APPARENT SPEED OF SAMPLED MOTION

Perceived speed was measured for stimuli moving unidirectionally in apparent motion with different sampling steps. The stimuli were displayed at successive locations for very brief durations (ontime = 1 msec). The basic result is an elevation of apparent speed produced by increasing the sampling step. This speed-up effect is maximal at low speeds (2 deg/sec), then progressively decreases with higher speeds until it disappears at medium velocities (8 deg/sec). In addition, the speed-up observed at low speeds declines when the ontime is gradually increased from 1 msec to larger values, the largest one corresponding to ''staircase motion''. These results are consistent with models assuming that speed-encoding is based on an antagonistic comparison of the activity in two broadly tuned temporal filters (low-pass and band-pass). The high temporal frequencies introduced by motion-sampling would activate the band-pass filter relatively more and would thus produce an overestimation of apparent speed.