Ternus Illusion

The Ternus illusion is an illusion of human visual perception involving apparent motion, in which the same underlying visual stimulus can be seen in one of two different ways depending on timing: either as element motion, in which a single disc appears to jump across a row of otherwise stationary discs, or as group motion, in which the entire row of discs appears to shift together as one unit, a form of bistable perception in which the same display supports two genuinely different perceived outcomes. The Gestalt psychologist Joseph Ternus identified and described the phenomenon in 1926 and again in 1938, building on earlier related work published by Pikler in 1917, and Ternus referred to the effect as the problem of phenomenal identity. The illusion is produced by presenting three brief visual frames in sequence, separated from one another by a blank gap called an interstimulus interval, and the length of that gap determines which kind of motion an observer perceives: a short gap of under about fifty milliseconds tends to produce element motion, while a longer gap of fifty milliseconds or more tends to produce group motion instead. Researchers consider the Ternus illusion valuable for vision research because the type of movement perceived does not follow directly from the physical, objective properties of the display itself, but instead depends on how the visual system actively processes and organizes that information over time.