What exactly is a blind spot, and why do we have one?

A blind spot is any region of the visual field (i.e. what your eye registers) that is obscured. In principle you can have more than one blind spot, or "scotoma," but when people refer to "the" blind spot they typically mean a region in the back of the eye that does not have light-sensitive cells. This is the "physiological" blind spot, and everyone has one in each eye.

The reason for a blind spot is rather surprising - the optic nerve, which collects data from the light-sensitive cells, actually comes through the back of the eye and collects data from the front of the retina. In other words, your retina looks backward, and the cable that collects the data has to go from the front through the retina to get to your brain. The spot where it goes back through is the blind spot.

To an engineer this really doesn't sound like a very good design, but all vertebrates (animals with backbones) have eyes that work like this.

Lest you think that there is some deep reason why an eye would have to work in this way, it might surprise you to find that octopi actually have their eyes set up sensibly. Light comes into the eye, hits the retina, and the signals are read out from the back. There's no need for the optic nerve to puncture the retina, and consequently no blind spot!

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.