Velocity Threshold for Relative and Uniform Motion

Abstract

We measured velocity thresholds for relative and uniform motion as functions of spatial frequency and contrast. Stimuli were two horizontal bands on top of each other, both filled with vertical sinusoidal gratings. The gratings drifted either to the right or to the left, in opposite directions in the relative-motion condition but in the same direction in the uniform-motion condition. Observers had to report the direction of motion, and the velocity was varied until a velocity threshold was obtained. The results showed that the shapes of the threshold function plotted against spatial frequency are quite different for uniform and relative motion. The threshold for relative motion had a minimum at around 5 cycles deg−1, whereas the threshold for uniform motion had no such minimum, at least at higher contrasts (10% or higher). The difference was unclear for lower-contrast stimuli, however. The threshold profile as a function of contrast was also different between relative and uniform motion. Although the threshold decreased with increasing contrast in both cases, this dependence saturated at around 10% contrast for uniform motion, while it continued up to the highest contrast (85%) for relative motion. This difference held for all the spatial frequencies examined (from 0.75 to 12.1 cycles deg−1). The results suggest that the detection mechanisms for relative motion and uniform motion are different.