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.
- Pattern disk (see Assembly section below)
- Scissors (not shown)
- A sheet of 8 1/2 x 11 (A4) white card stock (if card stock is not available or printer won't accept it, plain white paper can be used, but is slightly less satisfactory and durable; alternatively white copy paper can be used and then mounted on posterboard)
- Black marking pen (not shown)
- Variable-speed electric drill (works well because it can be reversed)
- Machine screw (e.g., #10-24, 3 in or 4 in length) plus two washers (e.g., SAE 10) and a wingnut (e.g., #10-24) to fit the machine screw, or you can use double-sided tape to attach your disk to your rotator
- Click on the pattern below to enlarge the image, then print on white card stock or plain white paper. You can reduce or enlarge the pattern disk if you like.
- Cut out the disk. If your printer does not reproduce good solid blacks, fill in the black areas with a black marking pen.
- Poke a hole in the center of the pattern disk and mount it on the machine screw so that it is between the two washers and held in place by the wingnut. Insert the screw into the drill and tighten the chuck to hold it in place.
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.