The role of the retina in color vision

With my library still in boxes in the US, I had to spend the day refreshing my old brain cells with information from Wikipedia. In 1913, Sir William Abney published "Researches in Colour Vision and the Trichromatic Theory." After a little over a century, much of what he says remains the same, while there have been dramatic changes in other areas, especially neurology and genetics.

While the names of changed, Sir William Abney defines the eight layers of the retina. What he calls the "peculiar layer"  is the the layer of amacrine cells. The following diagram shows just the retina layer of the eye:

While the rods play an important role in low light conditions (scotopic vision), they have no known role in color vision (photopic vision). The cones require higher luminosity, before they respond. In the above illustration, the cones are divided into the Long (red), Medium (green), and Short (blue) wavelength cones. The ratio of Long:Medium:Short is 40:20:1. While this ratio is a good statistical average, the actual ratio varies across the human population.

The rods and cones are both photoreceptor cells. The shapes of the cells match their names, as seen in the following diagram:

The light sensitive protein lies between the disks in the rods,  and the folds in the cones. In rods the protein is an amino acid chain called rhodopsin. In cones, the amino acid chain is dopsin. The dopsin surrounds the chromophore, which is the pigment that distinguishes color. Most explanations leave out the chromophore, but it is the light filter, not the opsin. It is the chemical reaction in the chromophore that triggers the opsin. Thus, what distinguishes the types of cones is the chemical composition of the chromophore.

Teleost fish, birds, and reptiles are tetrachromatic. These species have cones with chromophore that detect Ultraviolet, Short (blue), Medium (Green), and Long (Red) wavelengths. Placental animals are dichromatic in that their cones detect Medium (Green) and Short (blue) wavelengths. Primates, including humans, developed trichromatic vision. It is possible that gene duplication resulted in a chromophore that detects Long (red) wavelengths. This certainly explains the similarity in their wavelength curves.

For short (blue) wavelength cones the chromophore DNA sequence is on chromosome 7. This placement makes the gene sequence sex independent. The medium (green) and long (red) chromophore DNA sequences appear in contiguous regions on the X chromosome.

The following statement from Hereditary Ocular Disease summarizes the genetic issue for red-green color blindness:

"Red-green color perception is based on gene products called opsins which, combined with their chromophores, respond to photons of specific wavelengths. The OPN1LW and OPN1MW genes reside in a cluster with a head-to-tail configuration on the X chromosome at Xq28. Red-green color vision defects are therefore inherited in an X-linked recessive pattern. There is a single gene for the red cone opsin but there are multiple ones for the green pigment. Only the red gene and the immediately adjacent green pigment gene are expressed. All are under the control of a master switch called the locus control region, LCR.

These DNA segments undergo relatively frequent unequal crossovers which can disrupt the color sensitivity of the gene products so that red-green colorblindness in some form is the most common type of anomalous color vision. It is found in approximately 8% of males and perhaps 0.5% of females."

The same article provides the following definitions:

• Protanopia - only blue and green cones are functional (1 percent of Caucasian males)
• Deuteranopia - only blue and red cones are functional (1 percent of Caucasian males)
• Protanomaly - blue and some green cones are normal plus some anomalous green-like cones (1  percent of Caucasian males)
• Deuteranomaly - normal blue and some red cones are normal plus some anomalous red-like cones (5 percent of Caucasian males)

My questions are what exactly are the green-like and red-like cones, and how do the alter the response to different wavelengths? Even though there are unanswered questions, the above should help provide a better understanding of the color codes generated in my Colorblind Simulator app.

I leave you with a thought. Just as there are genetic variations that produce the diversity in the physical appearance of the human population, there could be genetic variations in the dopsin structures that determine color vision. Every human eye could be uniquely different. Israeli research has shown that each of us an olfactory fingerprint that is unique to every person. Why not our sense of color?

In 1913, Sir William Abney lived at a time when little was known about neurology and genetics. I enjoy reading old medical books, because they show how much the world has changed in a 100 years. While they didn't know about DNA sequences, they certainly knew about the genetic expression of color blindness.