Effects of temporal frequency on speed discrimination and perceived speed

We examined the effects of temporal frequency of moving stimuli on speed discrimination and perceived speed in human vision, using psychophysical methods. A speed-comparing paradigm displaying two drifting sinusoidal luminance gratings simultaneously was adopted in our experiments. In experimental 1, human vision was found to be most sensitive to the speeds of moving gratings with temporal frequency of about 12 Hz, when speed was above 3.4 deg/sec. It suggests that a mechanism tuned to 12 Hz was involved in speed perception. In experiment 2, stimuli of subjectively equivalent speed (SSES) corresponding to various standard stimuli were determined in the frequency domain. The results showed that the distribution of SSES located along a straight line representing the physical speed of the standard stimuli at low standard speed in the frequency domain, which is considered as depart from the speed-tuning property. However, the distributions began to depart from the speed-tuning property and to approach a temporal-frequency-tuning property at higher speeds above 17 deg/sec. Our conclusion is that perceived speed and speed discrimination have speed-tuning property with normal speeds, but have a non-speed-tuning property with high speeds above 17 deg/sec; a mechanism tuned to 12 Hz is used by speed-tuned mechanisms and used directly by speed perception concurrently.