Maintaining rejected distractors in working memory during visual search depends on search stimuli: Evidence from contralateral delay activity.

The presence of memory for rejected distractors during visual search has been heavily debated in the literature and has proven challenging to investigate behaviorally. In this research, we used an electrophysiological index of working memory (contralateral delay activity) to passively measure working memory activity during visual search. Participants were asked to indicate whether a novel target was present or absent in a lateralized search array with three visual set sizes (2, 4, or 6). If rejected distractors are maintained in working memory during search, working memory activity should increase with the number of distractors that need to be evaluated. Therefore, we predicted the amplitude of the contralateral delay activity would be larger for target-absent trials and would increase with visual set size until WM capacity was reached. In Experiment 1, we found no evidence for distractor maintenance in working memory during search for real-world stimuli. In Experiment 2, we found partial evidence in support of distractor maintenance during search for stimuli with high target/distractor similarity. In both experiments, working memory capacity did not appear to be a limiting factor during visual search. These results suggest the role of working memory during search may depend on the visual search task in question. Maintaining distractors in working memory appears to be unnecessary during search for realistic stimuli. However, there appears to be a limited role for distractor maintenance during search for artificial stimuli with a high degree of feature overlap.