SPATIAL AND TEMPORAL PROPERTIES OF ILLUSORY CONTOURS AND AMODAL BOUNDARY COMPLETION

Spatial and temporal properties of illusory contours and amodal comple tion were investigated using a shape discrimination task Performance w as characterized as accuracy of angular discrimination of the inducing figures (''pacmen'') in a two-alternative forced choice (2AFC) paradi gm. First, we compared performance when four ''pacmen'' were organized into Kanizsa-like figures (squares and small deformations of squares) which produced the percept of illusory contours (ICs), with performan ce obtained with all four ''pacmen'' facing in the same direction, whe n no illusory contours were seen. Then, we found that it was possible to interfere with boundary completion and degrade performance with mas king lines placed between the inducers of a Kanizsa figure. From these experiments we concluded that performance in the shape discrimination task depended on boundary completion. Next, the dependence of contour -dependent performance on the spatial scale of the figures was examine d. Threshold angular discrimination was approximately scale-invariant and subjects were able to integrate visual information across gaps as large as 13 deg of visual angle. Performance in the shape recognition task for illusory and amodally completed figures was also measured. Si milar accuracy was obtained either when the boundaries were modally or amodally completed. Finally, we used shape discrimination in conjunct ion with backward masking to explore the dynamics of boundary completi on. Two different phases of the boundary completion process were obser ved. The first phase was revealed when the inducers were locally maske d, and took approximate to 117 msec. A second phase lasted an addition al 140-200 msec after the inducers were masked. Copyright (C) 1996 Els evier Science Ltd.