Perceiving visual expansion without optic flow

When an observer moves forward in the environment, the image on his or her retina expands. The rate of this expansion conveys information about the ob server's speed(1) and the time to collision(2-4). Psychophysical(5-7) and p hysiological(8,9) studies have provided abundant evidence that these expans ionary motions are processed by specialized mechanisms in mammalian visual systems. It is commonly assumed that the rate of expansion is estimated fro m the divergence of the optic-flow field (the two-dimensional field of loca l translational velocities)(10-14). But this rate might also be estimated f rom changes in the size (or scale) of image features(15). To determine whet her human vision uses such scale-change information, we have synthesized st ochastic texture stimuli in which the scale of image elements increases gra dually over time, while the optic-flow pattern is random. Here we show, usi ng these stimuli, that observers can estimate expansion rates from scale-ch ange information alone, and that pure scale changes can produce motion afte r-effects. These two findings suggest that the visual system contains mecha nisms that are explicitly sensitive to changes in scale.