Retinal inhomogeneity and motion in depth.

The relation between retinal inhomogeneity and motion in depth was explored by means of a circularly symmetric but nonuniform stimulus pattern, designed to fit the inhomogeneity of the subject's retina. Radial motion of the quasi-sinusoidal rings of this target evokes an illusion of motion in depth: forward if the frontal-plane motion is centrifugal, backward if it is centripetal. Like the Plateau spiral, this stimulus produces strong motion aftereffects (MAE's) over a wide range of spatial and temporal frequencies. Over a much more limited frequency range, the forward MAE is significantly more adaptable than the backward MAE. This forward effect was found to occur only at very low spatial frequencies (approximately 0.05-0.5 cycles/deg at the fovea) and velocities near 20 deg/sec. The effect was destroyed by decentering the target or by varying its homogeneity so that it no longer matched the retina. These results suggest a hard-wired collision-avoidance system.