Abstract
Visual objects impose fragmented contours on our retina in almost every visual scene. Yet, in most cases, humans perceive continuous surfaces and coherent objects. The two most common cases of fragmented contours are of modal (illusory contours) and amodal (partially occluded contours) completion. Despite their different phenomenology, it has been strongly argued that the same local interpolation mechanism underlies both types of perceptual completion ('Identity Hypothesis'). Some recent studies provide evidence for distinct processes mediating the two types of completion, including differences in dependency of interpolation on geometrical factors and in their developmental rates, as well as suggestions for different involvement of cortical areas. We tested these suggestions directly exploring the involvement of lower visual areas in the interpolation of modal versus amodal completion. Subjects were asked to judge the curvature of an interpolated shape (fat or skinny) varying in degrees of support ratio (i.e., ratio of physically presented contours to interpolated contours). Stimuli were presented stereoscoply either as a whole to one eye or segregated between the eyes. Binocular disadvantage (weaker interpolation in the two eyes presentations) indicates involvement of lower visual areas in the process. The results replicated the stronger effect of support ratio on modal interpolation, and more critically, revealed a binocular disadvantage for modal but not for amodal stimuli. The results indicate two different mechanisms with modal completion involving a more local interpolation process comprising lower visual areas, while amodal completion is presumably mediated by higher visual input.
Meeting abstract presented at VSS 2017