If an object swings in the frontal plane and the observer places in front of one eye a light-absorbing filter (any value between 5% and 40%), that object will appear to move along an ellipse. This elliptical movement is virtually horizontal, with one part in front and the other behind the frontal plane. If the filter is placed in front of the other eye, the object will appear to swing along an ellipse but in the opposite direction. One explanation for the perception of depth of the moving object is a hypothesized difference in the latent periods of the two eyes caused by the filter, which induces a reduced level of illumination in one eye. The covered eye presents a position of the moving target, which lags behind that of the uncovered eye. Therefore at any given moment this unilateral delay gives rise to binocular disparity and depth perception (Fig. S13). Syn. Pulfrich effect; Pulfrich phenomenon.