Figure 10. Rolling illusion:

(A) shows a 2D circle on a line, translating from left to right. Our percept of the translating circle, however, is rolling clockwise. To perceive the rolling based on the sensory information, local motion units that direct tangential to the contour (B) are required. (C) and (D) show local motion selective units from motion energy (left) and feature tracking (right). In both cases, the vectors are inconsistent with the required vectors. (E): Average proportion of rolling percepts (8 observers). The color of the bar shows the different speed of stimulus (blue: 0.6 deg/sec, orange: 6.0 deg/sec, and green: 60.0 deg/sec). The shape of the stimulus is indicated on the x-axis. The proportion of rolling percepts increased with speed and decreased when features were added to the rings. (F): Rolling illusion and rotational symmetry. The non-rigidity (rolling) percepts increases with the order of rotational symmetry from left to right. (G): The relationship between rolling illusion and the strength of feature. As the number of corners increase from left to right, it gets harder to extract the corners and accordingly, the percept of rolling increases. (H): Model prediction with rotational symmetry and average strength of features versus average proportion of rolling percepts for slow (left), moderate (middle), and fast (right) speeds ($R^2=0.90,0.94$, and 0.79).