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Ikuya Murakami, Akiyoshi Kitaoka, Hiroshi Ashida; Artificial image oscillation enhances the rotating snakes illusion. Journal of Vision 2006;6(6):551. doi: 10.1167/6.6.551.
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© ARVO (1962-2015); The Authors (2016-present)
A stationary pattern with asymmetrical luminance gradients can appear to move. We hypothesized that this effect, also known as the “rotating snakes” illusion, is driven by retinal image jitters due to eye movements. Recently we have reported a positive inter-subject correlation between fixation instability and illusion strength (Murakami et al., SfN 2004). Here we show evidence for their causal link by comparing the amplitude of artificial image oscillation and illusion strength. The stimulus contained asymmetrical luminance gradients characterized by the sum of the first and third harmonic sinusoids in specific phase relationship. A radial grating of this waveform was presented on the monitor. By actually rotating the stimulus about the central fixation spot, the physical rotation velocity just to cancel illusory rotation was determined. In addition, the stimulus as a whole was randomly jittering in position, simulating image oscillation due to small eye movements. It was found that the cancellation velocity at the point of subjective stationarity increased as the simulated fixation instability increased. Thus, the influence of retinal-image instability on illusion strength we previously found in inter-subject data was also observed within a single observer. We also tested whether the illusion occurs in a stabilized retinal image. In a total darkness, a printed illusory figure was flashed by strobe illumination. The subject subsequently experienced the afterimage of the figure, but reported no illusory rotation. Based on these findings, we conclude that the rotating snakes illusion is caused by retinal image motions due to eye movements.
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