Mind Hacks

Hack #25. See Movement When All Is Still

Aftereffect illusions are caused by how cells represent motion in the brain.

Why, when the train stops, does the platform you are looking at out the window appear to creep backward? The answer tells us something important about the architecture of the visual system and about how, in general, information is represented in the brain.

The phenomenon is the motion aftereffect. Just as when you go from very bright sunlight to the indoors, everything looks dark, or if you are in a very quiet environment, loud noises seem even louder, so continuous motion in a certain direction leaves us with a bias in the other—an aftereffect.

In Action

Watch the video of a waterfall ( http://www.biols.susx.ac.uk/home/George_Mather/Motion/MAE.HTML ; QuickTime) for a minute or so, staring at the same position, then hit pause. You’ll have the illusion of the water flowing upward. It works best with a real waterfall, if you can find one, although pausing at the end is harder, so look at something that isn’t moving instead, like the cliff next to the waterfall.

The effect doesn’t work for just continuous downward motion. Any continuous motion will create an opposite aftereffect; that includes spiral motion, such as in the Flash demo at http://www.at-bristol.org.uk/Optical/AfterEffects_main.htm .

The effect works only if just part of your visual field is moving (like the world seen through the window of a train). It doesn’t occur if everything is moving, which is why, along ...