Abstract
When a test disk is surrounded by smaller disks, it appears larger than when it is surrounded by larger disks (Ebbinghaus Illusion). Mruczek et al. (2015) reported that the strength of this illusion was almost doubled when the stimulus was in motion as the surround disks changed from large to small then reversed direction as the surround disks expanded again. This stimulus has deep similarities to the expanding-contracting square illusion (Anstis & Cavanagh, 2017) where a background texture repeatedly expands and contracts while a test square was flashed when the background was at its smallest size and then again when the background was at its largest size. The apparent size of the test square was twice as large when flashed on the small background compared to when flashed on the large background. We previously showed that this expanding-contracting square illusion is driven principally by the motion after each flash, not before (Takao et al., 2021). Here we examine whether the Dynamic Ebbinghaus version also shows this strong dependence on motion after versus before the test. In the experiment, six surround disks expanded and contracted continuously while the whole configuration moved back and forth diagonally, being smallest in size at one end and largest at the other. Rather than leaving the central test disk on continuously, it flashed briefly at each reversal point. The trajectory was broken into half cycles and the test disk flashed only at one end or the other to evaluate the influence of motion before versus after. Contrary to the results for the expanding-contracting square illusion, no differences were found in the Dynamic Ebbinghaus illusion strength for motion after vs motion before. This study indicates that the two illusions, despite their strong similarities, have very different causes.