Amodal completion improves perception of illusory contours defined by motion

BARRAZA J.F.; CHEN V.J.

Sarsota, Florida, USA

Congreso; 5th Annual Meeting of the Vision Sciences Society; 2005

Vision Sciences Society

Purpose. Illusory contours can be produced by moving the modulation of luminance in a random dot pattern. We study here how amodal completion interacts with contours defined by motion in a vernier acuity task. Methods. The stimulus consisted of a rectangular patch containing a random dot pattern in which no dots ever moved relative to the background. The luminance of the dots was modulated to define a vertical imaginary bar. The patch was horizontally divided in two equal parts. We used a vernier acuity test between the upper and the lower bar to evaluate how sharp the contour was perceived in each condition. To do this, we manipulated the phase of the modulation in both parts. Three stimulus configurations were used. For the first one, the area surrounding the patch was set to a luminance of 19 cd/m2, and no amodal completion was used. For the second configuration, amodal completion was achieved by adding to the first configuration two rectangles to the ends of the bars (outside the patch area) with the same luminance as the dots bars. For the third configuration, the area surrounding the patch was set to the same luminance as the black dots, and amodal completion was achieved as in the previous configuration. Importantly, for all the three configurations, the patch background was set to a luminance of 19 cd/m2, and the contrast between dots defining the bars (white) and the rest of the dots (black) was 1. Results. Vernier thresholds were measured for four different dot densities and for a wide range of speeds. Results show that thresholds obtained with the first configuration (no amodal completion) are higher than those obtained with amodal completion. The effect is less notorious as speed and density increases. No differences were obtained between second and third configurations. Conclusions. Results show that amodal completion improves the perception of illusory contours defined by motion. This suggests that there is an interaction between these two boundary completion modes.