Task-irrelevant filler items alter the dynamics of electrical brain activity during visual search

In electroencephalography (EEG) studies of visual search, task-irrelevant fillers are included in displays to balance bottom-up stimulation across the visual field and generally considered as inconsequential for performance or EEG results. We examined the impact of fillers on target and distractor processing using lateralised event-related potentials (ERPs). Two task-relevant items (TRIs) were presented, with or without fillers. One TRI (target or target-colour distractor) was on the vertical midline and the other in a lateral position (left or right visual field) on an imaginary circle around fixation. An N2pc was elicited by lateral targets and task-relevant distractors, suggesting that attention was allocated to the lateral TRI because it possessed a target defining feature (colour). A Ptc was only elicited by lateral task-relevant distractors, in line with previous research suggesting that this component is associated with distractor processing. When fillers were also in the circular arrangement, alterations in performance and neural activity occurred. Fillers enhanced and delayed attentional deployment (N2pc) and delayed distractor processing (Ptc). Critically, we observed no difference in Ptc amplitude according to filler presence. Thus, if the Ptc reflects active suppression (or attentional disengagement), it was not required for fillers. ERPs were also modulated by the distance between TRIs (which could be separated by one or four filler positions), but differently according to the colour scheme (blue TRIs with grey fillers or vice versa). Our results suggest that fillers affect lateralised electrophysiological activity at multiple loci during visual search and should not be considered inconsequential.