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Peter J. Bex, Steven C. Dakin; Directional crowding. Journal of Vision 2004;4(8):852. doi: 10.1167/4.8.852.
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The integration of nearby features can mask their individual identities, an effect known as crowding. We used moving targets to evaluate different accounts of crowding. Observers identified the direction of motion (up/down/left/right) of a drifting narrow-band micro-pattern that was surrounded by four similar patches. The surrounding elements either moved in directions forming global patterns of translation, rotation, expansion, contraction or random motion, or they counter-phase flickered. In line with previous studies, crowding increased with eccentricity and interactions among moving elements were low-pass tuned for spatial frequency and high-pass tuned for temporal frequency. Complex motion is thought to be encoded by optic flow-selective units with large receptive fields, so on the assumption that crowding reflects normal grouping processes, non-random surround configurations might be expected to crowd over larger distances than random configurations. However the spatial extent of interference was approximately equal for all conditions. This finding also challenges the generality of a recent proposal that crowding arises from compulsory averaging of nearby texture (Parkes etal 2001, Nature Neuroscience, 739), which predicts that surrounding configurations whose mean directions cancel (e.g. rotation) should crowd less than those that don't (e.g. flicker). Models in which crowding is based on confusions of target and surrounding elements are also confounded by the observation that surrounds that translate in the same direction as the target crowd as effectively as those that move in orthogonal and opposite directions.
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