Electrophysiological evidence for failures of item individuation in crowded visual displays.

Visual perception is strongly impaired when peripheral targets are surrounded by nearby distractors, a phenomenon known as visual crowding. One common behavioral signature of visual crowding is an increased tendency for observers to mistakenly report the features of nearby distractors instead of the target item. Here, our goal was to distinguish between two possible explanations of such substitution errors. On the one hand, crowding may have its effects after the deployment of attention toward-and individuation of-targets and flankers, such that multiple individuated perceptual representations compete to guide the behavioral response. On the other hand, crowding may prevent the individuation of closely spaced stimuli, thereby reducing the number of apprehended items. We attempted to distinguish these alternatives using the N2pc, an ERP that has been shown to track the deployment of spatial attention and index the number of individuated items within a hemifield. N2pc amplitude increased monotonically with set size in uncrowded displays, but this set size effect was abolished in crowded visual displays. Moreover, these crowding-induced declines in N2pc amplitude predicted individual differences in the rate of substitution errors. Thus, crowding-induced confusions between targets and distractors may be a consequence of failures to individuate target and distractor stimuli during early stages of visual selection.