When they think of it at all, most people think of their retina as like the film in a camera. The lens focuses the light onto the retina, which sends the image along the optic nerve to the brain.

There's some truth to that picture, but not as much as once was thought. Today, scientists regard the retina as a specialized part of the brain, given the function of responding to light. The retina contains different kinds of neurons that process the raw light signals before sending them along.

Most notably, the retina detects black and white and color with specialized cells. Cone cells respond to different colors of light -- roughly, blue, green and red. Rod cells are more sensitive to light, and work under low light levels. But since rod cells don't detect color, our vision in low light is black and white.

One of the odd things about the retina in humans and other vertebrate animals (those with spinal columns) is that these light-detecting cells are built "backward." That is, the light-detecting part of the retina cells is in the back of the retina, while the nerve fibers of each cell point out toward the middle of the eye.

The tiny individual nerve fibers cross in front of the retina, converging into one large bundle of fibers that passes down through the retina, forming the optic nerve. This location is the "blind spot." People aren't aware of the blind spot because the brain fills in the gap with a guess of what's there. Detect your own blind spot with tests like one at this Web site: http://serendip.brynmawr.edu/bb/blindspot1.html.

Near the blind spot is its opposite, the fovea. This is the area with sharpest vision, right ahead where it's most needed. The fovea is packed with cone cells, enabling it to perceive objects very sharply, precisely and in full color.

Many diseases of the retina are caused by the blood vessels that supply it with food and oxygen. These blood vessels may grow so much that they block off light. This is especially serious when it happens in the fovea, which normally has no blood vessels at all.

The fovea is located in the central part of the retina, which is called the macula. When blood vessels grow and leak into the macula, this causes what's known as "wet" macular degeneration. Vision gradually gets misty and clouded over, much like in a cataract.

Drugs, such as Lucentis, from Genentech, have been developed that stop macular degeneration and restore sight. Lucentis blocks the growth of blood vessels, allowing the macula to clear.

Retinal detachment also ranks as a major vision risk. The retina partly or completely separates from the eye, causing vision loss. This can take place through a gradual weakening of the retina, or it can be caused by a sudden blow to the head. Those who are extremely nearsighted have a higher than normal chance of detached retina.

Eye doctors have several methods to reattach the retina, including laser surgery or cryotherapy (freezing parts of the retina in place). Signs of possible retinal detachment include a "veil" or "curtain" over peripheral vision, a sudden increase in "floaters," or fragments in the eye; or flashes of light.

Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com.