Background
In the image above the strong (and beautiful) rotation of the “wheels” occurs in relation to eye movements. On steady fixation the effect vanishes.
For an explanatory hypothesis, view the animation on the right. You may wish to press the ‘Stop’ button after a while … it gets on one’s nerves ;-). See also Backus & Oruc 2004 for their explanation.
For an explanatory hypothesis, view the animation on the right. You may wish to press the ‘Stop’ button after a while … it gets on one’s nerves ;-). See also Backus & Oruc 2004 for their explanation.
What to observe
Even when fixating steadily, illusory motion occurs on appearance/ disappearance
Illusion does not depend on colour
Rotation direction depends on the polarity of the luminance steps (I arranged these to evoke a “gearbox effect” here, by mirroring the images appropriately)
Strength of illusion depends strongly on background luminance. The background luminance slowly ramps from 0% to 100% automatically, or you can use the slider. The illusionary rotation is strongest for 50% gray (exact value will depend on your monitor ‘gamma’).
Even when fixating steadily, illusory motion occurs on appearance/ disappearance
Illusion does not depend on colour
Rotation direction depends on the polarity of the luminance steps (I arranged these to evoke a “gearbox effect” here, by mirroring the images appropriately)
Strength of illusion depends strongly on background luminance. The background luminance slowly ramps from 0% to 100% automatically, or you can use the slider. The illusionary rotation is strongest for 50% gray (exact value will depend on your monitor ‘gamma’).
Comment
As Kitaoka & Ashida (2003) describe, asymmetric luminance steps are required. Presumably appearance of these triggers motion detectors (as in the animation on the right). I assume that the actual mechanism is quite similar to the “Rotating Spokes” illusion, where asymmetric luminance steps occur as well. Gregory & Heard (1983) were the first to describe that asymmetric luminance steps cause illusory movement.
As Kitaoka & Ashida (2003) describe, asymmetric luminance steps are required. Presumably appearance of these triggers motion detectors (as in the animation on the right). I assume that the actual mechanism is quite similar to the “Rotating Spokes” illusion, where asymmetric luminance steps occur as well. Gregory & Heard (1983) were the first to describe that asymmetric luminance steps cause illusory movement.
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