Object Solidity Biases Motion Perception

The human visual system is endowed with prior expectations about physical regularities it routinely perceives. One basic regularity of our world is that of “object solidity”: Solid objects cannot pass through each other. Is object solidity internalized in the visual system? Here we provide convincing evidence across four behavioral experiments that the adult human visual system uses solidity to compute object motion. Our experiments demonstrate that when viewing ambiguous motion displays compatible with multiple interpretations, the visual system strongly favors interpretations which respect the physical law of solidity over those that violate it. Further demonstrating the robustness of this effect, we discovered that even in the presence of other motion and depth cues, solidity remains robust and continues to bias the perception of motion. Together, our results demonstrate for the first time that the visual system integrates solidity in its computations of object motion.