The role of contrast polarity in perceptual closure.

Perceptual closure is one of the principles by which the visual system groups disjointed contour segments belonging to a single object. Recently it has been reported that grouping based on perceptual closure operates only upon contour elements of a consistent contrast sign and is eliminated when closed contours contain contrast reversing segments [Elder, J. & Zucker, S. (1994) A measure of closure. Vision Research, 22, 3361-3369]. The present study re-examines the effect of contrast polarity reversals on perceptual closure with special emphasis on differentiating the effect of the presence of contrast polarity reversals from the effect of their placement along the bounding contour. Results show that perceptual closure exhibits a strong dependence on the distribution of contrast polarity reversals: closed configurations containing intra-figural contrast reversals can be processed rapidly when contrast sign does not change at the corners of two-dimensional shapes.