Stereokinetic effect, kinetic depth effect, and structure from motion

When we move, the flat projection of the three-dimensional world onto our retinae changes; these changes alone can induce a powerful sensation of depth. The effectiveness of this process is well illustrated by the stereokinetic effect and the kinetic depth effect (or structure from motion), which refer to illusions of depth induced by moving two-dimensional stimuli. Stereokinesis studies have classically employed flat curvilinear shapes, such as ellipses and nested circles, rotating about the observer’s line of sight. Structure-from-motion studies, instead, have used flat projections of three-dimensional forms rotating in space. In all these cases, the flat stimuli perceptually transform into three-dimensional bodies that move in depth. Here we review the literature on these two phenomena, that have been studied largely independently of each other, with different equipment and within separate paradigms, despite being related so closely.