Precise velocity discrimination despite random variations in temporal frequency and contrast.

Velocity discrimination is not affected by random changes in contrast or temporal frequency. Observers judged the relative velocity of a moving sinusoidal grating when target contrast was varied randomly from trial-to-trial over the range from 5 to 82%. The Weber fraction for the random mixture of interspersed contrast levels was about 0.06, comparable to velocity discrimination for targets presented at a fixed contrast. In a parallel experiment, the spatial frequency of the target was changed randomly from trial-to-trial, a procedure which produced concomitant random changes in the nominal temporal frequency. These variations had little effect on the velocity increment threshold; random changes in temporal frequency ranging from 2.25 to 8.25 Hz increased the Weber fraction from 0.05 to 0.07. Under identical experimental conditions, velocity discrimination was generally more precise than the discrimination of differences in temporal frequency, particularly when temporal frequency thresholds were measured with counterphase gratings. Our results indicate that velocity discrimination depends on velocity.