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When you watch a continuously rotating spiral, the motion detectors in your eyes become adapted to that motion. Then, when you look away, the world seems to move toward or away from you.
Note: For a very simple version of this activity, reduce the size of the disk on a copy machine and then mount it on the flat upper surface of a suitable toy top. You could also try spinning the mounted disk on a pencil point or on a pushpin stuck into a pencil eraser. Whatever you can devise to get the disk spinning safely should be fine.
Start the spiral rotating and stare at its center for about 15 seconds.
Look away from the disk and stare at a nearby person. Notice that the person will seem to be expanding or contracting, as though he or she is rushing toward you or away from you.
If you can, try rotating the spiral in the opposite direction. Now what happens when you look up from the spinning pattern?
Your visual system is sensitive to inward and outward motion. Some nerve cells in the visual cortex fire more when objects move outward from the center of your field of view, and others fire more when objects move inward. When you’re looking at something that’s standing still, the inward and outward channels are in balance with one another; they send equally strong signals to your brain. When you stare at this moving pattern, however, one detector channel adapts and its response is reduced. Then, when you stare at the person, the detector that hasn’t been active sends a stronger signal to your brain than the adapted one does.
If, for example, the spiral seemed to be moving away from you, the person will seem to be moving toward you when you look away from the disk. If you rotate the spiral in the other direction, so that it seems to be moving toward you, the person will then seem to be moving away when you look away.
Next time you’re near a waterfall, try staring at one point of the waterfall for a minute. Then look at a rock or another stationary object to the side of the waterfall. The solid object will seem to flow upward. This apparent motion is due to the adaptation of the channels in your visual system that detect linear upward and downward motion.
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Attribution: Exploratorium Teacher Institute