Color Blindness

Overview

The term “color blindness” is actually something of a misnomer, as complete absence of color vision is extremely rare and most color blind individuals do actually see color – just not as many shades. The most common form of color blindness is an inherited condition and since it is on the X chromosome, of which males have only one, color blindness is much more common in males than in females. Since females have two X chromosomes, both parents would have to pass on color blindness genes to their daughter: although possible, it is extremely unlikely to occur. In males, it is estimated than 1 in 12 has at least some color perception abnormality.

There are two main categories of color vision deficiency: anomalous trichromats, and dichromats. Of these two categories, each contains 3 different ways in which color blindness can occur, that are directly related to the types of photoreceptors in the retina. That being said, let's step back a little bit and get a firm grasp on what photoreceptors are.

Photoreceptors

Photoreceptors are the light-gathering component of the retina. There are 4 types of photoreceptors, three of which are cones, with rods being the fourth type. Only the cones are responsible for color vision, so we will concentrate on them for now. There are three types of cones: red, blue, and green. Combinations of the light gathering abilities of these cones to particular wavelengths of light is what gives us our perception of color. For instance, our perception of a yellow object is actually a result of similar responses by the red and green cones to the wavelengths of light reflected off the surface of that object. Admittedly yes, it can get quite confusing really fast (red + green = yellow!?!?!). Click here for a more detailed explanation of how color vision works.

Color blindness develops from either partial or complete disruption in the ability of a particular type of photoreceptor (whether it be red, green, or blue) to respond to the same wavelengths as a person with normal color vision. A person with abnormal red cones has a protan defect while a person with abnormal green cones has a deutan defect and someone with abnormal blue cones is considered a tritan. Red-green color blindness (protan and deutan) are much more common the blue-yellow color blindness. A dichromat is someone who has complete disruption of one photoreceptor and effectively has only two functional photoreceptors. Examples of dichromats are protanopes, deuteranopes, and tritanopes.

How Protanopes See Color

Protanopes are individuals who have complete disruption of red cone photoreceptor responses. Without this, the world appears to be colored in shades of blue and yellow. Difficulty comes in distinguishing reds and greens. Since the red cone photoreceptors normally respond to long wavelengths of light, protanopes are much less sensitive to deep shades of red, which consequently appear very dim or black.

How Deuteranopes See Color

Deuteranopes have complete disruption of green cone photoreceptor responses. Like protanopes, deuteranopes view the world in shades of blue and yellow (although in a different way that protanopes). Also like protanopes, red and green distinctions are difficult for deuteranopes.

How Tritanopes See Color

Tritanopia is a much less common color vision deficiency in which the world is viewed in shades of red and green due to a disruption of blue cone photoreceptor responses. Difficulties arise when attempting to distinguish shades of blue.

Color Blindness & Eye Health

While inherited color blindness can be a burden, it has no consequence on eye health. Individuals with a color vision deficiency can lead perfectly long, healthy lives.

There are acquired color vision deficiencies, however, that are progressive and often signal that disease processes are occurring in the eye(s). Examples are macular degeneration and optic neuritis. The normal yellowing of the crystalline lens, however, can cause changes in color vision, but this is not considered to be a disease and is perfectless harmless.

Detection & Diagnosis of Color Blindness

Inherited color blindness typically doesn't become apparent until a child reaches pre-school and kindergarten ages. It is then that art and painting reveals abnormal color vision. Children are unable to accurately portray a color wheel, or a rainbow, for instance. In such situations, it is the responsibility of the teacher to notify the parents that their child may be color blind.

Figure: Ishihara Color Vision Test. Courtesy of National Eye Institute.

When you visit your optometrist, he/she can perform a test known as the Ishihara test for color blindness. The Ishihara test requires that a patient look at numerous plates that contain hidden numbers or lines that can be seen by a person with normal vision but not by someone with a red-green color vision deficiency. Although there are many different color vision tests, the Ishihara is the most common.

At a young age, it is possible that an accurate test is not possible, in which case the optometrist will want to repeat the test in a few years. When the child is somewhat older, a second test can be performed, which is known as a D-15. This requires the patient to sort different caps containing varying colors by similar colors. The D-15 can pick up on severe color vision difficiencies and the Ishihara can pick up on more subtle red-green deficiencies.

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