Title: Rotating snakes and the failure of motion mechanisms to
compensate for early adaptation to luminance
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.
Abstract:"Rotating Snakes" (A. Kitaoka, 2003) is a static image,
containing a repeating pattern of white, yellow, black, and blue, that evokes a striking illusion of motion
(
http://www.ritsumei.ac.jp/~akitaoka/saishin-e.html).
The artist correctly classified the illusion as a variant of existing illusions, that repeat a dark to light gradient. One account explained motion in the latter as arising from differential retinal processing times for bright and dim image regions (Faubert & Herbert, 1999). However, this account is not consistent with the observation that after fixation, apparent rotation slows to a stop over several seconds.
Slowing to a stop implies a saturating adaptive process. We show that three known visual mechanisms can predict the illusion: (1) nonlinear retinal adaptation to luminance, in which the light response and dark response adapt most rapidly at the lightest and darkest parts of the image, respectively; followed (optionally) by (2) contrast normalization; and then by (3) a standard motion detector (e.g. Reichardt, 1961; Adelson & Bergen, 1985; Johnston, McOwan & Buxton, 1992) operating at an appropriate scale.
This general framework predicts opposing motions from dark and light adaptation in the standard gradient illusion; this could underlie the finding by Fraser & Wilcox (1979) that different people see different directions of rotation.
In the Rotating Snakes illusion, dark and light adaptation induce motion in the same direction, which makes the illusion more compelling. Inspired by "Rotating Snakes", we created a new gradient illusion, with alternating gradients from gray to white and gray to black, that is likewise more compelling than the standard gradient illusion.
Corroboration for the role of luminance adaptation is that after a
flashed exposure to the standard gradient illusion, a uniform white field
appears to have a short-lived, fast illusory motion, whereas a black field has weak motion in the opposite direction.
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’).
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.
Sources
Backus & Oruc 2004 VSS presentation abstract + background material
Kitaoka A, Ashida H (2003) Phenomenal characteristics of the peripheral drift illusion. VISION 15:261–262
Gregory RL, Heard PF (1983) Visual dissociations of movement, position, and stereo depth: Some phenomenal phenomena. Quarterly Journal of Experimental Psychology 35A:217–237
English
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The Rotating Snakes Illusion
MMOA honorary predator 
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