Achromatopsia

Biology 202
1999 Second Web Reports
On Serendip

Marion Howard

When I was young, I was told that "color blindness" did not mean that the person saw the world like an old movie, but rather it meant that they could not distinguish between green and red. I thought that this understanding was very advanced and would quickly share my knowledge with any less-informed children. After looking into the matter, I have been forced to reject this generalization in favor of a broader range of diseases resulting in very different types of inabilities to perceive color in a "normal" fashion. While the typical color blindness I was told about affects 8 percent of men and less than 1 percent of women in the United States (1), there are many other types. The most common types of color blindness, effecting red and green vision, are not too serious for the sufferers, who can function normally and do not have overly impaired vision other than an inability to distinguish between certain colors. There are, however, more serious forms of "color blindness", such as blue cone monochromatopsia, partial rod monochromatopsia, and total rod monochromatopsia (3). The rod monochromats are also known as achromats, meaning they see no color at all. Only about 1/33,000 Americans has this disease, and women and men are effected roughly equally (3). This most severe variety of color blindness has many interesting symptoms which reveal a lot about rod vision.

Achromats have what can be called "night vision" or rod vision. This means that the only functioning receptors in their retina are rod photoreceptors. The cones are either absent entirely or are present but the signals are not being processed for some reason. There are two types of achromatopsia, one is congenital and the other can be caused by brain damage, called cerebral achromatopsia (3). There are also varying degrees of achromatopsia, with some sufferers being able to distinguish some color and others absolutely none. I will focus on the congenital version since cerebral achromatopsia varies in its symptoms and cause quite a bit, as well as being much more rare and so fewer cases have been studied. Congenital color blindness has more regular symptoms which can be used to demonstrate how rod-vision works.

One of the most noticeable things about an achromat that reveals their vision deficiency is sensitivity to light (8). Cone vision is what most people use during the day, since cones have less light sensitivity, meaning they can absorb more light without discomfort, although they too can be overloaded with light (like looking into the sun). Rod vision is not used by those with normal vision until dark parts of the day and in dark places (4). Rods are more sensitive to light, requiring less illumination in order to form an image of the world. The rods of an achromat have normal levels of rhodopsin, which absorbs all wavelengths of light, so they have comparatively better vision at night than during the day (4). The achromat, having only rods, must use rod vision at all times, and since rods are very sensitive to light, they cannot tolerate high or even normal levels of light without discomfort (8). They generally wear dark lenses and broad-brimmed hats as well as habitually staying away from brightly lit places and sitting facing away from light sources to help alleviate the strain on their eyes. Blinking and squinting are the body's natural way of trying to restrict the amount of light, but this is not a very socially acceptable way of living, besides being uncomfortable (6).

While achormats cannot say that they see only white, black, and shades of gray since they have never seen color and have no concept of "gray" (3), it can be demonstrated that he can't see color because of the nature of rhodopsin and their inability to distinguish between colors. An achromat cannot distinguish color because rhodopsin gives a certain response to a given wavelength of light, but by varying the intensity of that light one color can be made to look exactly the same as another. There is not a complex interaction of multiple photopigments such as exists in cone receptors, so a dark red will look like black, or blues and greens will be confused (8). This is an added confusion to achromats since they cannot even depend on a certain shade of gray to correspond to what they have been told is a certain color (8).

Achromats in general have very poor vision, and most are considered legally blind (8). This can be partially explained by exploring the location of the photoreceptors that function in an achromat. Cone photoreceptors are located in the center of the retina, directly behind the iris which can restrict to limit the amount of light admitted into the eye. These receptors are not functioning in the achromat, instead it is the rest of the retina, the peripheral rod photoreceptors, which are used for all vision (3). This allows for only a fuzzy, indistinct view of the world since the normal distribution of photoreceptors required to make distinct pictures is missing.

Some symptoms do not seem to relate as directly to the basic differences between rod and cone vision. For instance, many achromats when they are only a few months old will not show any aversion to light, even though they are missing the cone vision then just as they are as adults. Perhaps this has to do with how the brain processes visual input when a person is very young. There have in fact been many accounts from complete achromats that when they were babies their mothers were frightened to find them looking directly into the sun (8). When the child is a bit older, he may all of a sudden acquire a nystigmus, meaning he moves his eyes back and forth quickly and uncontrollably. The connection between rod vision and a nystigmus is not obvious. Perhaps it is the next stage in the brain making sense of the strange, incomplete vision. It may be something like the nervous system causing the eye to move to try to find where the cones are, where the light can strike to make the picture the brain receives make sense. The nystigmus is much less noticeable in adults, and often goes away completely (3) (8). This may support the hypothesis that the brain is trying to have the picture of the world make sense because eventually the nystigmus goes away, perhaps because the brain has learned to compensate for the lack of cones.

The symptoms of achromatopsia provide an excellent illustration for the functioning of rod vision. In the absence of cone vision, achromats show how rod vision works in all situations. For those with normal vision, cone vision kicks in where rod vision fails, and vice-versa, but by studying achromats, many interesting things can be learned by observing what happens when rod vision is used under circumstances where it is not intended to be the primary mode of vision.

WWW Sources

2)FAQ about Color Blindness

3)Achromatopsia Network

4)Medline article on Achromatopsia

5)Tests for Colorblindness

6)Treatment for Achromatopsia

7)Achromatopsia

8)Vision in a Complete Achromat: A Personal Accountby Knut Nordby

9)NIH list of Abstracts on Achromatopsia