions, the contrast thresholds almost doubled when the number of possible target locations increased from 1 to 8.

Even though the orientation difference between target and distractors was maximal, a set-size effect was found. The effect could be explained by positional uncertainty.