How Melting Hand works

Your hand never moved. For a few seconds, the part of your brain that sees motion was out of balance.

Cells that see motion

Near the back of your brain is an area called MT. It is packed with nerve cells that each watch one small patch of what you see. Each cell fires most for one direction of motion in its patch, such as left, right, up or down. Your brain compares them to work out what is moving.

Left: while you stare, the stripes flow in toward the dot. Right: afterwards, your still hand seems to flow out and grow.

Tired cells

The spiral only turned. But its curved stripes seemed to slide in toward the dot, all the way around it. So in every patch, the cells for “toward the middle” fired nonstop for 25 seconds, and they got tired.

Then you looked at your hand. The tired cells stayed quieter than usual, while the cells for the opposite direction, away from the middle, were fresh. Away from the middle won in every patch, so your still hand seemed to swell. After a few seconds the tired cells recover, and the hand settles down.

The waterfall illusion

Scientists call this the motion aftereffect. In 1834, Robert Addams stared at a waterfall in Scotland, then at the rocks beside it. The rocks seemed to drift upward. Try it on a friend: after their stare, have them look at your face.