Eye Care Blog

Since most of the organs of the body are paired, you might have taken your two eyes for granted. If you stop to think about possible reasons, you might decide that with two eyes you can get a wider field of view to the sides. True enough. But to have a really terrific field of view, wouldn’t you design the system to have one eye mounted on a finger-like stalk atop your head to see all around? Wouldn’t it be nice to have the ability to see behind your back? The way it is now, there has to be a elaborate neck joint complex so that you can turn your head to get a reasonable near-360 degree panorama. Yet, nature evolved into our present set-up. Why?

Actually, by itself, a very wide field of view is not all that beneficial. While it’s good for noticing movement and general shapes, it’s very poor for seeing details. (Try threading a needle out of the “corner” of your eye.) Seeing movement is adequate for a frog since it merely triggers a reflex action to catch a fly, but for us it’s important to know what caused that movement. Is it a charging rhinoceros or only the shadow of a bird? To make this vital decision we must see it in detail. Why can’t we see in detail over the entire field of view? Very simply, the limited size of the brain.

The retina’s area of detailed sight is the macula which is about 1/20th of an inch in diameter. The flood of nerve signals from this tiny area alone keeps a large part of the brain occupied with interpreting the visual meaning. (The visual cortex allocates about 35 times as much space to the fovea as the rest of the retina.) If the entire retina had the sight property of the macula, the eye would have to be much bigger and the brain logarithmically larger (room-size). It’s doubtful if you could even “attend” to so much information.

Our vision is actually an adroit compromise. To the sides we have a reasonable field of view without much detail; at the macula we have a very limited view with marvelous detail.

It would be possible, though, to have this arrangement with just one eye bulging out of the center of the forehead. That’s impractical because the eye would be very susceptible to injury. So the eye is placed within a bony vault for maximum protection. If you put your mind to it, you could come up with several alternate placements for the eyes—fore and aft, for instance. But there is a very distinctive advantage to frontally placed eyes with overlapping fields of view—depth perception.

Before you say, “Is that all?,” remember that nature thinks so much of stereoscopic vision that a very elaborate system is involved to produce it. The retina is divided almost exactly down the middle with the nerve fibers from the outer half of each eye connecting to the same side of the brain; the nerve fibers from the nasal side cross over and connect on the opposite sides of the brain. This seems like a curious arrangement, but it’s not the crossing that’s curious (your left hand is controlled by the right side of the brain). The nerve fibers which don’t cross are part of the secret of depth perception.

Because the eyes are about two and one-half inches apart, each retina receives a slightly different image. You can easily prove this if you hold your finger 8 inches in front of your nose and alternately close each eye. The position of the finger will seem to shift back and forth. Within the brain there are special cells which match the offset images from the two eyes to yield the sense of solid depth.

• A diagramatic view, looking down at a cross-section of the eyes and brain, showing how the nerve fibers cross over.
• The nerve fibers from the two right halves of the retina end up on the right side of the brain; the left halves end up on the left side.
• The two half-images are “welded” into one by the brain. We do not normally see the half images as shown in the drawing.
• However, in certain cases of brain injury or disease,it is possible that only half the field of view will be seen.

Stereopsis. By crossing or uncorssing your eyes you can fuse these two images into a single solid pyramid with depth. To cross your eyes, make believe you’re staring at a spot. To uncross your eyes, look at a spot between the two images and make believe you’re looking far away. When crossing the eyes, the pyramid will appear smaller and the point of the base will be further away.

There are other ways to see depth which can be achieved with only one eye: perspective, size, surface texture, shadows, etc. which are all employed to depict a sense of three-dimension in pictures. But it is simply not quite the same as true stereopsis.

Is stereopsis all that important for survival? It’s difficult to reconstruct exactly how much advantage it conferred on our ancestors. Certainly, grasping and picking up objects is much simpler with 3-D. Whatever the reason, nature labored long and hard to perfect it, so let’s enjoy it.

As with any system, the more complicated, the more potential there is for errors. To appreciate the full value of depth perception (and for it to develop), very precise and intricate alignment of the eyes is essential. Horizontally, the alignment must be within a few degrees of arc; vertically, much less. If the six muscles controlling each eye cannot point the two eyes at the same spot within this narrow range, the stereoscopic effect will be diminished or lost. The very narrow range of vertical alignment explains why people who develop a condition wherein one eye sights slightly higher than the other, will often see double.

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