Radial motion looks faster

Current models of motion perception depend on unidirectional motion-sensitive mechanisms that provide local inputs for complex pattern motion, such as optic flow. To test the generality of such models, we asked observers to compare the speed of radial gratings with the translational speed of vertical gratings. The speed of the radial gratings was consistently overestimated by 20-60% relative to that of translating gratings that were identical in all other respects. The speed bias was not associated with a general spatial or temporal processing bias, nor with the high relative speed of points about the center of expansion/contraction. The bias increased non-linearly with the size of sectors of the radiating pattern exposed. As the motion of the two patterns was locally identical but judged differently, the apparent speed of both kinds of motion cannot be served by any mechanism, nor described by any model, that is based entirely on local motion signals. We speculate that the greater apparent speed of the radial motion has to do with apparent motion in depth.