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Paul F. Bulakowski, Robert B. Post, David Whitney; Differential spatial integration for perception and action revealed by perceptual and visuomotor crowding. Journal of Vision 2008;8(6):307. doi: 10.1167/8.6.307.
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While the spatial characteristics and perceptual effects of crowding are well documented, the influence of crowding on action, such as reaching and grasping, is largely unknown. Studies of saccades in a crowded visual search reveal that classic crowding manipulations (e.g. decreasing target-flanker distance & increasing target-flanker similarity) affect saccade size and duration (Vlaskamp & Hooge, Vision Research, 2006). This suggests that the mechanisms limiting perceptual resolution may similarly affect movement planning. The current study sought to directly compare perceptual and visuomotor effects of crowding in the upper and lower visual fields. A central target bar, flanked by randomly oriented distractor bars, was presented at an isoeccentric (30 deg) location in the upper, central, or lower visual field. Subjects discriminated the orientation of the target bar using a perceptual 3AFC key-press, or by reaching and grasping the target. An Optotrak tracking system recorded the angle of subjects' pincer grasp. The results demonstrated that the accuracy of perceptual and visuomotor judgments toward crowded targets were similar, with the lowest performance in the upper visual field, consistent with previous findings (He et al., Nature, 1996). Interestingly, while perceptual responses in highly crowded conditions were attracted to the mean or ensemble orientation (Parkes et al., Nature Neuro, 2001), the visuomotor responses displayed a repulsion or contrast effect relative to the mean flanker orientation (i.e. subjects tended to orient their grasp opposite the orientation of the average flanker orientation in all but the most crowded trials). The results suggest that while crowding may be an absolute bottleneck for both perceptual and visuomotor behavior, the spatial extent and/or weighting of visual space for perception and action are dissociable. A single antagonistic center-surround mechanism may account for this dissociation.
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