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Jungang Qin, Bosco S. Tjan; Size illusion and crowding. Journal of Vision 2010;10(7):1337. doi: 10.1167/10.7.1337.
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Crowding represents an essential bottleneck for form vision in the peripheral field (Levi, 2008). The severity of crowding depends on the center-to-center spacing between target and flankers and the critical spacing corresponds to a 6 mm separation on V1 cortex (Pelli, 2008). The perspective cue of a scene can induce changes in both the perceived object size and spacing. An increase in the perceived size leads to a corresponding increase in the spatial extent of fMRI activation in V1 (Murray, et al., 2006). Do the perceived object size and spacing affect crowding? We examined this question by presenting letters against a computer-rendered hallway scene with strong perspective cue. Letters of constant size appeared larger when presented at 3/8 the screen height measured from the top of the display (“far” condition) than at 6/8 the screen height(“near” condition). Contrast thresholds were measured for identifying a target letter when it was presented either alone or flanked by other letters. The letters were presented either at the “near” or “far” location, each with its own fixation to ensure a target eccentricity of 10°. In the control conditions, the hallway scene was replaced with a local patch of the scene covering the “near” or “far” letter area with a 1.43 x-height margin. Contrast thresholds were normalized with respect to the corresponding local-patch conditions. We found that the normalized threshold elevation due to crowding was reduced by an average of 0.24 log units at the “near” position (where letters appeared smaller) compared to the “far” position (where letters appeared larger). This counterintuitive finding suggests that the smaller perceived size might induce a narrower distribution of spatial attention or reduced positional uncertainty, which in turn reduces crowding.
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