A dynamic Ponzo illusion: induced direction of rotation in depth.

Observers viewed parallel projections of a five-rung gate rotating in depth about its vertical center rung and judged the direction of apparent rotation. The background of a Ponzo illusion superimposed upon these parallel motion projections affected the direction of apparent rotation, with that end of the gate approaching the apex of the background appearing to recede from the viewer. This result is consistent with the thesis that the static Ponzo illusion is, at least in part, an illusion of depth.