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Peter Thompson, Stuart Anstis; Retracing our footsteps: A revised theory of the footsteps illusion. Journal of Vision 2005;5(8):929. doi: 10.1167/5.8.929.
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In the footsteps illusion, Anstis (2001, 2003a, b, 2004) has shown that when a grey square drifts steadily across stationary black and white stripes, it appears to stop and start as its contrast varies. A dark grey square has high contrast when passing over a white stripe and appears to speed up. On a black stripe it has low contrast and appears to slow down. The opposite is true for a light grey square. To explain this effect, Anstis appealed to Thompson's finding that low contrast stimuli appear to move more slowly than high contrast stimuli, (Thompson 1976, 1982). However, in the Anstis effect the square can appear to stop moving completely, whilst Thompson's contrast effect rarely exceeds an apparent speed change of 25%. We now report that, if the moving square is made progressively smaller than the width of a background stripe, the illusion is reduced and is finally abolished. We propose that the footsteps illusion involves not only weak motion signals from its low-contrast moving edges, but also spurious signals of stationarity from its partly occluded lateral edges. These interacting signals from different parts of a grey square also generate Zollner-like zigzags in a stationary analogue (the Wenceslas illusion).
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