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Rumi Hisakata, Ikuya Murakami; Effects of eccentricity and retinal illuminance on the rotating snakes illusion. Journal of Vision 2007;7(9):983. doi: 10.1167/7.9.983.
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© ARVO (1962-2015); The Authors (2016-present)
Kitaoka recently reported a novel dramatic illusion, the Rotating Snakes (Kitaoka & Ashida, 2003), in which a stationary pattern appears to rotate constantly. The pattern is comprised of luminance defined micropatterns, each having four adjacent regions of different luminances, black, dark gray, white, and light gray. The direction of the illusory motion is perceived in this order. However, it remains a fundamental question as to why and how such an illusion occurs. Here we introduce a cancellation method to quantify this motion illusion, thereby determining the strength of illusion as the physical rotation velocity in the opposite direction that just cancels the illusion. 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 the stepwise luminance patterns mentioned above and was presented in the left visual field constantly. The right eye was used. The subject's task was to indicate in which direction, clockwise or counterclockwise, the stimulus ring appeared to rotate. With increasing eccentricity, the cancellation velocity increased, and was saturated at about 12 degrees. In the next experiment, we examined the effect of retinal illuminance. The stimulus was always presented at 12 degrees eccentricity. The illuminance was varied by putting neutral density filters in front of the subject's eye. Interestingly, the cancellation velocity decreased as the retinal illuminance was decreased. From these results, we argue that (1) the rotating snakes illusion maximally occurs at a certain spatial scale in relation to early retinotopic organization, and that (2) it is mediated by an early mechanism that is sensitive to illuminance level. More specifically, we discuss that this illusion is a result of some early process having different temporal response properties depending on photopic levels, presumably in the form of different temporal impulse response functions.
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