The effect of contrast change on the motion-in-depth perception

Contrast is a cue for depth perception; the farther away an object is, the less contrast appears in the surface plane. What happens if the contrast of an object varies in time? Do we perceive the object moves in depth? We examined whether a contrast change induces motion-in-depth perception. The stimulus was a circular sine wave grating of 1s duration whose phase was shifted by various angles every 250ms. The contrast of the grating either decreased, remained constant, or increased. The subjects judged whether the stimulus appeared to move towards them, or away from them. When the contrast of the stimulus decreased (increased) the number of 'receding' ('looming') responses increased. The results suggest that a contrast change produces motion-in-depth perception