Perception of Depth in Rotating Objects: 1. Stereokinesis and the Vertical-Horizontal Illusion

Os viewed under extreme reduction conditions either a rod or an ellipse as it rotated slowly (5 rpm) in the frontoparallel plane. They reported seeing a sequence of percepts of the luminous stimulus starting with veridical rotation, then expansion-contraction, and/or advance-retreat and finally ending with apparent rotation in a plane oblique to O. The percepts were the same with either monocular or binocular regard and with the head tilted 90° to the side. In the latter case, the apparent plane of rotation shifted with the head position showing that the visual field determined the effect. This stereo-kinetic effect was related to the vertical-horizontal illusion and was explained in terms of the asymmetry of the visual field.