HOW COME? Why we see a 3-D world

Try this experiment: Before you get up in the morning, close one eye and look out a window to the side of your bed. Take note of what you can see. Now, without moving your head, close the other eye instead. (Hmmm, did that bird feeder just disappear?)

Now look straight ahead at the room in front of you. Shut one eye, and then the other, holding your head still. You'll see that the image from your left eye includes more of the room on your left. Likewise, the image from your right eye reveals more of the room's right side. But in a kind of image overlap, each eye sees the room in front of you.

You'll notice that you must do a lot of annoying head-swiveling to see as much of your room as you did with two eyes open. And you may also notice something else: Everything seems flatter, more like a photo of a bedroom than the room itself.

Scientists call how we normally see "binocular vision." Just as with binoculars, we view the world through two lenses. Two eyes give us something that a single eye can't: stereo vision.

Human eyes are set only a little over 2 inches apart, but those few inches confer a big advantage over, say, a single, centered eye. When we look at something, each eye receives light bouncing off the object. So each eye sees from a slightly different viewpoint. Struck by light, the nerve cells in each eye's retina send signals to the brain along the optic nerve. Presto: The brain takes the two different images and neatly combines them.

Having two eyes makes for an expanded field of vision, so that we can take in more of the world around us without turning our head. Our binocular vision also allows us to perceive the world in three dimensions. Since we see objects from several angles at once, they seem to project out from their backgrounds.

We humans see objects most three-dimensionally, and are best at estimating their distance, when they no farther than 20 feet away. Why? The more distant the object, the more similar the image sent by each eye. Which is why a line of trees in the park down the street looks flatter than a chair across the room.

However, if vision in one eye is impaired or uncorrected, the brain will have trouble processing the two mismatched images. If eyes don't point in the same direction, there may not be enough image overlap. And if the part of the brain that does the processing is damaged, binocular vision may be impossible.

But even when everything works in concert, there are disadvantages to relatively close-set eyes. For example, it's hard to see what's sneaking up on you. So animals that must frequently escape predators often have eyes positioned on either side of the head. The result: An extra-wide field of vision that helps them notice threatening creatures, from stealthily slithering snakes to dive-bombing hawks.