Color Contrast Polarity of Boundary Edge Affects Amodal and Modal Surface Completion

Two loci on a natural surface are more likely to have the same than different colors. Does the visual system capitalize on this ecological regularity to integrate partially occluded fragments into a larger common surface? In particular, when the geometrical relationship between the boundary edges of two image fragments are appropriate for amodal surface completion between them, do these edges need to have the same color contrast polarity (CP)? To answer this, we investigated whether the color CP of equiluminous, spatially aligned rectangles affects the amodal surface completion between them, and the consequent formation of the modal surface that occludes the amodal surface. We found using three divergent psychophysical tasks that separated rectangles with the same color CP (red/red or green/green), rather than with opposite color CP (red/green or green/red), tend to integrate into a partially occluded surface. First, observers subjectively reported the perceived illusory contour (modal surface) as stronger when the separated rectangles had the same color CP. Second, observers were more efficient in discriminating the orientation of modally completed ellipses, formed from aligned rectangles with the same color CP. Orientation discrimination was worse when the aligned rectangles had opposite color CP, which negated the formation of the modally completed ellipse. Third, when motion signals were added to the edges of the separated rectangles with the same color CP, the rectangles were more likely to be perceptually integrated and seen to move in synchrony (global motion). In all experiments, we also varied the luminance contrast of the yellow background relative to the equiluminous rectangles. We found the contribution of color CP to surface completion remains substantial with either the brighter or darker background. This indicates that even when the aligned rectangles carry the same luminance CP information, color CP information can exert an effect on the surface completion process.