Reverse Spoke Illusion

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Reverse Spokes Illusion - click to see the animation

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The impression I get after pressing the Start button is that of rotating wheel defined by circular sectors whose spokes rotate in the opposite direction. Michael Bach on whose site I stumbled on this illusion claims that on closer inspections the spokes move only very little and then slowly move back. The latter phenomenon is harder to observe, but the circular motion is very persistent. You may keep cursor on one of the spokes and still see other spokes move. Even in the presence of dots placed on the spokes that do not appear to take part in the illusion, the spokes seem to be rotating.

Thickening the spokes or increasing their number seems to weaken the illusion. With a large number of spokes they no longer appear to move but instead brighten up over the passing dark part of the rotating wheel.

No doubt the explanation of the illusion lies in the effect the contrast has on our perception.

References

M. Bach, Reverse Spoke Illusion

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