SIMULTANEOUS DISCRIMINATION OF VELOCITY AND CONTRAST OF DRIFTING GRATINGS

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 k new on which dimension, velocity or contrast, the gratings would diffe r. Two vertically oriented grating patches were presented centered 2 d eg left and right of the fixation point. The stimuli drifted under a s tationary envelope to the right at a speed of either 1.25 deg/s (Exper iment 1) or 5 deg/s (Experiment 2). The reference contrast was varied over five interleaved staircases from 0.02 to 0.32 in equal logarithmi c steps. The results of two different tasks were compared. In the sing le-judgment task, the subject knew along which dimension the stimuli w ould change and was asked to judge which stimulus had the higher value along that dimension. In the dual-judgment task, the stimuli could di ffer in either velocity or contrast but not both. In this task the sub ject first indicated which dimension differed and second which stimulu s had the higher value along that dimension. The dual-/single-judgment threshold ratios remained constant over a wide range of stimulus cond itions. The mean value of these ratios, however, significantly exceeds that expected by the vector model presented by Greenlee and Thomas [J . Opt. Sec. Am. A 10, 395 (1993)]. A modification of the model, which assumes that velocity and contrast are not independently coded, appear s to be sufficient to account for the observed differences. The result s are in line with the known dependency of perceived speed on stimulus contrast. (C) 1998 Optical Society of America.