The effects of eccentricity and retinal illuminance on the illusory motion seen in a stationary luminance gradient.

Kitaoka recently reported a novel illusion named the Rotating Snakes [Kitaoka, A., & Ashida, H. (2003). Phenomenal characteristics of the peripheral drift illusion. Vision, 15, 261-262], in which a stationary pattern appears to rotate constantly. In the first experiment, we attempted to quantify the anecdote that this illusion is better perceived in the periphery. The stimulus was a ring composed of stepwise luminance patterns and was presented in the left visual field. With increasing eccentricity up to 10-14deg, the cancellation velocity required to establish perceptual stationarity increased. In the next experiment, we examined the effect of retinal illuminance. Interestingly, the cancellation velocity decreased as retinal illuminance was decreased. We also estimated the human temporal impulse response at some retinal illuminances by using the double-pulse method to confirm that the shape of the impulse response actually changes from biphasic to monophasic, which indicates that the transient processing system has weaker activities at lower illuminances. We conclude that some transient temporal processing system is necessary for the illusion.