Object solidity disambiguates ambiguous motion

The human visual system is endowed with 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. Surprisingly, not only is there no convincing evidence that solidity is embedded in automatic visual processes, the visual system has been shown to tolerate obvious violations of this physical constraint (Ames, 1951; Leslie, 1988; Wilson & Robinson, 1986). Here we provide novel evidence that the adult human visual system does in fact use solidity to guide perception – in particular interpretations of ambiguous motion. When viewing displays compatible with multiple interpretations, participants consistently favored interpretations which did not violate object solidity over those that did (for a demo, see www.daweibai.com/solidity_ambiguous_motion/demo_sep.html). Specifically, two ambiguously rotating rings that partially overlapped were predominantly perceived as moving in 180° rotation (which respects solidity), instead of 360° rotation (where the rings traverse each other). This preference diminished or disappeared if the rings were separated or holed, and could not be overridden by a contradictory disambiguating cue – 360° rotating shadows. Further demonstrating the robustness of this effect, we discovered that a linearly moving circle was more likely perceived as moving circularly in depth when a vertical bar was introduced in its path. This effect persisted even if the circle varied in size during its motion (as would a circling ball). Together, our results demonstrate that the visual system integrates solidity in its computations of ambiguous motions, preferring the interpretations respecting solidity.