Blind Spot or Scotoma

The blind spot or scotoma, as your eye doctor may say, is the "hole" or missing area of your retina where your optical nerve enters the eye. The optic nerve enters about 5 mm from the center of the fovea. This produces a blind spot at this position because of the lack of light detecting photoreceptor cells at that point. We cannot see a part of the field of vision since there are no cells there which can detect light. All vertebrates have a blind spot.

This is rarely a problem because our eyes work together as a team. Each covers for the missing information in the other eye's blind spot, automatically filling in what we expect to see in that area.

Observe this blind spot optical illusion. Simply focus your right eye at the "X" with your left eye closed. Move closer, then farther away. You may need to move quite close to the screen.

You should notice that at a certain distance, the red "O" disappears. Stay at that position and open your left eye. You'll see the "O" reappear.

You can do the same test with your left eye by looking at the "O" with your right eye closed. The "X" should disappear, then reappear when you open your right eye.

Read more about the blind spot.