The Wandering Circles: A Flicker-Rate and Contour Dependent Motion Illusion

Our understanding of the visual system can be informed by examining errors in perception. In this vein, we present a novel illusion that we call the Wandering Circles in which stationary circles undergoing contrast polarity reversals (i.e., flicker), when viewed peripherally, appear to move about in a random fashion. Here we report the results of two psychophysical experiments in which participants rated the strength of the perceived illusory motion under varying stimulus conditions. The illusory motion percept was strongest when there was a light/dark alternation at the circle’s edge and when the edge faded smoothly to the background gray (i.e., a circular arrangement of the Craik-O’Brien-Cornsweet Illusion). Additionally, the percept of illusory motion is flicker-rate dependent, appearing when the circles flickered at 9.44Hz and 28.33Hz, and was virtually non-existent at 1.98Hz. The Wandering Circles differ from many other classic motion illusions as the light/dark alternation is perfectly balanced in time and position around the edges of the circle, and thus, there is no net directional local or global motion energy in the stimulus. Furthermore, the direction of the illusory motion does not seem to be in a particular direction. Thus, it appears that the perceived motion may rely on factors internal to the viewer such as top-down influences, asymmetries in luminance and motion perception across the retina, adaptation combined with positional uncertainty due to peripheral viewing, eye movements, and/or low contrast edges.