Why do People Have a Blind Spot?

Most recent answer: 10/22/2007

Why do people have a blind spot?
- Arlene (age 17)
Bishop Allen Academy, Toronto
Our eyes have a blind spot for a very good reason. On the back of our eye, the retina is the stuff that detects the light. All the information that the retina picks up is sent to the brain through the optic nerve. The only problem is that the optic nerve needs a way to get out of the eye. The place where it leaves is where we have our blind spot. Since we have 2 eyes that can move around, we don't notice the blind spot often.

Here's an interesting way to see your blind spot. First, take a piece of paper and draw 2 small dots on it. Cover one of your eyes. Hold the paper so the dots are on the same level (parallel to the ground). Stare at one dot and slowly move the paper towards and away from you. You will see that the other dot will disappear. That is the dot in on your blind spot and your other eye cannot see it.


p.s. You may wonder why the nerves go into the eye and then need a hole to come back out as a bundle in the optic nerve. Why not just head straight back out, and bundle up behind the eye, with no hole? For that, there’s no good reason. It’s an accident of evolution, where  things happened to go wrong and then couldn’t get fixed in small evolutionary steps. Squids and octopuses happened to get it right, and have no blind spots.  Mike W.

(published on 10/22/2007)