Abstract
Our percept of an object's size arises from the integration of multiple sources of visual information including retinal size, perceived distance and relative size. This constructive process is revealed through a number of classic size illusions such as the Mueller-Lyer Illusion, the Ebbinghaus Illusion and others illustrating size constancy. Here we present a novel size illusion that we have named the Binding Ring Illusion that is not easily explained by existing models of size perception. The perceived radius of a circular array of elements is underestimated when superimposed by a circular contour—a binding ring. Here we characterized the stimulus conditions that lead to the illusion. Methods: Using the method of constant stimuli, observers were presented with two arrays (one with a binding ring and one without), and asked to indicate which was perceived to be larger. Results: The results of experiment 1 demonstrate the influence of the binding ring is quite robust and consistent across subjects. In experiments 2 and 3, we investigated the possible role of occlusion and depth ordering of the binding ring. In these latter experiments, the binding ring was either occluded by the array elements or only visible through the interiors of the array elements (as if viewed through a hole). Paradoxically, the results of experiments 2 and 3 suggest that the size distortion depends upon the entire binding ring being visible and superimposed on the array and not upon depth ordering cues, as is the case with other size illusions. Conclusion: The Binding Ring Illusion demonstrates that under certain circumstances perceived size may be computed prior to perceived distance.