Scarlet Letter

A common misconception of people is that being color-blind means that you can only see shades of gray, black, and white. But total color blindness is extremely rare, and virtually no one is truly color blind. More typically, is partial color blindness which is called color vision deficiency, a condition in which certain colors cannot be distinguished.

Color blindness may be a hereditary condition or caused by a disease of the optic nerve or retina. Acquired color vision problems only affect the eye with the disease and may become progressively worse over time. People with a color vision defect caused by disease usually have trouble distinguishing blue from yellow. However, inherited color blindness is most common; it affects both eyes, and does not worsen over time. People with inherited color blindness have more difficulties distinguishing red from green. This type is found in about 8% of males and 0.4% of females. Since men only have one X chromosome, the chances of color blindness showing up in men are much higher than in women, who have two X chromosomes so color blindness is almost always passed on from a mother to her son.

There are three groups of inherited color vision defects; anomalous trichromacy, dichromacy, and rod monochromacy.

Anomalous Trichromacy is the least severe of the three, and is by far the most common form of color blindness. It occurs in about one to five percent of males. Anomalous Trichromacy is caused by a shift in the sensitivity of one or more cone types. It reduces the ability to discriminate between colors but it does not eliminate color perception all together. Anomalous Trichromacy is divided into three subgroups; protanomaly, deuteranomaly, and tritanomaly. Protanomaly and deuteranomaly are both forms of red/green color deficit, and they are both usually inherited. The conditions Protanomaly and deuteranomoly are caused by the lack of “green cones” in the eye’s retina. The last form, tritanomaly is a blue/yellow color deficit, and is usually acquired. Tritanomaly is caused by the lack of blue cones in the retina.

Dichromacy is a more severe form of color vision deficiency. Dichromats lack one of the three cone types entirely, but can still tell some hues apart. Like anomalous trichromacy, dichromacy is also divided into three subgroups; protanopia, deuteranopia, and tritanopia. Protanopia is similar to protanomaly but is more harsh. Protanopia is one of the forms of red/green color deficit. The other form of red/green color deficit in dichromacy is deuteranopia which is similar to deuteranomoly, but again, more harsh. Lastly, there is tritanopia, which is a form of color blindness that prevents the person with that condition to be able to differentiate the colors blue and yellow.

Finally there is rod monochromacy, which refers to people who are truly color blind because they cannot distinguish any colors whatsoever. They only see different degrees of lightness. For them, the world appears to be in shades of gray, black and white. They also have poor visual acuity, aversion to bright light and often have nystagmus (an involuntary, rapid movement of the eyes.) To have rod monochromacy, someone must inherit a gene for the disorder from both parents. Only one in 30, 000 people have rod monochromacy.

Our eyes can see color when light stimulates the retina (a neuro membrane lining the inside back of the eye.) Covering the retina are millions of light sensitive cells called rods and cones. The cones pick up color and the rods control brightness. Nearly everyone has red, green, and blue cones which are sensitive to those colors and combinations. Color vision occurs within the visual part of the brain where the brain compares electrical signals from the different types of cones. The human eye and brain work together to translate light into color. People