Visual perception of rigidity of solid shape.

Organisms react to objective properties of bodies in their visual field instead of to the perpetually changing retinal images of those bodies. We show how such a faculty can be mechanized. The organism synthesizes an internal model of the external object, that is a bundle of expectations of how the visual input will transform in response to the organism's exploratory movements. We deduce the necessary structure of the internal model and we show how the organism can extract this structure from the invariant features of the sensory input transformations. With this internal model it is possible to predict the subsequent aspects (contours) of the visual object as the spatial relations of organism and object change.