Apparent speed and speed sensitivity during adaptation to motion

Adaptation, a change in response to a sustained stimulus, can be demonstrated in motion perception by velocity after effects-changes in the apparent speed of a moving pattern following adaptation. We measured changes in the apparent speed of sinusoidal gratings drifting at 4 or 7.5 deg/s during 30 s of adaptation followed by 30 s of recovery. The apparent speed of the patterns fell to approximately half the unadapted apparent speed, and the time constants of adaptation were much faster (5 s) than for recovery (22 s). Part of the loss of apparent speed (approximately 12%) was related to a loss of apparent contrast with adaptation. Sensitivity to speed increments and speed decrements increased during adaptation and was well described by a Weber fraction based on apparent speed. The results suggest that adaptation to motion, like light adaptation, may serve to improve an observer's sensitivity to the prevailing environment. (C) 1999 Optical Society of America [S0740-3232(99)00612-2].