What to see
The wheel above rotates 9 times per second by 25° to the right – just 5° short of a perfect fit, since the disks are spaced every 30°. Since our brain associates each new position with the closest preceding one, we see a counter-clockwise rotation. This is the classic wagon wheel effect, as seen in the cinema in spoke wheels and explained on the preceding page. Nothing new so far – but wait: What is this yellow tail, and how can it convince us that the wheel really is rotating clockwise?
What to do
• You can use the speed slider to convince yourself of the basic wagon wheel effect. You can also manipulate step speed (preset at 9 Hz) with the stepper at the right.
• Press “Reset” (to ensure we're at 25° steps and standard speed), and from the popup control (bottom centre) choose “One segment coloured”. Now one (curved) “spoke” is special, and the motion interpretation will not be to nearest neighbour, but to the same colour – and the rightwards rotation is obvious, at least for the yellow segment. This is a systematic version of the wobble I had alluded to on the preceding page.
• Now for a really weird or beautiful effect: Choose “Alternating colours” from the popup. Now the segments are alternatingly coloured in a rather bright green or a darkish magenta. Rather a jumble for my eyes. Now move the slider that has appeared on the right, adjusting background luminance. At the upper 3rd, roughly, the green disks and the background have the same luminance and only differ in hue – equiluminance (or isoluminance). Interesting things happen there, of interest here is that the motion system is (nearly) colour blind, and so the green changing disk positions do not trigger motion perception any more, while the magenta disks, being much darker, trigger motion detectors and a coherent rightwards rotation can be seen again, as above with the yellow segment. Related phenomena occur for the bottom 3rd background slider position.
This beautiful extension of the basic Wagon Wheel Effect was awarded 3rd prize in the 2012 Illusion of the Year contest. My demonstration above is similar to theirs (with kind permission by Arthur Shapiro), but is based on a different animation technology.
Wikipedia entry on the Wagon Wheel Effect