Electrophysiological evidence for target facilitation without distractor suppression in two-stimulus search displays

This study used electrophysiological measures to investigate how attention is deployed to target and distractor stimuli during visual search using search displays with a small set-size. Participants viewed randomized sequences of two-item displays that consisted of either a target and a distractor (differing in color) or a pair of task-irrelevant filler stimuli having a third color, all presented in an ongoing stream of flickering gray circles. The allocation of attention was assessed by concurrent recordings of steady-state visual evoked potentials (SSVEPs) elicited by the flickering displays and perturbations of the endogenous alpha rhythm following each type of display. The aim was to test a central prediction of the signal suppression hypothesis, namely that the processing of distractors will be proactively suppressed below the level of filler stimuli. Amplitude modulations of both the SSVEP and the lateralized alpha rhythm provided converging evidence against early proactive suppression of highly salient distractors. Instead, these electrophysiological measures were consistent with the view that in this type of two-stimulus search task there is an initial capture of attention by all color-change stimuli (targets, distractors, and fillers) followed by a further focusing of attention upon the target, with no evidence for suppression of the distractor.