Reverse Phi Illusion

from Michael’s Visual Phenomena & Optical Illusions


What to observe

Above you see a movie, and something moves, obviously, but doesn’t really advance, does it?

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


The movie consists of altogether 4 frames. The first 2 frames are consecutive frames from a movie, the next 2 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”. The disconcerting effect when watching this arises from a conflict: (1) our motion detectors indicate continous motion, but (2) feature tracking indicates a forward-backward jump.

You can slowly step through the 4 frames using the Step+/Step– buttons.

[By the way: The scene is in the Swiss Alpes (near Flims/Laax) while snowboarding.]

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