ILLUSORY-CONTOUR FORMATION AFFECTED BY LUMINANCE CONTRAST POLARITY

We report a new type of illusory contour (Illusory-O) whose formation is contingent upon the contrast polarity of its juxtaposed inducing el ements being similar, ie both elements must either be positive or nega tive in contrast sign. To test the hypothesis that this contingency is primarily dictated by factors that determine amodal surface completio n (occlusion) between the inducing elements we conducted a series of e xperiments employing known spatial properties of the amodal completion mechanism, to show that spatial conditions unfavorable to occlusion l ead to a concurrent weakening of the Illusory-O formation. For instanc e, we found that when the juxtaposed inducing elements (solid rectangl es) were spatially misaligned, or when their spatial separation increa sed, our observers rated the perception of the Illusory-O as reduced. We also showed that, in addition to using solid-form inducing elements , the Illusory-O can be induced by line terminals, as long as these li nes respect the requirements of the amodal completion mechanism such a s similar contrast polarity and spatial alignment. Then we demonstrate d that the role of the amodal completion mechanism is not limited to o ur particular arrangement of inducing elements by showing that the for mation of the illusory Necker cube also relies on similar contrast pol arity. Finally, to explain why some illusory contours like the Illusor y-O are dependent on contrast polarity while others (eg Kanizsa square ) are not, we propose that the key rests upon the visual system's pres umption of occlusion. That is, in forming the illusory contour, if the visual system infers that it is a byproduct of the inducing elements being occluded, then having inducing elements of similar contrast pola rity becomes a prerequisite. This assumption can be traced to the occu rrence in the real world where partially occluded objects usually have visible parts (on both ends) with similar contrast polarity. Along th is line of thinking, we suggest a plausible neural circuitry that may be implemented to form both contrast polarity sensitive and insensitiv e types of illusory contours.