A lot of people think, if you suffer from color blindness you can not see any colors at all apart from black, white, and shades of gray. But they are wrong.
This form of complete color blindness is a very rare subtype of color vision deficiency among much more common ones like the the well known red-green color blindness. You might say, in this case the wording color blindness isn’t chosen appropriate—and you’re right.
Color Blindness begins when you don’t have normal human vision which might be only a very light change of color perception. And it ends with complete color blindness combined under the umbrella term monochromacy.
If you are suffering under monochromacy, you perceive everything just in shades of gray. This makes it really hard to accomplish many everyday tasks, because colors are such a central part of our life. For example, people suffering form monochromacy might mix up the following colors:
- green and blue
- red and black
- yellow and white
Monochromacy—also referred to as monochromatism—can be observed in different forms and even worse, many different names are in use:
- rod monochromacy: typical or complete achromatopsia, total color blindness, day blindness
- blue-cone monochromacy: S-cone monochromacy, incomplete achromatopsia
- cone monochromacy: complete achromatopsia with normal visual acuity
- cerebral achromatopsia: atypical achromatopsia
Before we dig a little deeper into the different forms of complete color blindness you should know the basics of vision and specially color vision.
The human eye has two different receptors inside the eye to perceive light. They are called photoreceptor cells. The rods are very sensitive on brightness and used for night vision. On the other side we have three different types of cones (trichromacy) which are responsible for colored day vision. The three different forms of cones have different peaks on color perception: red, green and blue. And the mixture of those three base colors makes us perceive such a huge variety of colors—or at least almost all of us. Compared to cones, rods play no role in color vision.
In this case your vision relies solely on the rods and the cones are usually not working at all. Some cases also report that the cones are absent, have some irregular distribution or that they show abnormal shapes. Occurrences are estimated to be between 1 : 30,000 and 1 : 50,000. But even if this numbers look very small, rod monochromacy is still by far the most common type of complete color blindness.
Rod monochromatism is often referred to as achromatopsia and is based on autosomal recessive inheritance. In other words, you need two defective chromosomes and it is equally distributed among men and women. Achromatopsia is characterized by the following symptoms:
- complete inability to differentiate colors,
- severe light sensitivity (photophobia),
- involuntary eye movements (nystagmus),
- and central depressed vision (scotoma).
Also rod monochromats are completely colorblind, they often learn to associate certain colors with objects and to differentiate some colors by their brightness.
This rare form of monochromatism is caused by loss or rearrangement of the genes encoding L- and M-cones. Because of that only blue color receptors (S-cones) and rods are transmitting color and/or brightness information. If you suffer S-cone monochromacy your are usually completely colorblind. But in twilight situations—where rods and S-cones are working—color perception can be compared with dichromatic vision.
There are reports about very different forms of blue-cone monochromacy and it is sometimes not easy to differentiate it from rod monochromacy. No exact number of incidences is available. Scientists estimate the frequency at 1 : 100,000—for men. Because red and green cones are encoded on the sex chromosome, inheritance patterns are similar to red-green color blindness. Occurrences in women are unknown.
Also known as complete achromatopsia, this form is very rare. Only a few cases have been reported and none of them is fully accepted. It is assumed that they are either have working L- or M-cones and inactive or absent S-cones. Compared to blue-cone or rod monochromacy, cone monochromacy doesn’t have reduced visual acuity or any special light sensitivity.
In this case optical examinations show normally functioning cones and rods. It is not understood in detail yet, but it is thought that the color receptors are working properly only the information is not reaching the brain or is not processed. Only a few cases have been investigated. Cerebral achromatopsia is unlike the other types of monochromacy not inherited but and acquired color vision defect. It may be caused by trauma or illness.
The academical background and many references on all types of monochromacy can be found in Opsin genes, cone photopigments, color vision, and color blindness by Sharpe et al. An internet information portal on achromatopsia is provided by the The Achromatopsia Network.