Can you see your Hole hand?

Introduction

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! 

Background

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! 

Materials

Preparation

Procedure

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:

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.

More to Explore

• Now You See It, Now You Don't: A Chromatic Adaptation Project, from Science Buddies
• Why Do We See in 3D?, from LiveScience

Credits