Why does staring at a river make you dizzy?

The full narration of the video.

You stand on a bridge and stare down at the river for a dozen seconds or so. Then the bridge seems to slide slowly upstream, and you grab the railing. The water moves. The bridge doesn't. Neither do you. So why are you the one who feels dizzy?

It's not that you're timid. Your brain is having an argument. To tell whether you're moving, the brain listens to two messengers. The first sits deep inside your ear: a few tiny loops of tube filled with fluid.

When you start, stop or turn, the fluid sloshes, and it sends a message. But it only reports speeding up, slowing down and turning. At a steady speed, it says nothing. You know this from a smooth train ride: once it's cruising, close your eyes and you can't tell if it's moving or parked.

So the brain leans on the second messenger: your eyes. When a big chunk of what you see slides one way, the brain guesses: I must be moving the other way. You've felt it at a station: your train is still, the one next to it pulls out, and you feel yourself rolling backward.

Back to the bridge. The river is that other train. One difference: at the station, your train might really be moving. On the bridge, the ground under you never moves at all. But the water fills your view, flowing one way, nonstop.

Your eyes say: you're drifting upstream. Your ears say: nothing's changed. Your feet say: we're planted. Three messengers disagree, the brain can't decide, and you feel dizzy. Stranger still, the brain tries to correct a drift that isn't happening, and your body actually sways.

In 1974, a British experiment put toddlers who had just learned to walk inside a little room, then gently moved the walls toward them. The floor never moved. Yet many of them sat down hard, backward. There's one more layer: the higher you stand, the farther away the nearest still thing is.

On flat ground, if you sway a little, your eyes catch it at once. Up high, the far bank barely shifts, so your sway goes unnoticed and you feel unsteady. A survey of over three thousand people in Germany found that nearly three in ten get dizzy or weak-kneed up high. Staring at water also leaves an aftertaste. In 1834, a man watched a waterfall in Scotland for a while, then looked at the rocks beside it, and the rocks seemed to creep upward.

The brain cells that track downward motion get tired and go quiet. Look at something still, and the upward side wins out. So that dizzy spell by the water isn't something wrong with your body. Your brain mistook the moving water for you moving. Next time, look at a tree on the bank and hold the railing. Give your eyes something that stays still.

The last time a bridge seemed to move under you, which river were you looking at?