Why Do Still Rocks Seem to Rise After You Watch a Waterfall?

After watching water fall steadily for a while, you may look at the stationary rocks beside it and see them drift upwards. This is the motion aftereffect, often called the waterfall illusion. The rocks have not changed position, and the eyes need not actually rotate upwards. The change lies in how the visual system encodes motion direction.

Groups of neurons in the visual pathway respond preferentially to different directions of motion. Prolonged downward movement adapts the pathways responding to that direction. When the view shifts to a stationary surface, direction signals that would normally balance one another are temporarily uneven, so the relative upward signal wins and the brain reports motion that is not present. Saying that the downward-motion neurons have become “tired” is a useful introduction but an incomplete account. Research indicates that motion aftereffects can arise at several stages of visual processing, while their strength and duration depend on the pattern, speed and test image.

The illusion does not simply show that vision is unreliable. It shows that perception recalibrates to the recent environment. What we experience as stillness is not an absence of signals, but a temporary balance among competing motion evidence.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3087115/
https://www.illusionsindex.org/i/waterfall-illusion