The Ternus illusion is an illusion of human visual perception involving apparent motion. In one form of the illusion, two dots (let's call them L for left and C for centre) are shown side by side as the first frame of a movie, then two similar dots (let's call them C for centre and R for right) are shown on the second frame. Depending on various factors including the time between the first and second frames, people see either element motion, in which L appears to move to R while C remains stationary, or group motion, in which L and C appear to move together to C and R.
