Abstract
One basic physical law is that of “object solidity”: Solid objects cannot pass through each other. Here we discovered a new visual illusion showing that this physical constraint is automatically integrated into processing of ambiguous motion patterns. When two structure-from-motion rings are presented separately side-by-side, they create an unstable percept, appearing to move in 180° or 360°, clockwise or counterclockwise rotations. However, if the two rings overlap (e.g. their centers are one radius’ distance apart), they appear to move in stable 180° rotations, bouncing back and forth – as if they “avoid” traversing each other. Remarkably, if one ring is gapped such that the other ring can pass through the gaps, the unstable percept is restored. You can experience this yourself: https://osf.io/zm8dx/wiki/home/?view_only=658452ac87324473addcbc298e590d82. We conducted an online experiment which confirmed this effect. In each trial, participants were shown one of the three displays (separated, overlapping or gapped rings), and indicated what they were seeing by choosing from four unambiguous displays (180° counter-rotation, 180° co-rotation, 360° counter-rotation and 360° co-rotation). They also gave a confidence rating after each trial. The results showed that when viewing separated rings, participants’ responded at chance. When viewing overlapping (full) rings, they predominantly chose 180° co-rotation (45% of the time, vs. around 19% for the other options). For gapped rings, they preferred 180° co-rotation (34% of the time, vs. around 22% for the other options). Importantly, this preference was weaker than with overlapping rings. Moreover, when viewing overlapping rings, people were more confident answering 180° co-rotation than other options. No difference in confidence was found between options for the separated and gapped conditions. These results suggest that when viewing ambiguous displays that are compatible with multiple interpretations, the visual system integrates solidity to compute object motion, disallowing the interpretations in which solidity is violated.