Debbie Lansing, age 13, of Mundelein, Illinois, for her question:

How do we see in three dimensions?

This problem baffled scientists for centuries and researchers solved it only during the past five years. Our visual equipment consists of two small screens, a multitude of nerves and nerve threads and several small computers. It seems impossible that these items can create three dimensional colored movies    but they do. And at long.., last the mystery has been explained, at least in a general way.

The left and right eyes observe the scenery from two slightly different angles, or points of view. They should show a pair of almost matching picture postcards and we are told that these pictures are upside down. Instead, we see a single picture, right side up and in three dimensions. This is because, in a fantastic split second, the data from the eyes is transformed behind the scenes. The miraculous teamwork involves the eyes, five computer centers in the brain, plus about 330 million light sensitive nerve cells and their fibers. The countless items of information are transmitted from center to center along nerve threads by chemical electrical impulses.

Within each eye, a transparent lens adjusts to focus the angles of light from objects near and far. The picture falls on the retina, a curved screen lining the back of the eyeball. Its 166 million nerve cells are sensitive to the color and intensity of light. Each eye captures the scene from a slightly different angle and together they give a small peek around the corners. With angles from the lenses and retinas, they provide the basic date for binocular, or two sided vision, plus the clues for judging distances and spaces between objects.

But a fantastic amount of sorting and computing must be done to assemble and decode these millions of separate items. The retina cells relay their data along separate neuron threads that merge to form the optic nerve behind each eye. This nerve cable meets its neighbor in the optic chiasm, a sorting center in front of the brain. Here, about half the fibers from each eye are crossed to the opposite side. Those that come from the outer sides of each retina are not crossed over.

Two optic tracks leave the optic chiasm, both carrying an assortment of fibers from each eye. Each of these nerve cables goes farther back to a lateral geniculate nucleus. In this amazing center, the fibers are sorted in layers where they swap data. When they leave, the separate fibers carry packages of information, somewhat like jigsaw pieces with coded signals for color, shape and depth.

The assembly job is done when these neuron fibers flash their signals to the two visual cortex centers in the back of the brain. Singly and in groups, they trigger sensitive cortex cells. In a split second the complete binocular picture is assembled in color and the countless angles registered by the eyes are arranged to give three dimensional depth to the scene. Between blinks, streaming multitudes of items from the eyes are processed by the complex visual tract through the brain to give a continuous colored movie in three dimensions.