The "Double Ring Illusion": The physical constraint of solidity shapes visual processing

Who does not like visual illusions? Not only do they offer a wondrous experience to viewers, but they also compellingly reveal the inner workings of our visual system. Out of the myriad of existing illusions, however, rarely are they produced thanks to priors about the physics of our world. Here, we present a novel illusion of this kind — the Double Ring Illusion — which demonstrates that a representation of the physical constraint of solidity (i.e., objects cannot pass through one another) shapes perception. Thus, when viewing ambiguously rotating rings that are compatible with multiple interpretations, the percept is strongly altered by the solidity constraint: Observers predominantly perceive the interpretation respecting solidity rather than the alternative interpretation where solidity is violated. A series of experiments first confirmed that observers reliably experienced this illusion. We then demonstrated that the effects of the illusion influence orthogonal perceptual judgments of object width, thus ruling out decision-level processes as a driver of these effects. Furthermore, we showed that the solidity constraint shapes visual processing even when the stimuli are unambiguous thanks to additional motion and depth information. And finally, we found that the visual system predictably makes use of solidity even in contexts other than the Double Ring Illusion. Together, these results demonstrate the existence of a robust prior for solidity in visual processing, guiding the computations of object motion and interaction.