What to observe

Above you see a movie, and something moves, obviously, but doesn't really advance, does it?
By the way: You see a scene in the Swiss Alpes (near Flims) which I took while snowboarding.

Speed can be adjusted by the inter-frame delay in multiples of 17 ms.


The movie consists of altogether four frames. Two frames are consecutive frames from a movie, the next two frames repeat the first two, but in the negative. The reason for the seemingly continuous forward motion is that our motion detectors are sign-dependent. Simultanous inversion of contrast and motion direction does not reverse perceived motion direction. This effect is also known as “four stroke motion”.

This version won George Mather the second place in the Illusion Contest 2005.



Anstis S (1970) Phi movement as a subtraction process. Vision Res 10:1411–1430

Anstis SM, Rogers BJ (1975) Illusory reversals of visual depth and movement during changes in contrast. Vision Res 15:957–961

Anstis SM, Rogers BJ (1986) Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception. Perception 15:627–640

George Mather (several motion demos)


Created: 2003-Jul-08

Impressum·Legal notice   |  
Last update 2018-01-11 by Michael Bach