Motion Aftereffect Spiral

How it works

This is the motion aftereffect, one of the oldest and most reliable illusions in vision science. It reveals that your brain does not have a single detector that measures motion directly. Instead, it compares the activity of many populations of neurons, each tuned to a particular direction.

In the visual cortex, especially an area called MT (also known as V5), there are neurons that fire most strongly for motion in one direction: some prefer up, some down, some outward expansion, some inward contraction. When you look at something still, these opposing groups fire at a low, balanced background rate, and the balance reads as zero motion.

While you stare at the spinning spiral, the neurons tuned to its direction fire hard for half a minute. Like any hard-working cell, they adapt: their response weakens, partly through fatigue and partly through active gain control that keeps the system sensitive to change. When the spiral stops, the adapted neurons fall below their normal resting rate, while the neurons tuned to the opposite direction are fresh. The balance tips, and the brain reads the imbalance as motion in the reverse direction.

A spiral is a powerful adapter because its rotation is seen locally as expansion or contraction. That is why the aftereffect transfers so well to faces and hands, which are not spirals at all: the adaptation is about the direction of motion in each patch of the visual field, not about the shape that caused it.

Some curious details support this explanation. The aftereffect is usually limited to the part of the visual field that was adapted, so if you keep your gaze on the dot, only the region the spiral covered will appear to move. It also has a paradoxical feel: things appear to move without changing position, which tells us that the brain's sense of motion and its sense of location are computed partly separately. The effect fades over several seconds as the neurons recover.

History

Aristotle described a version of it, and the Czech scientist Jan Purkinje noted it after watching a cavalry parade. The best-known early report came from Robert Addams in 1834, who stared at the Falls of Foyers in Scotland and then saw the rocks beside the waterfall drift upward, which is why it is often called the waterfall illusion. In 1849 Joseph Plateau used a rotating spiral to produce it, the same design used here.

Written by K.R. for OpticalIllusions.com · Updated October 26, 2026. Illusion artwork © The Wizard Of Sales, LLC. Feel free to share a link; see our terms for reuse.