Concurrent measurement of perceived speed and speed discrimination threshold using the method of single stimuli

Velocity matching using the method of Constant Stimuli shows that perceived velocity varies with contrast [Thompson, P. (1982). Perceived rate of movement depends upon contrast. Vision Research, 22, 377-380]. Random contrast jitter would therefore be expected to increase the slopes of psychometric functions, and thus the velocity discrimination threshold. However, McKee, S., Silverman, G., and Nakayama, K. [(1986) Precise velocity discrimination despite random variation in temporal frequency. Vision Research, 26, 609-620] found no effect of contrast jitter on thresholds, using the method of single stimuli. To determine whether this apparent discrepancy is due to the difference in methodology, or to the different ranges of temporal frequencies used in the two studies, we used the method of single stimuli to measure psychometric functions at three different velocities (0.5, 2.0 and 4.0 degrees/s). We found that contrast jitter increased thresholds at low but not at high velocities. Separate analysis of the psychometric functions at each contrast level showed that increases in contrast increased perceived velocity at low standard speeds (0.5 degree/s) but not at high. We conclude that the effect of contrast on perceived speed is real, and not a methodological artefact, but that it is found only at low temporal frequencies.