"Crowding" in normal and amblyopic vision assessed with Gaussian and Gabor C's.

The purpose of this study was to investigate the extent and specificity of crowding in the normal fovea and periphery, and the central field of amblyopes, using "C"-like patterns. In the first experiment we measured the extent of crowding for C-patterns comprised of Gaussian patches, over a range of target sizes using a four-alternative forced-choice (up, down, left, right) method. We found that the extent of foveal crowding is proportional to target size. In contrast, in normal periphery and in the central field of amblyopes, crowding extends over large spatial distances and is not size dependent. Crowding for our stimuli occurred with both same-polarity and opposite polarity patches. To test whether the extended crowding in amblyopia resulted from a shift in the spatial scale of analysis, we measured crowding with band-limited C-patterns (comprised of Gabor patches) in a gap localization task (2-AFC). With band-limited stimuli, and a task that does not involve judging the orientation of the gap, the amblyopic eyes showed crowding over a longer distance than that of normal observers. We also tested the orientation specificity of crowding by varying the orientation of the flanks. In normal fovea, crowding is orientation specific: in amblyopia it is not. While crowding in normal fovea can be explained by simple pattern masking, crowding seen in normal periphery and amblyopes cannot. Instead we suggest that crowding in amblyopic and peripheral vision is a result of extended pooling at a stage following the stage of feature detection.