The Pd component reflects suppression of salient distractors below baseline

Both enhancement of relevant and suppression of irrelevant information contribute to visual selective attention. Suppression of irrelevant salient stimuli is particularly helpful as it prevents attention to be captured and thus attentional resources to be wasted. It was suggested that the Pd (distractor positivity) component in the event-related potential of the EEG reflects such active suppression. This study aimed at directly connecting the Pd component to failures to suppress salient distractors. Participants performed a visual search task in which they had to report the digit of a diamond-shaped target while ignoring circular distractors. One of the distractors could have a unique color (an additional singleton), rendering it particularly salient. Not just the target, but all distractors had a unique digit embedded. Thus we could deduct from a participant’s response which item they attended. Reports of target digits served as a behavioral index of enhancement, and reports of color distractor digits served as a behavioral index of failed suppression, each measured against reports of neutral distractor digits serving as a baseline. We measured participants’ EEG signal while they performed the task to identify the underlying neural mechanisms of suppression. In line with previous results, participants reported the target identity more often than any distractor identity and the color singleton identity least often, suggesting suppression of the singleton below baseline. Importantly, when the singleton identity was not suppressed but reported, the Pd was observed in a later time window. This suggests that the Pd reflects suppression of salient distractors below baseline. Interestingly, singletons also elicited a contralateral delay activity (CDA) when they were reported, indicating that the erroneous encoding of singletons into working memory may contribute to behavioral errors. Our results provide evidence for the signal suppression hypothesis that states salient items have to be actively suppressed to avoid attentional capture.