Strength and variability of the backscroll illusion.

When a walking person is presented in a movie, the background image appears to move in a direction opposite to that of the person's locomotion. This study aimed to quantify the strength of this backscroll illusion and to examine interobserver and intraobserver variability. Stimuli were movie clips that presented a walking person in profile against a background of dynamic grating composed of two vertical sinusoidal gratings moving in opposite directions. Employing a motion-nulling method, we controlled the ratio of luminance contrasts of the component gratings to determine points that canceled the percept of unidirectional motion in the grating background. Results across 50 observers showed that the backscroll illusion disappeared when a luminance contrast of moving grating components consistent with a walker's direction was about twice as high as that for the opposite motion direction. Intraobserver variability was relatively small. However, nulling points for individual observers were more variable under conditions with dynamically moving walkers than conditions presenting only a static picture of a walker. We speculated on the underlying mechanisms of the backscroll illusion in relation to similar phenomena.