The Place You Can't See

This week's experiment is an old classic, but it is one that always amazes me. Each of your eye's has a blind spot. A small area where you cannot see. What amazes me is not that there is an area that you can't see, but that you can't see the "hole" in your vision. Instead, your brain fills in the space, so that you don't notice it. The only way to see it is to position a small object directly into that spot and notice that it disappears. To try this, you will need:

- this e-mail
- your eyes

In the space below, you will see an X and an O. Sit with the e-mail (either printed or on your screen) about 24 inches from your eyes. Cover your left eye and with the right one, stare directly at the X. Move the X towards your face (or your face towards the screen), always staring directly at the X. When you reach a certain point, the O will vanish. If your shift your eyes to look for it, it reappears. Look back at the X and it is gone again.

X O

You can try this with your left eye too, using the O instead of the X.

What is happening? On the back of your eye is a layer of light sensitive cells called the retina. When light hits these cells, it stimulates your brain and you see. To carry the information from the retina to your brain, you need a large nerve. It is called the optic nerve. At the point where the optic nerve comes through the retina, there is a hole, a spot with none of the light sensitive cells. That is your blind spot. It is off to the side of your field of view, in the area where things are not in sharp focus. Instead of seeing a "hole" your brain fills in the space. For this reason, you don't notice that things disappear when they are in this part of your vision, unless you are really watching for it to happen.