A visual illusion of apparent movement created by gazing for a period of time at a fixed point in a waterfall and then looking at a stationary object, which appears to move upwards.

The "Waterfall Illusion," technically known as the Motion Aftereffect (MAE), is a powerful visual phenomenon where, after staring at a stimulus moving in one direction for a prolonged period, stationary objects appear to move in the opposite direction. For example, if you watch the downward flow of water in a waterfall for about a minute and then look at the stationary rocks beside it, the rocks will appear to drift upward. This happens because the neurons in your brain that detect downward motion become "fatigued" or habituated. When you shift your gaze to a static scene, the "upward-detecting" neurons, which are still fresh, have a higher relative firing rate than the fatigued downward ones. Your brain interprets this imbalance as actual movement in the opposite direction. It is one of the most studied illusions in neuroscience because it reveals how our sensory systems use "opponent processing" to perceive the world around us.