A rotating spokes wheel defined only by disparity cues appears stationnary when fixating the center of rotation. With peripheral fixation, one can infer the wheel’s rotation by tracking single spokes. – While there exist cleary stereo motion detectors, stereo rotation detectors are either missing or inhibited by the presence of a stationary texture.
The interesting thing about the Pinball Wizard illusion is that it breaks the ‘rules’. Whilst the classic Rubin Vase illusion demonstrates how we automatically segregate foreground and background in an image, in this illusion a single image is seen acting simultaneously as both, giving rise to an illusory sense of rotation.
Using visual cues to create an impression of depth and carefully chosen colour values, a static screen is combined with an animation of horizontally traversing spheres. The screen appears simultaneously as background and as foreground surface on the spheres – inducing a sense of rotation as the spheres move.
You will see a rotating wheel that pulsates each time it aligns with two stationary shapes. You may also see that, at the same time, the inner stationary shape wiggles. The pulsations seem to be caused by color assimilation, and the other effects by ambiguous figure-ground segregation.
Read more about the illusion and possible explanations at Peter van der Helm’s website
For another interactive version of this Illusion, see Michael Bach’s “Optical Illusions & Visual Phenomena” website.
Kaleidoscopic motion and velocity illusions Peter A. van der Helm Vision Research, 2007. 47:460–465
When sets of disks with tangential greylevel gradients are arranged in concentric circles (see image above, most observers perceive these disks moving around the centre, similar to Kitaoka’s ‘snake illusion’. This motion illusion is enhanced for large-scale and bright images and depends to a large extent to dynamic changes in the stimulus such as elicited by involuntary eye movements or blinks – fixating the centre of the pattern does abolish the illusion, whereas scanning the picture the motion sensation. A reliably effective version of this illusion, which does not require eye movements (i.e. persists when observers fixate the target in the centre of the image), can be generated by modulating the background luminance of the array of disks (see attached animated gif file ‘spin_disks.gif’). This stimulus offers the opportunity of studying this motion illusion – the percept of spinning disks in the absence of any physical displacement – in a highly controlled manner in psychophysical and physiological experiments, because it is not depending on involuntary eye movements or eye blinks. Work in preparation (Zanker 2005) will demonstrate how this illusion can be explained in terms of a two-dimensional motion detector network (2DMD, cf. Zanker & Walker, Naturwissenschaften 91, 149 – 156, 2004).