Motion Aftereffect (Waterfall Illusion)

The waterfall illusion

Watch a waterfall for half a minute, keeping your eyes still, 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 see they aren't going anywhere. Then the effect fades and the scene looks normal again.

The demo uses the modern lab version: a rotating spiral. Watch its centre for 30 seconds while it turns; when it stops, the still spiral seems to shrink or swell in the opposite direction. Press the button when the movement disappears and the page times how long your aftereffect lasted and compares it with other visitors'.

Tired motion detectors

The visual cortex contains neurons tuned to motion in particular directions: some respond best to downward motion, others to upward, and so on. When you look at something still, the opposing groups balance 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. The brain reads that imbalance not as stillness but as motion in the opposite direction (Anstis, Verstraten & Mather, 1998).

Why it fades

As soon as the adapting motion stops, the adapted neurons start to recover, and the balance returns. That's why the drift always fades rather than lasting. In general, the longer you watch the moving pattern, the longer the aftereffect lasts afterwards, which fits 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 looking 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, though today 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 the brain's motion detectors, but they need no real motion at all; the motion aftereffect is the after-effect of real motion that has stopped.