Salient-but-irrelevant stimuli cause attentional capture in difficult, but attentional suppression in easy visual search.

Search for a shape target is difficult when its shape is similar to the shape of the surrounding nontargets and easy when it is dissimilar. We asked whether interference from a salient but irrelevant color singleton depended on search difficulty as manipulated by target-nontarget similarity. We found that interference was strong in difficult searches, and the occurrence of an electrophysiological index of attentional selectivity (the N2pc component) confirmed that attention was captured by the distractor. In contrast, interference from the distractor was weak with easy searches, and the occurrence of the PD component confirmed that saliency signals from the distractor were suppressed. The results suggest that attentional suppression of salient but irrelevant distractors is only possible when search is efficient (i.e., with low target-nontarget similarity), otherwise, attentional capture occurs. Further, we analyzed the Ppc component, a positivity occurring between 100 and 200 ms after stimulus onset contralateral to a salient feature discontinuity. It has been suggested that the Ppc reflects bottom-up saliency signals, but we found the Ppc to occur only when the salient stimulus was unlikely to be selected, as in efficient searches. Thus, attentional requirements modulate the Ppc component.