Ebbinghaus illusion depends more on the retinal than perceived size of surrounding stimuli

A stimulus surrounded by smaller/larger stimuli appears larger/smaller (Ebbinghaus illusion). We examined whether the Ebbinghaus illusion would depend on the retinal or perceived size of the surrounding stimuli. The flash-lag effect, where a flashed stimulus perceptually lags moving stimuli, was used to dissociate the retinal from perceived size of the surrounding stimuli. Two sets of four surrounding disks changed their size smoothly: one with larger disks shrinking, the other with smaller disks expanding. Two identical central disks were presented briefly at various timings relative to the moment when the surrounding disks were physically identical in their size (coincidence time). A significant flash-lag effect was observed for size change (Experiment 1). Participants reported the two central disks being in equal size when they appeared only slightly before the coincidence time. However, this asynchrony was not significantly different from zero and was significantly smaller than the perceptual delay expected from the flash-lag effect (Experiment 2). These results suggest that the Ebbinghaus illusion depends more on the retinal than perceived size of the surrounding stimuli.