Interactive motion aftereffect (waterfall illusion) simulation. Stare at the centre of the rotating spiral for 20 seconds while it adapts your motion-sensitive neurons, then press Stop. The static spiral will appear to move in the opposite direction — demonstrating neural fatigue and opponent-process motion channels in the visual cortex.
This simulation recreates the classic motion aftereffect, often called the waterfall illusion. You stare at the centre of a slowly rotating spiral so that direction-selective neurons in your visual cortex become adapted through prolonged firing. When the spiral is stopped, the now-static pattern appears to drift in the opposite direction. The spiral is drawn pixel-by-pixel from a sinusoid, sin(r·rings·2π/R − θ·arms + phase), so the illusion emerges from genuine neural adaptation rather than any movement on screen.
A rotating Archimedean-style spiral generates continuous inward or outward motion. The brightness of each pixel comes from the sign of a sinusoid combining radius, angle and a moving phase, with the number of arms and ring count controlling the pattern. After about 20 seconds of fixation, motion-sensitive channels fatigue, producing the illusory reverse drift when motion stops.
Press Start, then fix your gaze on the central dot without moving your eyes. The Speed slider (0.5×–4×) sets rotation rate, Arms (1–5) the spiral density, Duration (10–30 s) the adaptation time, and Direction toggles inward or outward. The telemetry panel tracks phase, countdown and adaptation percentage. Press Stop to freeze the pattern and watch the aftereffect.
The effect was described in antiquity and named the waterfall illusion by Robert Addams in 1834 after he stared at the Falls of Foyers in Scotland and saw the nearby rocks appear to flow upward. It is strong evidence that the brain encodes motion direction in dedicated, separately adaptable channels.
It is a visual illusion in which, after watching steady motion in one direction for a while, a stationary scene briefly appears to move in the opposite direction. Because the prolonged motion here is a rotating spiral, stopping it makes the static spiral seem to rotate or expand the other way. It is also known as the waterfall illusion.
Your visual cortex has neurons tuned to specific motion directions. Watching the spiral rotate one way over-stimulates those neurons, so they fatigue and reduce their firing. When the motion stops, the opposing direction channels are momentarily more active, and your brain interprets that imbalance as motion in the reverse direction even though nothing is actually moving.
Speed sets the rotation rate from 0.5× to 4×, Arms changes the spiral density from 1 to 5 arms, and Duration sets how long you adapt, from 10 to 30 seconds. Direction switches between inward (contracting) and outward (expanding) motion, which determines which way the aftereffect appears to flow.
Longer adaptation generally produces a stronger and longer-lasting aftereffect, which is why the default duration is 20 seconds and the slider reaches 30. Keeping your eyes fixed on the central dot is important, because moving your gaze spreads the adaptation across different parts of the retina and weakens the illusion.
It is a genuine perceptual phenomenon rooted in how the brain processes motion, not imagination or eye movement. It can be measured objectively and has been studied for centuries as evidence for opponent, direction-selective motion channels in the visual system. The effect is temporary and fades within a few seconds as the adapted neurons recover.