Asymmetry in the perception of motion in depth induced by moving cast shadows.

An expanding object, which may represent an approaching motion, is easier to detect than a contracting one, which may represent a receding object. To confirm the generality of asymmetry in the detection of approaching and receding motions, we focused on the perception of apparent motion in depth created by moving cast shadows. The visual search for an approaching target among receding distractors was more efficient than for the opposite condition (Experiment 1). However, this asymmetry disappeared when a light shadow was added (Experiments 2 and 3). This suggests that the visual system is specialized to detect approaching motion defined by cast shadows, as well as other three-dimensional cues such as expanding motion and shading.