What to do – Fixate the blue cross in the centre.
What to observe – While the shrinking/expanding goes on, note that the rings subtly rotate relative to each other. But they are not rotating at all!
What else to do & observe – Press the ‘stop’ button. Then move your head towards and away from the screen while keeping your gaze on the blue cross. Again, illusory rotation can be seen, possibly more compelling compared to the animation, because apart from you nothing really moves here. If you have a mobile computer (e.g. tablet), try rotating the screen – you will see a strong zoom effet on the rings (thanks, Tomas).
Comment – Two factors are at play here: (1) The luminance profile of the local elements triggers motion detectors also at 45° to the veridical orientation through an interaction of direction and motion responsiveness. (2) A secondary Gestalt effect causes a ‘whole’ percept of ring rotation.
The version I show above is based on Pierre Bayerl’s variant, which is a little stronger than the original (right). The next page will demonstrate effects of eye movements.
Gurnsey R, Sally SL, Potechin C, Mancini S (2002) Optimising the Pinna-Brelstaff illusion. Perception 31:1275–1280 [PubMed]
Pierre Bayerl has an even more striking version.