Color blindness is a term that most of us are familiar with, but do not fully understand. Color blindness, or color deficiency, refers to any condition in which a person has difficulty distinguishing colors. Most forms of color deficiency are inherited and affect the cones of the retina of the eye. Cones are receptors that transform light energy into neuronal activity, thus preserving information about wavelength, which is necessary for color perception. Three categories of cones record different wavelength ranges: long (perceived as red), medium (green), and short (blue). The combinations of activity in the three types of cones allow the perception of the entire spectrum of colors.
Monochromaticism is a rare condition in which there are no working cones. Monochromatics can perceive only shades of gray and are therefore truly color blind. They also have poor visual acuity and are sensitive to strong light.
Dichromatics have only two functional categories of cones instead of three. Dichromatism is a recessive trait inherited through a gene on the X chromosome. Therefore, it is more common in men, who have only one X chromosome, than in women, who have two. There are three forms of dichromatism, each of which corresponds to the loss of one type of cone red green colorblind test. Protanopia involves insensitivity to long wavelengths of light, causing the inability to distinguish reds. Protanopes only see blue and yellow. Deuteranopes are insensitive to medium wavelengths of light, causing the inability to perceive greens. Like protanopes, they only perceive blues and yellows. Both protanopia and deuteranopia occur in approximately 1% of males and 0.01 to 0.02% of females. Tritanopia involves the loss of short wavelength cones and is very rare. Tritanopes perceive the world in bluish greens and reds.
Other forms of color deficiency are more subtle and complex, such as abnormal trichromatism. Another example is achromatopsia, which involves the inability to perceive color due to dysfunctions in the brain. Achromatopsia can be inherited or acquired through injury, inflammation, or disease. The existence of achromatopsia reveals that color is processed in the brain, not the eyes, and that the perception of color occurs through mechanisms separate from those that produce the perception of shadow, contour and movement.
While color deficiencies are not considered debilitating, there are many situations where color distinctions are important. Tasks like choosing clothes, identifying road signs, or reading colored text often require normal color vision. Psychologists are just beginning to explore the cognitive and socio-emotional implications of color deficiencies.