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Edward M. Hubbard, Naveen Krishnan, Vilayanur S. Ramachandran; Reduced crowding with illusory contours supports an attentional locus for crowding. Journal of Vision 2002;2(7):6. doi: 10.1167/2.7.6.
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What is the neural basis of conscious awareness? Studies of blindsight patients (due to circumscribed V1 lesions) have suggested that V1 is the locus of visual awareness. However, the existence of extensive visual processing without awareness suggests that V1, while necessary for consciousness may not be sufficient. We explored the amount of processing that can occur in the absence of conscious awareness by means of the “crowding” effect, in which flanking distractors make it harder to identify a peripherally presented target. He et al. (1996) and Ramachandran et al. (1998) have demonstrated extensive processing of crowded items in the absence of conscious awareness. He et al. therefore attribute crowding to the limited resolution of visual attention. Von der Heydt (1995) showed that illusory contours can arise as early as V1 and V2, and studies of patients with neglect suggest that illusory contours may arise pre-attentively. To determine whether crowding is due to attentional limitations, we tested twenty subjects with four open or four closed pac-men stimuli that would form either an illusory contour (IC) or an amodally completed (AC) rectangle, with crowders either present or absent. If IC perception precedes crowding, subjects should be better in the IC condition than in the AC condition. To assess the magnitude of crowding, our observers judged whether rectangles of different aspect ratios were “tall” or “wide” (aspect ratio discrimination). Without crowders, performance was similar on IC trials and AC trials. With crowders present, performance on AC trials was significantly impaired relative to the no crowder condition. However, in the IC crowded condition, performance was significantly better than in the AC crowded condition. These results are consistent with the claim that crowding occurs late in processing and may result from the limited resolution of attention. Supported by NIMH 1 RO1 MH60474 to V.S.R> and NIMH 1 F31 MH63585-01 to E.M.H.
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