Prioritizing new over old: an fMRI study of the preview search task

Abstract

In visual search, observers can successfully ignore temporally separated distractors that are presented as a preview before onset of the search display. Previous behavioral studies have demonstrated the involvement of top-down selection mechanisms in preview search, biasing attention against the old set in favor of the more relevant new set. Using functional magnetic resonance imaging, we replicate and extend findings showing the involvement of superior and inferior parietal areas in the preview task when compared to both a relatively easy single-set search task and a more effortful full-set search task. In contrast, the effortful full-set search showed activation in the dorsolateral prefrontal cortex when compared to the single-set search, suggesting that this area is involved in rejecting additional distractors that could not be separated in time.

Figure 1

Stimuli and experimental procedure. In all conditions, a preview display was followed by a search display. In the preview condition, previewed distractors remained on screen when search items were added. In the single‐set and full‐set conditions, the previewed items disappeared when the search set arrived. The final total set in the full‐set condition was twice as large as that in the single‐set condition and as large as that in the preview condition. The target is an N, and the final set size is 16 (8 in the single set). The target could also be a Z, and the set size could also be 8 (4 in the single set). Displays were followed by a 10‐s blank period.

Figure 2

Behavioral data: Average reaction time search functions for the single‐set, full‐set and preview conditions.

Figure 3

Areas of activation for various contrasts averaged across 12 participants presented on sagittal, coronal, and axial slices of their normalized and averaged structural scans. Graphs were not chosen for maximum level of activity, but for maximum overview of the activated areas. There were no significantly deactivated areas.