P_D components and distractor inhibition in visual search: New evidence for the signal suppression hypothesis

The hypothesis that salient distractors in visual search are actively suppressed is supported by the fact that these objects elicit P_D components believed to be associated with inhibition. This account was challenged by researchers who found that a P_D to lateral color singleton distractors was followed by a contralateral negativity, which they interpreted as an N2pc indicative of attentional capture. As this would be at odds with successful distractor suppression, they proposed an alternative lateral-first serial scanning hypothesis, which assumes that the P_D might actually be an N2pc elicited when a lateral context item is selected. We tested this hypothesis by measuring lateralized ERP components to search displays with two lateral and two vertical midline items, including a color singleton and a shape-defined target. Color singletons triggered P_D components not only in blocks where attention was unfocused because target location was unpredictable, but critically also in blocks where targets only appeared on the midline and participants had no reason to attend to lateral items. This is inconsistent with the serial scanning hypothesis and supports the idea that the P_D reflects signal suppression. P_D components to singleton distractors were followed by a contralateral negativity, which we interpreted as a second P_D elicited by nonsalient distractors on the opposite side. Our sequential inhibition account reconciles conflicting results of recent studies and emphasizes the role of inhibitory processes during attentional target selection in visual search.