ON THE PERCEPTION OF ILLUSORY CONTOURS

Illusory contours are invoked by the visual system to account for othe rwise inexplicable gaps in the image. We report three sets of novel ob servations on illusory contours. First, when an illusory square is sup erimposed on a checkerboard pattern there is a considerable enhancemen t of the contours so long as they are exactly coincident with the bord ers of the checks. If the checks are misaligned, on the other hand, th e illusory contours associated with the pacman edges disappear and a n ovel percept emerges: the contours of the checks nearest to the illuso ry square appear enhanced. This result implies that subjective contour s are generated by intermediate-level contour interactions rather than the top-down processes of three-dimensional interpretation. Second, w e find that steady fixation for as little as 4 sec leads to a complete disappearance of the enhanced illusory contours caused, presumably, b y adaptation or ''fatigue'' of cells that signal these contours. Such adaptation occurred even when the illusory contours were rendered invi sible by displaying them on a misaligned checkerboard, suggesting that the adaptation occurs prior to the vetoing of the signal by the check s. Third, we found that illusory contours persist for a surprisingly l ong time (0.3 sec) after the inducing elements have been switched off. These results suggest that the stimuli we have designed (''enhanced i llusory contours'') might provide a novel probe for dissecting differe nt stages involved in the processing of illusory contours and for unde rstanding how the visual system combines different types of contours t o construct object boundaries.