The Effect of Target-Flanker Similarity in Dense Crowding

Visual crowding is the impairment in recognising an object in the presence of nearby clutter. Two prominent characteristics of crowding are that flankers similar to the target cause more crowding than dissimilar ones, and radially located flankers cause more crowding than tangential ones. These observations are based on studies where a target is surrounded by a small number of flankers. Recent evidence, however, suggests that crowding behaves differently in the presence of a dense set of flankers. Here, we examined the effect of target-flanker similarity and radial-tangential anisotropy in dense crowding in order to probe the mechanisms underlying crowding. Experiment 1 found that the presence of dissimilar flankers reduced crowding in sparse settings, particularly in radial locations, as expected, but paradoxically increased crowding in dense settings. This effect was not attributable to perceived grouping. Experiment 2A manipulated the number of similar flankers adjacent to the target embedded in a dense set of dissimilar flankers and found that just one pair of nearby similar flankers drastically impaired target identification. Experiment 2B manipulated the number of dissimilar flankers adjacent to a target embedded within similar flankers, which revealed a modest improvement in performance with increasing number of dissimilar flankers. There was no evidence of radial-tangential anisotropy in Experiment 2A or 2B. These findings are discussed in relation to prevalent theories of crowding. The results, particularly that of Experiment 2A, lend support to the nearest neighbour hypothesis, which explains crowding as a failure of target segmentation from its immediately adjacent neighbours.