Roget's ‘Palisade’ Illusion

from “Visual Phenomena & Optical Illusions”

What to do & see

1. We certainly all agree that on the right side a wheel is seen rotating. 
2. On pressing the “show” button, a brown picket fence (a palisade) will fade in. When it is fully opaque, it largely occludes the rotating wheel, but one can still perceive its rotation.  
3. On pressing the “move” button, the palisade begins to, well, move. 
4. What do the spokes of wheel look like now? A little distorted, similar to the icon on the top right?
   Yes, it is somewhat difficult to perceive this, due to the timing distortions (“anti-aliasing”) by computer presentation. 
5. When fading out the palisade, the distortion vanishes as soon as a little of the wheel becomes visible through the semi-transparent palisade. 
6. The effect changes drastically when changing the relative speeds. When you inverse rotation direction or palisade movement (by going to negative numbers), the “bends” move to the top of the wheel.
7. If you observe the wheel for a while an then stop, you see opposite rotation. That’s the motion after effect
8. When you inverse rotation direction or palisade movement (by going to negative numbers), the “bends” move to the top of the wheel.

Comment

Peter Mark Roget (1779–1869), author of the famous Thesaurus, first described this illusion, hence known as the “Roget” or “Palisade” illusion.

His own explanation is not convincing from our current understanding. Essentially it is a sampling proplem. Carpenter detailed a geometrical derivation in 1868, which was recently formalized by Jim Hunt (2003). The intersections of gaps and spokes over time indeed form curves. Due to the afterimage (“persistance of vision”) our perception connects these and we perceive the illusionary shape.

Note added 2009-04: Interestingly, this effect can also occur in “finish photos” of bike racing, where the spatio-temporal intricacies of the camera shutter “bend the spikes” – and quite strongly too. Furthermore, the Palisade illusion probably also causes an interesting shape distortion in archery, when capturing the string release with a photo just at the right time. Beautiful things can be learned from the web!

References

I am grateful to James L Hunt for pointing me to this illusion and providing material.

Roget PM (1825) Explanation of an Optical Deception in the Appearance of the Spokes of a Wheel Seen through Vertical Apertures. Phil Trans Royal Soc London 115:131–140

Carpenter WB (1868) On the Zoetrope and its antecedents. The Student and Intellectual Observer 1:427–444

Hunt JL (2003) The Roget Illusion, the Anorthoscope and the persistence of vision. Am J Phys 71:774–777

Wade NJ, Heller D (2003) Visual motion illusions, eye movements, and the search for objectivity. J Hist Neurosci 12:376–395