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Paul F. Bulakowski, Robert B. Post, Michael D. K. Nguyen, David Whitney; Visual and visuomotor crowding. Journal of Vision 2007;7(9):415. doi: 10.1167/7.9.415.
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Although crowding is a well-established perceptual phenomenon, the degree to which it impacts action, such as reaching and grasping, is unclear. The current study sought to characterize visual and visuomotor crowding across a range of target/flanker separations and visual eccentricities. Using a 3AFC perceptual paradigm, subjects judged the orientation of a central bar presented within a radial display of 6 non-overlapping flankers. Utilizing the same stimuli, subjects' visuomotor responses were recorded as the approach angle of a pincer grasp directed toward the target bar. Visuomotor accuracy was operationalized as the correspondence between the orientation of the target bar and the angle between the index finger and thumb. As the separation between the target and distracters decreased, performance on both perceptual and visuomotor tasks declined at comparable rates. This crowding effect occurred more strongly at larger eccentricities and followed the half-eccentricity rule (e.g. Bouma, 1970; Pelli et al, 2004), which states that multiple objects separated by less than half their visual eccentricity crowd each other. These results extend crowding into the visuomotor domain, and suggest an overlapping mechanism between perceptual and visuomotor systems. Interestingly, however, the influence of the average flanker orientation differed across visual and visuomotor trials. Specifically, in very crowded trials, perceptual judgments of the target orientation more highly correlated with the mean ensemble orientation (e.g. Parkes et al, 2001). This finding suggests that visuomotor coordination of a grasp may not utilize ensemble information in the same way that it drives perceptual judgments in crowded scenes.
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