The vast majority of people who describe themselves as color blind are not, in fact, completely unable to see color. Rather, color blindness is usually poor color vision: either a partial or complete inability to see one or more colors that humans can normally see.
Human Color Vision
As described in Neuroscience, there are two general types of photoreceptor cells responsible for light vision: rods and cones. Rods detect light with no regard to color, which is determined by wavelength. Cones are responsible for detecting specific colors of light within a certain range of wavelengths. Humans have three types of cones: blue, green, and red. All other colors are perceived by humans as combinations of these three colors. For example, while we generally think of yellow as a primary color—with which you can mix blue to make green, or red to make orange—but in terms of wavelength, yellow and orange are both between red (the longer end of our visual spectrum) and green (the middle). When both red and green cones are stimulated, a normal eye sees either yellow or orange, depending on the ratio.
Other animals have different color vision: dogs, for example, have only two types of cones, while many birds have a fourth. And, as the Encyclopedia of Life notes, mantis shrimp have at least ten!
Types of Color Blindness
- Achromatopsia, the complete inability to see color, is extremely rare, according to EyeSmart.
- Red-green color blindness is more common among males. The most common defect occurs in the X chromosome, causing red-green blindness. Because males have only one X chromosome, they are more likely to be color blind; females need to have the color gene in only one or the other of their two X chromosomes, while males only have one chance. Red-green color blindness can be caused by protanopia (the absence of the red cone), protanomaly (a defect of the red cone), deuteranopia (the absence of the green cone), or deuteranomaly (a defect of the green cone). Some people cannot distinguish between red and green at all, while others have difficulty in low light, or when there is only a thin line or small area of the color.
- Blue-yellow color blindness is caused by tritanopia (an absence of the blue cone) or tritanomaly (a deficiency of the blue cone). Both are rare and result in difficulty or inability to distinguish between certain blues and greens, as well as between certain yellows and reds.
Difficulties
Our society frequently uses colors to communicate, and those with reduced color vision must learn to compensate. However, this is rarely seriously disabling. For example, those with reduced red-green vision simply must remember the sequence of colors on a traffic light. On the positive side, a study published in Proceedings of the Royal Society B reveals evidence that reduced color vision may heighten other aspects of vision; color blind individuals were found to be able to see certain types of camouflaged objects more easily than those with typical vision.
Causes of Reduced Color Vision
Color blindness is most commonly caused by a genetic defect, but can also be acquired through trauma to the eye, brain, or optic nerve, through incidents like blows to the head, exposure to chemicals, or shaken baby syndrome. It can also be caused by diseases like diabetes, glaucoma, and macular degeneration.
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