Many people assume that what they see is actually what's out there. That's not entirely true. Each human eye has a blind spot, and the brain sometimes has to fill in what is there by looking at the surrounding area.

There's a way to find your blind spot. Cover your left eye and look at the dot on the left in this image. Be aware of the cross on the right, but don't look at it - just keep your eye on the dot.

Move your face closer to the monitor, and farther away. At some point, you should see the cross disappear. Stay at that point and close your right eye. Stare at the cross, and you should see that the dot has disappeared.
It doesn't just happen with a white background. Try the same with colored paper, and your mind will fill in the background color of the paper when the mark gets in your blind spot.

You don't see as much of the world as you think.



For the most part, the human eye gives the brain an accurate picture of what's going on in the world. There are limitations. Although many birds and insects can see ultraviolet, and some creatures can see infrared, humans are stuck looking at so-called 'visible' light only. This cuts down human's view of the world, not letting them see the urine trails left behind by some mammals, and not letting them fully appreciate the colors of certain flowers, which have evolved to put on quite a show in ultraviolet while remaining plain in visible light.

The human eye also can't distinguish between polarized and nonpolarized light, while many cephalopods and some birds can.  Still, the eye sends back signals that let humans navigate through the world pretty successfully. Although we assume that what we see is actually "reality," that's not entirely true. Each human eye has a blind spot, and the brain sometimes has to "make up" what is missing by looking at the surrounding area. So, a under certain circumstances, the brain is actually manufacturing a "reality" that is not visible.


Light gets into they eye by passing through the pupil. It hits the retina at the back of the eye. The retina is covered with light-sensing proteins. They relay what they sense to the optic nerve, which carries the information back into the brain. The problem is, the optic nerve ends in the field of the retina itself. This is a little like having to plug the power cable for a TV directly into the screen. It creates a dark spot. Most of the time, the other eye will see what's happening in its partner's blind, but if the blind spots overlap while looking at a certain object, or if the person is only looking through one eye, the brain just fills in the spot looking at the surrounding picture.