It's time for another bracing reminder that your brain is a screw-up and your eyes are liars. This particular illusion is known as the Motion Aftereffect Illusion. It's pretty simple. Look at enough motion steadily and your brain will get tired and punk you for fun. You can even do it with a real waterfall. Find out how.
This visual illusion was first noted by Aristotle, so it has an illustrious history. He noticed that, after looking at a moving stream fixedly for some time, if he looked at stationary rocks beside it, they appeared to be floating upstream. The Motion Aftereffect has puzzled psychologists and neuroscientists ever since.
The facts, as far as we can tell, are this. When something moves in our visual field, retinal cells inform the brain of that fact. They first inform the brain vigorously, but after many seconds of the same motion over and over, the rate at which they fire slackens. After the moving stimulus is taken away, the cells signaling motion in that direction are still fatigued. They fire below even baseline levels. However, cells firing at baseline levels indicating motion in other directions are still on the go, and so false motion appears in the opposite direction.
That's one theory, at least. But not all theories a physiological. It appears that the effect of the illusion doesn't entirely match the timing of the firing and recovery period of cells. It's thought that part of the illusion, then, is just the brain adapts to that kind of motion, and then has to drop that adaptation when the motion is removed.
People are still looking into (eh? eh?) this visual illusion. In the meantime, check out the video and see if the still picture of the stream moves for you.
Via UC San Diego.