Discrimination thresholds for velocity and contrast were measured as a function of (1) the stimulus duration, (2) the reference contrast of the stimuli, (3) the stimulus velocity, and (4) whether the observer knew on which dimension, velocity or contrast, the gratings would differ. Two vertically oriented grating patches were presented centered 2 deg left and right of the fixation point. The stimuli drifted under a stationary envelope to the right at a speed of either 1.25 deg/s (Experiment 1) or 5 deg/s (Experiment 2). The reference contrast was varied over five interleaved staircases from 0.02 to 0.32 in equal logarithmic steps. The results of two different tasks were compared. In the single-judgment task, the subject knew along which dimension the stimuli would change and was asked to judge which stimulus had the higher value along that dimension. In the dual-judgment task, the stimuli could differ in either velocity or contrast but not both. In this task the subject first indicated which dimension differed and second which stimulus had the higher value along that dimension. The dual-/single-judgment threshold ratios remained constant over a wide range of stimulus conditions. The mean value of these ratios, however, significantly exceeds that expected by the vector model presented by Greenlee and Thomas [J. Opt. Soc. Am. A 10, 395 (1993)]. A modification of the model, which assumes that velocity and contrast are not independently coded, appears to be sufficient to account for the observed differences. The results are in line with the known dependency of perceived speed on stimulus contrast.
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