Key concepts
Biology
Vision
Binocular vision
Depth perception

Introduction
Did you know that there is a type of giant clam that has thousands of small eyes covering its body? It's called Tridacna gigas, and it lives in the South Pacific Ocean. Although it has a lot of them, T. gigas's eyes are fairly primitive compared with ours. (So don’t be too jealous that you don’t have thousands of eyes, too!)

All vertebrate animals (including humans) have two eyes these days. Humans have developed an amazingly complex visual processing system that allows us to collect and process a tremendous amount of information—in the blink of an eye! Our visual system is so good at processing information for us that we usually don’t even notice what it’s doing. In this activity we’re going to explore some of the amazing abilities of that system. So get ready to say, “thanks, eyes!”.

Background
Having two eyes gives humans (and other animals) many advantages in our daily lives. For example, having two eyes allows us to see a larger field of view. (To test this, cover your left eye and try to see something on that side of your head. Without moving—you can’t!) In addition, the eyes of humans and some other predators are special because they are positioned on the front of our heads. This gives us what is known as binocular vision, where our two eyes work together to see the world around us. Some animals, particularly prey like rabbits and mice, have eyes positioned on either side of their heads. These animals have what is known as monocular vision, where the information coming from each eye is different, and there is little to no overlap between the visual fields of the two eyes.

Having binocular vision gives humans and other animals much better depth perception, which is our ability to perceive our world in three dimensions and estimate the distance between objects. For example, when you reach across the table to pick up a glass, you use your depth perception to tell you where that glass is—and when to stop moving your hand toward it, so you don’t knock it over.

In this activity we'll be exploring exactly how much binocular vision contributes to our depth perception. Keep your eyes open!

Materials

  • Ping-pong ball
  • Fabric, scarf or eye patch (Something that can be used to cover one eye.)
  • Partner
  • Pencil or pen
  • Paper
  • Yardstick or measuring tape
  • Removable tape (such as painter's tape)
  • An open space of at least 10 by five feet (Select a location, preferably indoors, so it won’t be affected by wind.)

 

Preparation

  • Use your yardstick or measuring tape to measure a distance of 10 feet. Mark your starting point with a piece of removable tape. Mark every two feet with a piece of removable tape.
  • At the 10-foot marker, place a sheet of paper on the ground that says “10 FEET” in large letters that can be read from your starting point.
  • On your paper, make a table that includes rows for each of the two-foot increments (two feet, four feet, etcetera) and a row at the bottom for the total. This column will be labeled “Distance.” Add three more columns to the right: “Both eyes—successful catches (out of 10)”; “Right eye—successful catches (out of 10)”; Left eye—successful catches (out of 10)”; “Total (distance); and Total (eye).”

 

Procedure

  • Stand at the starting point. Have your partner stand on the two-foot marker, holding the ping-pong ball.
  • Have your partner gently toss the ball to you 10 times. Ask your partner to observe you as you catch it. For yourself, notice how you track the ball with your eyes. Do you watch it from the moment it leaves your partner's hands to the moment it lands in yours?
  • In the column “Both eyes—successful catches” record the number of times you caught the ball at a distance of two feet.
  • Repeat these steps again, with both eyes open, at each distance you marked on the floor. Record your successful catches in the corresponding row. Does your success rate change as the distance changes?
  • Use your makeshift eye patch to completely cover your left eye.
  • Repeat the first step by standing at the starting point again. Have your partner stand on the two-foot marker holding the ping-pong ball.
  • Take a moment to notice how your vision is affected by covering one eye. Try reading the sign on the floor at the 10-foot marker. Is the sign more difficult to read? When you look around, do you see things less clearly?
  • Once again, have your partner gently toss the ball to you 10 times. Ask them to observe you as you catch it. Is it easier or more difficult to catch the ball with one eye covered? Do you notice any changes in how you catch it? For example, do you keep your hands up higher or lower? Do you turn your head at all while you're tracking the movement of the ball? Does your partner notice you doing anything different to catch the ball?
  • In the column “Right eye—successful catches” record the number of times you caught the ball at a distance of two feet.
  • Repeat these steps with your left eye covered, at each distance you marked on the floor. Record your successful catches in the corresponding row. Does your success rate change as the distance changes? Is the distance more challenging with only one eye open? If so, why do you think that might be the case?
  • Remove the eye patch from your left eye and use it to completely cover your right eye.
  • Take a moment to notice how your vision is affected by covering your right eye. Try reading the sign on the floor at the 10-foot marker again. Is the sign more difficult to read than it was when your left eye was covered? When you look around, do you see things less clearly?
  • Repeat the first step by standing at the starting point again. Have your partner stand on the two-foot marker, holding the ball.
  • Again, have your partner gently toss the ball to you 10 times. Ask them to observe you as you catch it. Is trying to catch the ball different with only your left eye open compared with when only your right eye was open? Does your partner notice that you do anything different to catch the ball?
  • In the column “Left eye—successful catches” record the number of times you caught the ball at a distance of two feet.
  • Repeat the steps with your right eye covered at each distance you marked on the floor. Record your successful catches in the corresponding row. Does your success rate change as the distance changes? Is the distance more challenging with only one eye open? If so, why do you think that might be the case?
  • In the “Total (distance)” column add up the total number of successful catches you had for each distance.
  • Add up the total for each row.
  • Compare the “Total (distance)” results for each row. At which distance did you have the most successful catches? At which did you have the least number of successful catches?
  • In your last “Total (eye)” row add up the total number of successful catches you had for each eye.
  • Add up the total for each column.
  • Compare the “Total (eye)” sums from each column. Did you have more catches for “both eyes open” compared with “right eye” or “left eye”? Was there a difference between how many successful catches you had with your right eye open compared with your left?
  • Extra: Repeat the procedure with your partner catching the ball. Pay attention to how they catch the ball when both of their eyes are open compared to just one. Did they have different results with a different eye covered than you did?
  • Extra: It’s easy to test depth perception! Place a few items on a table in front of you, then close (or cover) one eye. Try to reach for the objects quickly, without trying to feel where they are. Make sure you don’t use anything fragile!
  • Extra: Another way is to told a pen or pencil in each hand. Moving just your hands, tap the points together gently. Now try doing the same thing with one eye covered.

 

Observations and results
In this activity you explored some of the functions of our binocular visual system. When you covered one eye you probably noticed it was more difficult to catch the ping-pong ball compared with both eyes open. If you look at the “Total (eye)” row and compare the number of successful catches for both eyes open versus right eye open versus left eye open, you probably can see that you were much better at catching the ball with both eyes open.

When you compare the right- and left-eye totals, you might have also noticed that you were better at catching the ball when just your right eye was open compared with your left—or vice versa. This is common and can be a sign of eye dominance. Just like most people have one dominant hand, many of us also have a dominant eye. This is the eye that our brain relies on most for visual information. Many people are slightly better at processing precise positional information (such as the trajectory of a ping-pong ball) that comes from their dominant eye. So if you notice a difference in the data you collected between your right and left eyes, you might have discovered which of your eyes is dominant.

In addition to eye dominance, this activity demonstrates the importance of binocular vision in depth perception. When you covered one eye, you probably didn’t notice a big change in how clearly you could see things around you. You could probably still read things at a distance (as well as you could with both eyes open). Even though you could see just as clearly with one eye covered, however, you still struggled to catch the ball. This is because depth perception is improved when the brain receives information from both eyes. One reason for this improvement is the binocular visual cue known as stereopsis, or binocular retinal disparity. In short, having two eyes focused on an object allows us to triangulate its position with a higher degree of accuracy. Because of the distance between your right and left eyes, each perceives the object from a slightly different angle. If an object is far away, the disparity of that image falling on both retinas will be small. If the object is close, the disparity will be large.

To see this in action, hold your finger up in front of your eyes, about six to eight inches from your face. Focus your eyes on something behind your finger, so that you see a double image of your finger. Once you see two (slightly transparent) fingers, slowly move your hand away from your face. As you do this, the distance between the “two” fingers will appear to decrease. Cool!

Having two eyes allows us to do many things more easily, including judge the distances between objects. So next time you’re playing a sport or running toward something or even reaching for your water glass—make sure to keep both eyes open!

More to explore
See Change: 2 Eyes, 1 Picture, from Scientific American
Are Your Eyes Playing Tricks on You? Discover the Science behind Afterimages!, from Science Buddies
Motion Aftereffects in Vision, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies