Lori Wong,'age 12, of San Francisco, California, for her question:
Why do we often see color with our eyes closed?
Color vision is a highly complex gift performed by special light sensitive cells in the human eye. The process is run by chemical electrical energy. Light beams start a series of reactions in the cells and sometimes the results continue after the triggering beams have gone.
Most of us must take along a swatch of fabric when we go to buy a spool of matching thread. A few gifted people can guess a perfect match from memory. Many people can recall colors, almost as if they were still seeing them. These and other special talents are rather rare and at present the biologists have no complete explanation of how they work. All of us, however, have experienced the sensations of colors when we close our eyes after gazing at a colorful picture or pattern. Part, but not all, of this color sensing can be explained.
The basic color receptors are light sensitive cells called cones. There are about 130 million of these cones behind the retina screen in the back of the eye. They respond to the different wave lengths of light and flash their reactions along nerves to the brain's center of vision. Part of the coding work needed to translate wave lengths into colors may or may not be done by the cones themselves. The major artwork of assembling the colored picture is done at the headquarters in the brain.
The most widely accepted modern theory on how this process is performed stresses the tendency of the vision to associate certain colors in pairs. This pairing is evident in direct vision and also in the afterimage that appears when the eyelids are closed. In gen¬eral, the cones tend to pair red and green and also yellow and blue. There may be a factor of cell fatigue. When the eye has been staring at a vivid color for a long time, it appears to fade and lose its brilliance.
Color pairing and perhaps also cell fatigue seem to be responsible for colors that seem to appear on the inside of the eyelids. One test for the afterimage uses a gray card adorned with a vivid green heart bordered in bright yellow. In the center of the heart is a black dot. The eye is allowed to stare at the picture for 20 seconds: Then it is re¬placed by a stark white card. For a moment, the eye may visualize the colored card. Then the afterimage changes to the paired opposites. It becomes a red heart,bordered with blue.
The modern theory suggests that certain cores handle the decoding of both reds and greens. Others cope with blues anc yellows. When the triggering light waves are removed, the electrical activity of the sensitive cone cells continues for a while. Sometimes the same colors are sensed on the inside of the closed eyelids. Sometimes an afterimage of the paired opposite colors is sensed.
In any case, the triggering action stops when the colors are removed from the field of vision. The stimulated cells, however, may continue to react to the colored wave lengths. Then we sense, rather than actually see colors with the eyes closed. Sometimes we can visu¬alize colors and design them in imaginative patterns. Emotions and other factors can stimu¬late these artistic memories and colorful creations. Some people have the gift of stimulat¬ing their color vision while asleep. They dream colorful dreams.