Pinna-Brelstaff Illusion
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.
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).
Sources
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
Created: 2003-09-20
Last update: 2013-10-04