Stimulus uncertainty affects velocity discrimination

Velocity discrimination thresholds were determined for 1 c/deg drifting gratings when uncertainty about the reference velocity was introduced by interleaving stimuli with different reference velocities from trial to trial. When drifting gratings with reference velocities spanning 4 octaves (1-16 deg/sec) were mixed randomly within a series of trials, the velocity discrimination threshold for a 4 deg/sec stimulus increased by more than a factor of 3. The threshold elevation decreased as the range of interleaved velocities was reduced from 4 to approx. 0.75 octaves, below which velocity interleaving had little effect. In contrast, when gratings that spanned a 4-octave range in spatial frequency were interleaved on successive trials, velocity discrimination for 4 deg/sec was essentially unaffected. Our results indicate that the psychophysical mechanisms underlying velocity discrimination are not spatial-frequency specific, but are turned to the velocity or speed of the stimulus.