Background – The image above is modified just a little from the original “Rotating Snakes” by Akiyoshi Kitaoka. It demonstrates strong (and beautiful) rotation of the “wheels”, occurring in relation to eye movements. On steady fixation the effect vanishes.
As Kitaoka & Ashida (2003) describe, asymmetric luminance steps are required. Presumably appearance of these triggers our 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.
A “Stress Test”? – No!
Repeatedly, I was sent such pictures with the assertion that they comprise a stress test (and some of the people sending me this were deeply worried). And I just found a web page entitled “test online the level of stress” (I will not link to it) which contains these statements “For a normal person, they should all move at a slow pace, barely rotating. The slower the pictures rotate, the better your ability of handling stress: Allegedly, criminals see stress test images moving and spinning around madly, while seniors and children see them still…”
This is utter BS! Don’t get alarmed. For one, the effect depends on eye movements, and these are known to differ markedly between subjects without relating clearly to psychological traits. Further, a few people do not see it at all (could be around 5%, among them a very renowned vision scientist), in spite of appropriate eye movements. There are no actual data showing relations to stress (or age), so don't distress yourself when you see it rotating strongly or not at all.
Kitaoka A, Ashida H (2003) Phenomenal characteristics of the peripheral drift illusion. VISION 15:261–262
Conway BR, Kitaoka A, Yazdanbakhsh A, Pack CC, & Livingstone MS (2005) Neural basis for a powerful static motion illusion. J Neurosci 25:5651–5656