Mind Hacks

Hack #27. Show Motion Without Anything Moving

Find out why static pictures can make up a moving image on your TV screen.

The motion aftereffect [[Hack #25]] shows that motion is computed in your brain separately from location. For instance, becoming accustomed to the moving surface of a waterfall causes you to see stationary surfaces as moving the other way, although they’re quite still. In theory, motion can be calculated from position and time information, but that’s not how your brain does it—there’s a specialized brain region for detecting motion directly. Since location and motion are perceived separately, this can lead to some odd illusions, the motion aftereffect chief among them: you get the illusion of motion without anything actually changing position.

The motion aftereffect relies on an initial moving scene to set it up, but we can go one better and get an impression of movement when there’s been no actual thing present, moving or otherwise. The effect is apparent motion, and even if you haven’t heard of it, you’ll have experienced it.

Look at two pictures one after the other, very rapidly, showing objects in slightly different positions. Get the timing right, and your brain fills in the gap: You get an illusion of the objects in the first picture moving smoothly to their position in the second. There’s no single, moving object out there in the world, but your brain’s filling in of the assumed path of movement gives you that impression.

Sound familiar? It should; it’s the ...