motion aftereffect · so1omon.net

Aristotle noticed it watching a river: after staring at flowing water, the rocks on the bank seemed to slide upstream. The same effect works with rotating spirals, drifting gratings, falling snow, or any sustained directional motion.

The moving spiral below will adapt your motion detectors. When it stops, the static image will appear to move in the opposite direction — not because anything is moving, but because the sensors that report stillness have been recalibrated by the preceding motion.

Knowing this does not prevent it.

demonstration

idle

Fixate on the red dot at the center throughout. Press Start to begin the 30-second adaptation.

Keep your eyes on the red dot. Try not to look away during adaptation — the effect is much stronger with steady fixation.

rotation speed 0.9×

what happened

The visual cortex contains neurons tuned to specific directions of motion. When you watch the spiral rotate, the detectors for that direction fire continuously. After ~20–30 seconds, they fatigue — their baseline response rate drops.

The static spiral looks still because motion perception is a balance between opponent detectors. Clockwise and counterclockwise detectors have approximately equal baseline firing rates; the difference is zero; the percept is stillness.

After adaptation, the detectors for the adapting direction are suppressed. The balance tips. The opponent detectors — tuned to the opposite direction — now have a higher relative firing rate. The difference is no longer zero. The percept is motion, even though nothing is moving.

This is the same logic as color afterimages: stare at red, then look at gray, and you see green. The chromatic opponent system is recalibrated by prolonged input. The system reports deviation from its current baseline — not deviation from the truth.

The illusion lasts 5–15 seconds for most people. It is unaffected by knowing about it. The report "it is moving" comes from a layer of processing that doesn't have access to the meta-knowledge that it was adapted.

See also: McCollough effect (color adaptation tied to orientation), graveyard spiral (vestibular adaptation with lethal consequences), entry-578: The Smaller Radius (Mach's question about whether sensation is primary).

the spiral that keeps moving