Motion aftereffect

How it works

The visual cortex contains cells that respond most strongly to movement in one direction. Some fire for upward motion, some for downward, and so on for every direction. When nothing moves, these cells are in balance. The brain reads the balance as “still”¹.

Stare at a waterfall for about a minute and the cells for downward motion fire constantly¹. They adapt and become less responsive. Now look at the rocks beside the water. The downward cells are still tired, so the upward cells win by a small margin. The brain reads the imbalance as slow upward drift. The rocks appear to climb¹.

The effect is strange because the rocks do not seem to go anywhere. They drift, but they stay in place. The brain reports motion without a change of position. Adaptation happens at several levels of the visual cortex³. One of them is an area called MT or V5, which has many cells tuned to direction².

The aftereffect lasts for a few seconds. Longer viewing gives a longer effect⁴. It can be stored: if you close your eyes right after adapting, the aftereffect is still there when you open them⁵.

Try it

1. Search for a video of a spiral that rotates, or a waterfall filmed from a fixed camera.
2. Stare at the centre of the image for 30 to 60 seconds. Do not follow the motion with your eyes.
3. Look at your hand, a wall, or a friend’s face.
4. The still surface bulges, shrinks, or drifts in the opposite direction for a few seconds.

A real waterfall, a passing train, or a moving escalator works too.

Is it a problem?

No. The waterfall illusion is a normal side effect of healthy motion-detecting cells in the visual cortex, not a symptom¹². It fades within seconds and leaves nothing behind. It is not a sign of conditions that cause ongoing false motion or trailing images. See an eye doctor or neurologist if you see things moving, trailing, or shimmering without having just stared at real motion, since that is a different and unrelated symptom.

History and culture

Aristotle mentioned the effect in about 350 BC, using the example of a fast-flowing river¹⁶. Jan Purkyně described it clearly in 1820 after watching a cavalry parade¹. Robert Addams reported it in 1834 after watching the Falls of Foyers in Scotland¹. The name “waterfall illusion” dates from 1880¹.