  Why do moving objects appear rigid when projected retinal images are deformed
nonrigidly? We used rotating rigid objects that can appear rigid or non-rigid
to test whether shape features contribute to rigidity perception. When two
circular rings were rigidly linked at an angle and jointly rotated at moderate
speeds, observers reported that the rings wobbled and were not linked rigidly
but rigid rotation was reported at slow speeds. When gaps, paint or vertices
were added, the rings appeared rigidly rotating even at moderate speeds. At
high speeds, all configurations appeared non-rigid. Salient features thus
contribute to rigidity at slow and moderate speeds, but not at high speeds.
Simulated responses of arrays of motion-energy cells showed that motion flow
vectors are predominantly orthogonal to the contours of the rings, not parallel
to the rotation direction. A convolutional neural network trained to
distinguish flow patterns for wobbling versus rotation, gave a high probability
of wobbling for the motion-energy flows. However, the CNN gave high
probabilities of rotation for motion flows generated by tracking features with
arrays of MT pattern-motion cells and corner detectors. In addition, circular
rings can appear to spin and roll despite the absence of any sensory evidence,
and this illusion is prevented by vertices, gaps, and painted segments, showing
the effects of rotational symmetry and shape. Combining CNN outputs that give
greater weight to motion energy at fast speeds and to feature tracking at slow,
with the shape-based priors for wobbling and rolling, explained rigid and
nonrigid percepts across shapes and speeds (R2=0.95). The results demonstrate
how cooperation and competition between different neuronal classes lead to
specific states of visual perception and to transitions between the states.
