When I get dressed, my wife checks that the colors I choose match, says Michael. At breakfast she selects a piece of fruit for me because I can’t see if the fruit is ripe. At work I can’t always see where to click on the computer screen, since items are often distinguished by color. When I’m driving, red and green traffic lights appear the same to me, so I observe whether the illuminated light is on top or on the bottom. Horizontal lights, however, can present a problem.
HOW DO WE SEE COLORS
Light from object passes through the CORNEA and the LENS and is focused on the RETINA. THE RETINA contains CONE CELLS and ROD CELLS. Together they give the full range of vision. THE CONE CELLS are sensitive to RED, GREEN, OR BLUE light. The OPTIC NERVE carries visual impulses to the brain. The image is inverted but corrected later by the brain.
TEST FOR COLOR-VISION DEFICIENCY
Tests to discover the type and degree of color-vision deficiency that a person has often employ patterns of dots in various hues and shades. The widely used Ishihara test consists of up to 38 different patterns. For example, when viewing one of the test patterns in daylight, a person with normal vision should see the numbers 42 and 74, while someone having a red-green deficiency –the most common –may see no numbers at the top and 21 at the bottom.
If testing reveals a defect, an eye doctor may recommend further tests to determine whether it was inherited or has some other cause.
WHY MAINLY MALES?
Inherited color deficiencies are carried on the X chromosome. Women have two X chromosomes, while men have one X and one Y. Thus, if a woman inherits a visual defect in an X chromosome, the normal gene in the other chromosome will likely override it, and her vision will be fine. But a man who inherits a defect on his X chromosome has no other X chromosome to fall back on.
Michael has color-vision deficiency, also called color deficiency or color blindness. He inherited a genetic flaw that causes a defect in the retina –the light-sensitive inner lining of the eyes. Michael shares this condition with about 1 of every 12 males of European ancestry and about 1 of every 200 females.
Like the vast majority of sufferers, Michael can see different colors –he does not see only black and white. But some colors do not look the same to him as they do to people with normal vision.
In the human eye, the retina normally contains three kinds of CONE-SHAPED color-sensitive cells. Each kind is tuned to the wavelength of a different primary color of light –blue, green, or red. Light of different wavelengths triggers the corresponding cones, which signal the brain and enable one to perceive colors.
In people with color-vision deficiency, however, the sensitivity of the cones to one or more colors is weak or shifted in wavelength, so that their response to color is altered. Most sufferers have difficulty distinguished between yellow, green, orange, red, and brown. This effect can make it hard to see green mold on brown bread or on yellow cheese or to distinguish a blue-eyed blonde from a green-eyed redhead. If a person’s red-sensitive cones are very weak, a red rose appears black. Very few sufferers cannot see blue.
Color-vision deficiencies can be found in all racial groups, but it is most common among Caucasians. Many animals can discern colors, although their color vision differs from ours. Dogs, for example, have only two kinds of cones in their retinas –one for blue and the other for a hue between red and green. Some birds, on the other hand, have four kinds of cones and can detect ultraviolet light, which is beyond the human range.
COLOR-VISION DEFICIENCY AND CHILDREN
Defects in color vision are usually inherited and present at birth, and children with the condition often learn unconsciously to compensate. For instance, even if they cannot see the difference between certain hues, they may perceive differences in contrast and brightness and associate these variations with the names given to the colors.
They may also learn to identify objects by surface patterns and textures instead of by color. In fact, many young people remain unaware of their disability throughout childhood.
Because schools often use color-coded teaching tools, especially in the early grades, parents and teachers may mistakenly think that a child has a learning disability when in fact, he may have a color-vision deficiency. One teacher even punished a five-year old boy for painting a picture that had pink clouds, green people, and trees with brown leaves. To a child with color-vision deficiency, these colors may seem perfectly normal. For good reason, therefore, some authorities recommend routine color-vision testing in early childhood.
Color-vision deficiency can sometimes be caused by disease if you notice changes in your color vision later in life.
Although there is no known cure for this condition, it neither worsens with age nor increases the risk for other defects in vision. Still, color-vision deficiency is a disability that can be frustrating.