Waterfall Illusion

The waterfall illusion is a specific, historically important example of the motion aftereffect, in which a person who has stared for a period of time at continuously falling water, such as a waterfall, then looks at a nearby stationary surface, such as the surrounding rocks, and perceives that stationary surface as appearing to drift slowly upward, opposite to the direction of the water's actual motion. The effect arises because neurons in the visual system that specialize in detecting motion in one direction reduce their activity after being driven continuously by real motion in that direction, so that once the true motion stops, the relative balance of activity across motion-detecting neurons shifts and produces an illusory sense of movement in the opposite direction. The ancient Greek philosopher Aristotle was the first to record an illusion of this general kind around 350 BC, and the physiologist Jan Evangelista Purkyne gave the first clear directional description of the effect in 1820 after watching a passing cavalry parade. The Scottish naturalist Robert Addams documented the waterfall version of the illusion specifically in 1834, after viewing the Falls of Foyers in Scotland, and the English psychologist Silvanus Thompson coined the term waterfall illusion in 1880; Gustav Adolf Wohlgemuth later produced what is considered the most comprehensive single early study of the phenomenon in 1911. A related version of the same underlying effect, the spiral aftereffect, produces an illusory sense of expansion or contraction after a viewer watches a rotating spiral pattern.