Optical Illusions Etc...: Four-Stroke Apparent Motion

Description Presented here is a vast gallery of many different forms of Optical Illusions. Whether these optical illusions are conveyed through fractals, photographs, architecture, art, or old fashioned pen and ink, they are bound to remind you that "seeing is not believing."

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Monday

Four-Stroke Apparent Motion

The below clip gives us the illusion of continuous motion. In fact there are only four frames that loop continuously. Two of the frames move forward the other two move backwards.

Copyright George Mather, 2003

George Mather, the creator of this piece, had this to say about this illusion.

This animation involves a repeating cycle of four different frames (hence "four-stroke"). The motorcycle moves forward and then backward again (the scenery moves backward and then forward again), but the backward step is accompanied by a reversal in contrast:

The effect of contrast reversal is to reverse the direction of perceived motion, so instead of appearing to move backwards between frames 2 and 3, and between frames 4 and 1, the motorcycle still appears to move forward. Consequently, the impression is of continuous forward motion. This kind of display was first described by Anstis & Rogers (1986), and is closely related to 'quadrature motion' displays (Carney & Shadlen, 1993).

Go to his site for a more in dept analysis of this and other illusions.

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