Motion Aftereffect (Waterfall Illusion)

The waterfall illusion

Watch a waterfall for half a minute, keeping your eyes still, and then look at the rocks beside it. For a few seconds the rocks seem to creep upwards, against the direction of the falling water, even though you can plainly see they aren't going anywhere. Then the effect fades and the scene looks normal again.

The demo uses the modern lab version, which is a rotating spiral. Press "Start (30 seconds)" and keep your eyes on its centre while it turns. When it stops, the still spiral will seem to shrink or swell in the opposite direction. Press "It's stopped moving" as soon as the movement disappears, and the page will time how long your aftereffect lasted and compare it with other visitors'.

Tired motion detectors

Your visual cortex contains neurons tuned to motion in particular directions: some respond best to downward motion, others to upward motion, and so on. When you look at something still, the opposing groups balance each other out and the overall signal says "no motion".

Watching steady motion in one direction makes the neurons tuned to that direction adapt. Their response weakens with prolonged stimulation, much as you stop noticing a smell after a while. When you then look at something still, the adapted "downward" neurons respond less than normal while the "upward" ones respond as usual, and your brain reads that imbalance as motion in the opposite direction (Anstis, Verstraten & Mather, 1998).

Why it fades

As soon as the motion you've been watching stops, the adapted neurons begin to recover and the balance returns. That's why the drift always fades away instead of lasting. In general, the longer you watch the moving pattern, the longer the aftereffect lasts afterwards, which fits nicely with the idea of a temporary imbalance that takes time to wear off (Mather, Verstraten & Anstis, 1998).

Known since Aristotle

The motion aftereffect was noticed long before anyone knew what a neuron was. Aristotle described how, after you've looked at a fast-flowing river, things that are still seem to move. It entered modern science in 1834, when the Scottish scientist Robert Addams described watching the Falls of Foyers near Loch Ness and then seeing the rocks beside them appear to move upwards (Addams, 1834). That's where the nickname "waterfall illusion" comes from, although these days it's studied with spirals and moving stripes.

Because its strength and duration can be measured, the aftereffect has been one of the most useful tools for studying how the brain codes motion. The rotating snakes and the peripheral drift illusion also fool your brain's motion detectors, but they need no real motion at all. The motion aftereffect is different, because it's what real motion leaves behind once it has stopped.