Motion aftereffect

How it works

The visual cortex contains cells that respond to movement in one direction only. 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 30 seconds 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 position¹. The cells involved are in an area of the visual cortex called MT or V5².

The aftereffect lasts for a few seconds. Longer viewing gives a longer effect. It can be stored: if you close your eyes right after adapting, the aftereffect is still there when you open them¹.

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.

History and culture

Aristotle mentioned the effect in about 350 BC, after looking at a flowing river¹. Robert Addams described the modern version in 1834 after watching the Falls of Foyers in Scotland, which is why it is still called the waterfall illusion¹.