Abstract
Crowding is a breakdown in object recognition that occurs in cluttered visual environments. Though often attributed to inhibitory or noisy processes that result in information loss, we show that crowding actually produces a systematic change in the appearance of target objects. In particular, patches of isotropic bandpass-filtered noise appear oriented when crowded by four identically oriented Gabor patches. This was assessed with a novel change-detection paradigm, where either the target or flanking stimuli (counterphasing at 2 Hz) could be changed midway through the 1-second presentation. Observers were required to detect changes in the target noise, positioned at 15° eccentricity. On trials in which a Gabor replaced the noise element, subjects' ability to detect change was directly related to the similarity between flanks and the replacement Gabor: while dissimilar substitutions were easy to detect, similar substitutions were rarely noticed. This indicates that the perceived orientation induced by crowding is indistinguishable from physically oriented stimuli. The strength of this induced orientation was also apparent on trials when flank orientation changed (leaving target noise unaltered), with observers frequently indicating an erroneous rotation of the target. When this crowded noise was used as an adaptor, the perceived orientation of subsequently presented Gabor stimuli showed a strong tilt aftereffect (TAE), while contrast-detection thresholds were unaffected. This TAE cannot be solely attributed to spatially non-specific adaptation at the flank locations, as adaptation to the flanks in isolation induced a much smaller TAE. Rather, these results demonstrate that crowding arises from the superimposition of target and flank signals in the target location, via interactions that occur after the initial feature-detection stage. It is through these interactions that crowding exerts what amounts to an organisational influence, combining information from adjacent locations in the peripheral visual field.
Funded by the Wellcome Trust.