The Ternus Illusion is an optical illusion that was discovered in the early 20th century by German psychologist Max Wertheimer. It is based on the idea that when two or more objects are presented in rapid succession, they appear to move together as a single object. The illusion is named
The Ternus illusion or Ternus effect is an illusion related to human visual perception involving apparent motion. In a simplified explanation of one form of the illusion, two discs, are shown side by side as the first frame in a sequence of three frames. Next a blank frame is presented for a very short, variable duration. In the final frame, two similar discs are then shown in a shifted position. Depending on various factors including the time intervals between frames as well as spacing and layout, observers perceive either element motion, in which L appears to move to R while C remains stationary or they report experiencing group motion, in which L and C appear to move together to C and R. Both element motion and group motion can be observed in animated examples to the right in Figures 1 and 2.