What to observe – While the shrinking/expanding goes on, note that the rings subtly rotate.
What else to do & observe – Press the ‘stop’ button when the figure is large. Then move your head towards and away from the screen. Again, illusory rotation can be seen, possibly more compelling compared to the animation.
Comment – Two factors are at play here: (1) The luminance profile of the lozenges triggers motion detectors also 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.
Pinna B, Brelstaff GJ (2000) A new visual illusion of relative motion. Vision Res 40:2091–2096 [PubMed]
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.
Michael Morgan was the first –I think– to create an expanding/contracting movie of this phenomenon