The perception of movement and depth in moiré patterns

Moiré patterns can produce striking movement effects and in more complex stimuli can induce vivid stereoscopic depth. The physical rules underlying these phenomena are reviewed and their relationship to psychophysics is discussed. First, it is shown how moirés in 'optical line interference' patterns are created by superimposing periodic visual stimuli, eg gratings, and shifting them relative to each other. When two gratings are presented in this manner, small differences in spatial frequency, orientation, and speed are magnified. This magnification has prompted the use of moiré patterns both in industry and in art where their enhanced sensitivity to misalignment and spatial distortion has been widely exploited. Next, it is demonstrated how enhanced depth in 'stereoscopic interference' patterns is produced by presenting grating stimuli in two (or more) depth planes. The perceived depth effect in the resulting moiré pattern can be elicited similarly by binocular disparity and motion parallax. Finally, it is described how perceived movements occurring in different directions and at different depths are the basis for the perceptual 'irritations' that fascinate observers in complex moiré patterns. The use of moirés for the noninvasive examination of the human retina by aliasing is discussed.