Visual saliency pushes around the locus of attentional selection: Evidence from occipito-parietal ERP asymmetries

We manipulated the relative distance of a varying number of colored targets that were displayed unilaterally among bilaterally distributed colored distractors. By varying luminance, targets were made as salient as distractors in Experiment 1, or more salient than distractors in Experiment 2. Subjects were asked to examine streams of six such displays/frames, and report at the end of each stream the number of frames including a pre-specified number of targets. When targets and distractors were equiluminant, increasing the number of targets and decreasing their relative distance were both reflected in an amplitude increase of the N2pc, an established electrophysiological marker of attention selection. When targets were more salient than distractors, effects of target numerosity and relative distance identical to those found with equiluminant stimuli were substantially anticipated, and reflected in analogous amplitude variations of the N1pc. In Experiment 3, subjects were exposed to the same physical stimulation as that used in Experiment 2, and instructed to monitor a central fixation point for a possible color change while disregarding all other stimuli in each frame. Although target onset still elicited an N1pc, effects of target numerosity and inter-target distance on its amplitude were reduced to nil. These results are discussed with reference to models positing a locus of attentional selection whose flexibility is contingent on the balanced contribution of downstream/sensory and upstream/cognitive factors.