Can you see your Hole hand?
Have you ever stopped to wonder why we have two eyes, but we only see one image? Usually you only see one image because your brain takes the information from your left eye and your right eye and combines them, without you even noticing! But sometimes your brain is too smart for its own good; it makes assumptions (or guesses) about the things we are seeing. When your brain makes guesses it can sometimes make mistakes, as it does in this experiment!
Why do you need two eyes to make one image? It turns out that you actually don't need two eyes-you can close one eye and still see pretty well-but having binocular vision (two eyes working together) has some advantages. For instance, binocular vision gives us much better depth perception, increases the size of our visual field, and increases the accuracy of our vision.
When you want to look at something in front of you, you focus your eyes so that they are both pointing toward that object (if the object is very close to your face, you may even go a little cross-eyed!). The cells at the back of your eye—known as the retina—send signals to your brain about how much light and color they are seeing. Your brain processes these signals to determine information, such as the object's shape, distance from your face, location in your visual field, and the amount/direction of light in the field. By establishing these parameters for the object you are looking at, your brain is able to combine the information coming from your left and right eye into one cohesive image.
Your brain is good (and fast) at what it does because it is designed to make intuitive assumptions. For example, your brain assumes that your eyes are always focused on the same thing in your visual field. This is a very smart guess for your brain to make, because it is almost always true.
For this activity, you are going to trick your brain by having your eyes send different information about what you are seeing. When your brain tries to combine the information it's getting from your left and right eye, it will come up with some pretty interesting results!
- A piece of white paper, 8 ½ x 11 inches
- Clear tape
- Roll the paper, along the longer side, into a tube; the tube should be about the diameter of a quarter.
- Use a piece of tape to hold the paper in place.
- To start, make sure that whatever is in front of you (the wall, a door, etc.) is not white. This experiment works best with a non-white background, in a well- lit room.
- With both eyes open, raise your right hand so that your palm is facing toward your face, about 1 foot away from you.
- Look at your hand with both eyes open, then use your left hand to alternately cover your left eye, then your right eye, while you continue to look at your right hand through the uncovered eye. Do this slowly at first. (Just for fun: If you switch eyes fast enough, it may look like your right hand is moving back and forth a tiny bit!) As you slowly switch eyes, what do you notice first, each time you switch which eye is uncovered? Do you notice what is behind your hand? Do you notice the lines on your hand?
- Open both your eyes and keep them open. Lower your right hand, and using your left hand, place the paper tube up to your left eye, so that you are looking through it as you would a telescope. What do you see? Do you notice the paper tube first, or the hole at the end of it? What do you notice about what your left eye sees compared to what your right eye sees?
- Raise your right hand so that your palm is facing toward you, then place it against the tube, so that the outside of your pinky finger is touching the tube, about halfway down its length. Look straight ahead with both eyes open. What do you see? Do you notice anything about your right hand? Can you change what you see by moving your right hand closer or farther away from your face? What about if you move your right hand closer or farther away from the tube?
- Keeping the tube and your right hand in place, try closing your right eye, then your left eye.
- What is different about what your left and right eyes are seeing? What is the same?
- If you saw something strange, try this: Without moving your right hand, slowly move the tube away from your face. See if you can continue to see the illusion, even as the tube gets farther away. How far away does the tube have to be before you stop seeing the illusion?
- Test what happens if you repeat this experiment, but this time look through the tube with your right eye. Does this work for you with one eye, but not the other, or does it work for both eyes?
Extra: Ask your parents and friends to try this experiment, and see if the illusion works better if they use their left or right eye to look through the tube.
Observations and Results
When you looked through the tube, did you see a hole in the hand pressed up against the tube? This is what is expected.
So why do you see the hole in your hand? Whenever your eyes are open, your brain is working to combine the information coming from your left eye and your right eye into one image. This usually works remarkably well, and you don't even notice, because your left and right eyes are usually looking at the same thing. However, in this experiment you are changing that one little fact: your left and right eye are seeing two different things! Your left eye is seeing a small circle of the world at the end of a tube, whereas your right eye is seeing your right hand.
When your brain combines the information coming from the left eye and the right eye, it looks something like this:
Small circle of the world (from left eye) + Right hand (from right eye)
Small circle of the world going through your right hand!
If you tried it with both eyes, did you notice that one eye worked better than the other? Some people have a ‘dominant eye,’ similar to how they have a dominant hand. If you have a dominant eye, information coming from the dominant eye will take precedent over information coming from the non-dominant eye. In this case, the tube illusion will work better when the person is looking through the tube using their dominant eye.
Ask an Expert
- Now You See It, Now You Don't: A Chromatic Adaptation Project, from Science Buddies
- Why Do We See in 3D?, from LiveScience