Seeing circles: what limits shape perception?

Humans are remarkably adept at judging shapes and discriminating forms. For ms and shapes are initially sampled by discrete localized visual filters (o l receptive fields) in 'early' stages of visual processing. However, more c omplex higher level filters which integrate ol pool information from many l ocal filters may be needed to discern shapes. In order to understand the me chanisms that limit shape perception we asked observers to detect distortio ns in the shape of briefly presented circles constructed out of samples (Ga bor patches, which are well matched to the early visual filters), and varie d the radius of the circle, and the number and orientation of the samples. Our results show that shape perception is determined by two factors. the pr imary determinant is the separation between the samples, however, the orien tation of the samples can modulate performance. At small separations, perfo rmance is best when the samples are aligned with the global shape, poorer w hen they are orthogonal, and intermediate when they ale all horizontal or v ertical. At larger separations these contextual differences disappear; howe ver at all separations, performance is reduced when the orientations of the samples are mixed (i.e. each sample is randomly either aligned or orthogon al, or randomly either horizontal or vertical.). These results suggest an i mportant role for sample separation in shape perception for sampled shapes and suggest that the mechanisms involved in feature binding may modulate th e responses of the mechanisms underlying shape perception. (C) 2000 Elsevie r Science Ltd. All rights reserved.