Abstract
I presented the “freezing rotation illusion” at the 2006 “Best Visual Illusion of the Year Contest”. When a foreground object (center) is continuously rotating while its surround is rotating sinusoidally back and forth, the rotation of the center is perceived as periodically slowing down. The slow-down occurs when the center rotates in the same direction as the surround. It is seen best, when the center's rotational speed is below the surround speed. If the speed of the center is close to speed of the swaying surround, a different percept reminiscent of motion capture may arise: the center sticks to the surround throughout the half-period, when center and surround are rotating in the same direction. The motion capture effect is stronger with small translucent objects and eccentric fixation. The roles of surround and center cannot be exchanged: an oscillating center does not alter the perceived speed of a continuously turning surround.
I measured the effects of surround rotation and center rotations on the rotational speed perception of center and surround. Test stimuli where composed of a central disk and an equal-sized annular surround painted with random dot patterns. Probe stimuli were similar to the test stimuli: the random dot pattern of the surrounding annulus was replaced with a uniformly gray; in control experiments, the central disk was gray. To minimize motion after-effects seen in pilot experiments, I adapted a stair-case method to my experimental paradigm allowing for short stimulus presentation times. Subjects had to compare the rotational speeds of corresponding stimulus elements. In all four healthy subjects, the changes of rotational speed induced by the surround (range of averages: 2.0 °/s to 15.5 °/s) were significantly higher (p [[lt]] 0.005) than the changes induced by the center (range of averages: 0.0 °/s to 1.3°/s). The results confirm the above observations.