When should you not believe what your eyes and ears are telling you? How do amusement ride designers create the illusions that make you think you're doing something when you're not?

Insights

Have you ever tried one of those motion simulator rides at an amusement park? How do the designers manage to fool you into thinking you're on an incredible journey?

For the answer, look at how your own eyes play tricks on you. Anthropologists believe humans developed peripheral vision to survive in the wild-we needed to see predators out of the corners of our eyes. Designers of simulators and virtual reality rides make use of this to create illusions. When the lights go off, your seat moves in conjunction with peripheral images projected around you.

Ride designers also take advantage of our ability to sense the distance of objects as we move. Our eyes perceive nearby objects moving more than objects in the distance. This visual cue to depth is called motion parallax. Sixty years ago, Disney Studios developed a camera to make cartoon characters look real. They placed several layers of celluloid drawings at different distances from the camera lens. When the layer of objects in front of a character was moved to the right and the layer behind to the left, the viewer's eyes sensed the characters moving in three dimensions. Today, designers use motion parallax in rides.

Another visual trick involves persistence of vision. Your brain retains an image for about one-tenth of a second after the image is gone. If you see 10 still pictures per second, your brain will tell you that you're seeing moving pictures. When you watch a movie, you're seeing at least 24 individual pictures per second.

Even your ears can be fooled. In your inner ear, three semicircular canals contain fluid and tiny hairs connected to nerve endings of the auditory nerve. Rapid movement causes the fluid to move back and forth through these canals, bending the hairs. This triggers the nerve endings, sending electrical signals to the brain that demand you move your eyes and head to adjust to rotating movements.

Some nerve endings are very sensitive to gravity, helping the brain know what postion your body is in. On a ride that uses hydraulic lifts to move your seat or that spins your body, your inner ear adjusts to provide a sense of orientation and movement. After you've stopped moving, the Coriolis illusion causes you to feel like you're still spinning for awhile. That's why it's hard to walk-your brain is trying to adjust to the spinning or abrupt movements rather than the ground.

Connections

1. What makes your favorite ride scary?
2. Nerves in your ears that are sensitive to gravity send messages to your brain about your body's position. Do you think astronauts in space would feel better floating or attached to a corner of the spacecraft?

Key Words

celluloid old-style film made from camphor and pyroxylin
Coriolis illusion an illusion caused by the apparent spinning of a body
hydraulic lift equipment that creates movement in rides with pressure from liquids forced through tubes
motion parallax ability to sense how far things are from us as we move
peripheral vision what we see at the edges of our field of vision
simulator device or ride that creates the illusion of an environment
virtual reality illusion of reality created by computers and other technology

Resources

1. Adams, J. (1991) The American amusement park industry: A history of technology and thrills. Boston: Twayne.
2. Funston, S. & Ingram, J. (1994) It's all in your brain. New York: Grosset and Dunlap.
3. Heweston, S. (1994) Eye magic. New York: Western Publishing Company.
4. Koenig, D. (1994) Mouse tales: A behind-the-ears look at Disneyland. Irvine: Bonaventure Press.
5. Lefkon, W. (Ed.) (1993) Birnbaum's Walt Disney World for kids by kids. New York: Hyperion and Hearst Business Publications.
6. Salzsider, J. (1994, Feb) Exposing the bathtub Coriolis myth. The Physics Teacher, p. 107.
7. The surprise behind the rides. (1994, Sept) Discover, pp. 44-52.

Additional resources

1. 3M Learning Software:What's the secret? CD-ROMs for Macintosh or Windows. (800) 336-2481.
2. NEWTON'S APPLE Shows 907 (amusement parks) and 1113 (virtual reality). GPN: (800) 228-4630. Or call your local PBS station to find out when it will be rerun.
3. Scholastic: Magic School Bus explores the human body. CD-ROM for Windows. (800) 426-9400.

Main Activity

Your inner eyeball is lined with a light-sensitive membrane called the retina, which is connected to the brain by the optic nerve. The retina has lots of cells-120 million rods and 8 million cones. The periphery of the retina contains rods and does not see images clearly but sees motion and where things are. The fovea of the retina contains mostly cones, and you use it to see what something is. Try this activity to see how these two areas of your retina differ.

Materials

• partner
  
• chair
  
• several objects of different colors

1. Ask your partner to sit on the chair and look straight ahead. You should stand behind the chair.
   
2. Raise two fingers and, starting from behind your friend's head, slowly bring your hand into view.
   
3. Ask your partner to say "stop" as soon as she or he sees your hand. Stop moving your hand, and ask how many fingers you have raised. The number will probably be wrong because the periphery only sees the movement of the hand-the fovea can see how many fingers.
   
4. Keep moving your hand forward until your partner can count the fingers correctly.
   
5. Switch roles, and see how you do.
   
6. Switch again. This time, as your partner stares straight ahead, slowly bring a colored object forward from behind that person's head.
   
7. Tell him or her to say "stop" as soon as he or she sees the object. Ask what color it is. The rods of the periphery see motion, but not color. Only the cones, found in the fovea, can see color.
   
8. Continue moving the object forward until your partner can tell you the correct color.

Questions
1. The rods in your retina see movement better in the dark, but they don't see as clearly as the cones. You think you are focusing on the pattern, but your eyes may be moving enough to make the pattern seem to move. Think about the scary things you may have imagined seeing in the dark. Does this help explain why you saw them?

Adapted from an activity in You Won't Believe Your Eyes by Nancy Crystal and Milan Tytla. Published by Annick Press, Ltd., Toronto (1992).

Think about the best parts of every ride you remember and design a ride that would be the coolest you and your friends have ever been on. Use construction materials to create a model of your ride.

Get a book of drawings by M. C. Escher. Study the illusions in the drawings. Do you see connections between the animals that turn into other animals, or people that seem to be suddenly walking up the down stairs, or down the up stairs? Try to create a pictorial illusion of your own. (First you might have to copy one of his.)

Test your peripheral vision and find your blind spot with the apple and the 3M logo on opposite ends of this page. Turn the page so that the apple is on the left and the 3M logo is on the right. Hold the page in front of your face; your nose should face the middle of the page. Cover your left eye with your hand and stare at the apple. Slowly move the page closer to your face. At some point you will no longer see the red 3M. Next, cover your right eye and repeat the exercise. Now, hold the page about one foot in front of you. Cover one eye and focus on the apple. Move the page from right to left and back to find your blind spot, the place where the 3M logo disappears.

Newton's Apple is a production of KTCA Twin Cities Public Television.
Made possible by a grant from 3M.
Educational materials developed with the National Science Teachers Association.