Apparent motion and the Pulfrich effect

The Pulfrich pendulum effect, obtained by viewing a moving object with a filter over one eye, was examined with target stimuli in apparent, rather than continuous, motion. The filter-induced depth effect persisted until a certain degree of intermittency in the presentations of the target was reached, and then it broke down. The degree of intermittency that could be tolerated before the depth effect broke down increased with the density of the filter. It could be argued that the filter determined a shift in the pairing of successive inputs to the eyes, such that the target position in the unfiltered eye was fused with the preceding target position in the filtered eye. However, it appears that the shifted-pairing effect cannot account for the depth impression seen when the target intermittency is less than about 30 ms. Below this value of intermittency a filter can produce a depth effect even when the delay it introduces is small in comparison to the intermittency of the input. The depth effect seen with intermittencies less than 30 ms appears to be of the same magnitude as that obtained with stimuli in continuous motion. It is concluded that a filter can cause two different kinds of depth shift with apparently moving stimuli.