Substitution and pooling in visual crowding induced by similar and dissimilar distractors.

Visual crowding refers to a phenomenon whereby objects that appear in the periphery of the visual field are more difficult to identify when embedded within clutter. Pooling models assert that crowding results from an obligatory averaging or other combination of target and distractor features that occurs prior to awareness. One well-known manifestation of pooling is feature averaging, with which the features of target and nontarget stimuli are combined at an early stage of visual processing. Conversely, substitution models assert that crowding results from binding a target and nearby distractors to incorrect spatial locations. Recent evidence suggests that substitution predominates when target-flanker feature similarity is low, but it is unclear whether averaging or substitution best explains crowding when similarity is high. Here, we examined participants' orientation report errors for targets crowded by similar or dissimilar flankers. In two experiments, we found evidence inconsistent with feature averaging regardless of target-flanker similarity. However, the observed data could be accommodated by a probabilistic substitution model in which participants occasionally "swap" a target for a distractor. Thus, we conclude that-at least for the displays used here-crowding likely results from a probabilistic substitution of targets and distractors, regardless of target-distractor feature similarity.