Asymmetric Effects of the Ebbinghaus Illusion on Depth Judgments

The Ebbinghaus illusion, also known as Titchner Circles, is a well- known perceptual illusion affecting the perceived size of a disc enclosed by an annulus of either larger or smaller discs. Though many have found highly consistent results with regard to the effect of the illusion on size perception, there have been mixed results when studying its effect on action-based tasks. In this paper, we present a study utilizing a head-worn virtual environment to examine the effect of the Ebbinghaus illusion on depth judgments as measured using a blind-reaching task. We found that participants’ size judgments were symmetrically affected by the classic "large annulus" and "small annulus" configurations, but their distance judgments were asymmetrically affected. Large annulus configurations had no significant effect on distance judgments while small annulus configurations resulted in significant underestimation of target distances. Despite this asymmetry, both configurations resulted in response times of similar magnitude that were significantly longer than those of the non-illusory control condition.