Effects of task-irrelevant or filler items on brain mechanisms of visual spatial attention.

Many visual search paradigms use color to distinguish task-relevant items from those considered fillers (e.g., blue task-relevant items and grey fillers). Hilimire and colleagues suggested that the N2pc, a lateralized electrophysiological component typically observed in visual attention, is a neural correlate for localized attentional interference, which postulates that target selection is degraded by nearby competing stimuli. In their study, N2pc amplitude decreased with decreasing distance between task-relevant items presented among fillers. With an increase in distance, however, there was also an increase in the number of fillers between task-relevant items. We tested whether this distance effect could be explained by the presence of fillers near task-relevant items rather than their proximity per se. We manipulated the distance between task-relevant items (adjacent, separated by two, or by four positions) and the presence/absence of fillers orthogonally. We used two color schemes: blue task-relevant items and grey fillers or grey task-relevant items and blue fillers (manipulated between-subjects) to control for color interactions. N2pc amplitude increased with increasing distance, but only when fillers were present, suggesting that the results of Hilimire et al. may be due to increasing fillers interference. Exploratory analyses also suggested that the colors selected to be task-relevant and task-irrelevant could play a role in our ability to filter task-irrelevant information. Our results suggest that fillers are not as inconsequential as sometimes assumed and generally support the Ambiguity Resolution Theory, where nearby items increase N2pc amplitude because of a greater need for focused attention.