Orientation anisotropy in the Ternus phenomenon.

The Ternus display is an ambiguous, multi-element, apparent-movement animation which can be perceived in two alternative organizations. One of these involves the simultaneous movement of several elements and is called group movement. The other involves the movement of one element only and is called element movement. The present study examined the influence of retinal eccentricity and organizational orientation on the percentage of group-movement reports obtained across a range of interstimulus intervals. Six participants observed 48 variations of the Ternus display over 100 replications and judged the type of movement seen on each trial. Analysis showed that retinal eccentricity had no effect on the appearance of the display. However, compared with horizontally organized elements, obliquely organized and vertically organized elements yielded progressively more group movement across all but the shortest interstimulus intervals. These results represent an orientational anisotropy in the processing of Ternus apparent movement.