When one object blocks another from view, as might two papers lying upon a desk, observers immediately infer which object is on top. Such occlusion relations are well-known cues to depth in static images (Ratoosh,
1949), but in dynamic displays, they have historically been considered as a source of error or ambiguity (Wallach,
1953). When occlusion is not properly inferred in displays of contour-defined motion, erroneous or ambiguous perception can result, but when occlusion is properly inferred, it disambiguates a variety of motion displays (e.g., McDermott & Adelson,
2004; Shimojo, Silverman, & Nakayama,
1989).
No study to date, however, has investigated whether occlusion, by itself, can produce a percept of motion. Below, we describe an illusion that strongly suggests that it can.