Correlation between perceived size and depth changes in the Dynamic Ebbinghaus illusion

In the Ebbinghaus illusion, a central disk surrounded by smaller disks appears larger than the same size disk surrounded by larger disks. This illusion surprisingly doubles in magnitude when the stimulus translates while changing continuously (Dynamic Ebbinghaus illusion: Mruczek et al., 2015). Here we test whether the looming motion of the Dynamic Ebbinghaus illusion generates an impression of changing depth that would increase the standard size contrast effect by adding a size-depth scaling. To examine this, we compared the magnitudes of the illusion and perceived depth across three versions of the Ebbinghaus stimulus: a static version (the classic Ebbinghaus illusion), the static version superimposed on a corridor background, and the dynamic version. Participants first nulled any size difference in the central test disk to measure the illusion strength in the three different versions. Participants then made a depth judgment of the whole stimulus comparing the left hand location to the right hand location with the central disk corrected to appear of equal size throughout (using the results of the first experiment). The depth was reported by adjusting the vertical separation of two markers to match the perceived depth as if viewed top-down. There was a positive correlation between the magnitudes of the illusion and the perceived depth. These results suggest that perceived depth, whether from the corridor illusion or the dynamic version adds an additional size change to the effect of the classic, static Ebbinghaus illusion.