Pulfrich Effect

The Pulfrich effect is a psychophysical percept in which the lateral motion of an object moving across a person's field of view is interpreted by the visual cortex as having an added depth component, caused by a relative difference in the timing of the visual signal arriving from the two eyes. The effect is most often demonstrated by placing a dark filter over one eye while watching a swinging pendulum, which then appears to trace an elliptical, three-dimensional path rather than a flat, side-to-side one, but it can also occur naturally in conditions such as cataracts and optic neuritis, which slow the signal from the affected eye in the same way a dark filter does. The German physicist Carl Pulfrich, who was the brother-in-law of the physicist Heinrich Hertz, first described the effect in 1922, and it has since been put to practical use as a basis for producing three-dimensional depth effects in television, film and video games.