Staring at a waterfall can make stationary objects appear to drift upward.

This phenomenon is scientifically known as the Motion Aftereffect (MAE) or the 'waterfall illusion.' It was famously described by Robert Adams in 1834 after he observed the effect at the Fall of Foyers in Scotland. The illusion is a result of neural adaptation in the visual cortex, specifically within the V5 or MT area of the brain.When you watch a waterfall, neurons tuned to downward motion fire continuously. Over time, these specific cells become exhausted and their firing rate decreases. Meanwhile, neurons tuned to the opposite direction, upward motion, maintain their baseline level of activity without being stimulated.When you shift your gaze to a stationary object like a rock, the fatigued downward neurons fire even less than usual. Because the upward-detecting neurons are now firing at a higher relative rate than the tired downward ones, the brain interprets this imbalance as actual upward movement. This sensory trick typically lasts between 10 to 30 seconds before the neurons return to their normal equilibrium.Modern research using functional Magnetic Resonance Imaging (fMRI) has confirmed that this process occurs in the early stages of visual processing. Studies at institutions like the University of California, San Diego, have shown that this adaptation helps the brain stay sensitive to new changes in the environment. By filtering out constant, predictable motion, the visual system remains primed to detect sudden, unexpected movements.