Stopped visual motion [26]

Abstract

WHILE undertaking another experiment, we discovered a surprising and dramatic visual phenomenon. We were observing two gratings of high contrast and different spatial frequency (see Fig. 1 inset) while both gratings were rotating at a rate of 1 revolution per min. Both gratings were easily visible at the viewing distances used. The coarse grating was seen to revolve as expected; however, the fine grating appeared to revolve very slowly. This phenomenon has been shown to over 100 people1 and all were startled by the observation. The individual reaction was to assert that the two gratings must be rotating at different physical speeds. This disbelief was readily corrected by removing the two gratings from their turntables and replacing each of them in the position of the other. Another entertaining variation of the observation is to observe the fine grating at the distance where it is perceived as a uniform disk; naturally, no movement can be seen at this distance. If one now advances slowly towards the grating it is initially seen as stationary; as one approaches closer it is seen to rotate progressively faster. At a particular distance it seems to rotate faster than the coarse grating. We have quantified these causal observations and show here that the perception of slow rotation movement is related to the spatial frequency and contrast of the pattern which is rotated. © 1979 Nature Publishing Group.