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Cast Shadow Illusions

Shadows are everywhere, but they usually go unnoticed. We do, however, use their consequences all the time. Figure 1A shows two pictures each with a green square in front of a checkerboard background. The two pictures differ only in the cast shadows of the square. Because of these differences, the right square appears to be further from the background than the left square. By adding motion to the cast shadow, the effect on the perception of depth can be made quite striking. Moving the shadow, leaving the square fixed in the image, results in the square appearing to move in depth (see: http://youtu.be/ig2CV1TLn5A). It is only with scrutiny that one is convinced that the square is not moving at all within the picture frame. In particular, the square is not changing size, as it should if it was indeed moving in depth. Shadows seem to create a paradox for the visual system. While on the one hand they can be quite useful as cues for depth, their unobtrusiveness suggests that there may be good reasons for a visual system to get rid of them.1 Cast shadows can be distinguished from “attached” or “form” shadows (see Mamassian et al., 1998; Castiello, 2001). An attached shadow on an object is the darkened surface that faces away from the light source. A cast shadow on a surface is the dark area created by an object blocking light onto that surface. A cast shadow can also be created when part of an object blocks light onto another part of the same object. For simplicity, in this chapter, the word “shadow” will usually refer to “cast shadow”.