Color Blind: Who's to Say?

Color vision is an evolutionary adaptation that has assisted the survival of vertebrates in many ways. From choosing a fit mate to heeding warning signs to finding food, color vision is a neurological property that has many benefits. In considering specifically humans, the question arises as to how color blind individuals view the world around them, and how their condition affects their perception of reality. From our discussions in class, we have discovered that the notion of reality, when it comes to sight, is very subjective. Our brains see a very different picture from that which is taken in through our retinas. One of the most astounding differences is that the world does not have color until the light that it gives off comes through our retinas and is processed within our photoreceptors. With such a difference between the image of the world outside of the brain and inside of the brain, there can be no question that there is at least some variation between individuals when it comes to color. With that said, I find it difficult to classify anyone as color blind. There are so many degrees of color deficiencies added to the inevitable variation between those with “normal” color vision, that too classify color blindness as a disability seems rather ambiguous.

In understanding how the color blind individual sees the world, we must first understand how those with said “normal” vision process color information. Most human color vision is trichromatic, meaning it consists of three variables: red, blue, and green. Coinciding to these three variables are the three types of cone photoreceptor cells within the retina that each contain a different photopigment. These short (S)-, middle (M)-, and long (L)-wave sensitive cones are each receptive to specific (although overlapping) wavelengths of light.(1)

Within the brain, the visual pathway has developed into three subsystems that distinguish light from dark, yellow from blue, and red from green. The system, however, is limited by the number of types of photopigments within the retina. The individual will experience the condition anomalous trichromacy if one of the three cone pigments is altered in its sensitivity, dichromacy if one of the cone pigments is missing, or monochromacy if two or all of the three cone pigments are missing. There are also rare cases in which an individual has four cone photopigments. This results, however, in trichromatic vision, due to the inability of the three subsystems to convey more that three independent color signals.(2)

The number of variations within those classified as colorblind, increases even more when we consider that the type (or types) of cone photopigment affected also contributes to differences among individuals, along with the number of cone photopigments affected. To complicate things even more, we must also consider the fact that cone photopigments can be either altered or completely void. An individual may have protanopia or protanomaly, the void of or abnormality within the L-wave sensitive cones; deuteranopia or deuteranomaly, the void of or abnormality within the M-wave sensitive cones; or tritanopia or tritanamoly, the void of or abnormality within the S-wave sensitive cones. There also exists the case in which an individual is unilaterally color blind, meaning that they have one eye with normal color vision and one that is color deficient (which it could be in a variety of ways). Besides alteration in the retina’s types of cone photopigments there can also be alterations in the components of cone structure and function, which can result in the loss of all cone function, also known as the condition rod monochromacy. (2)

As we can see, there is an incredible amount of variation within the visual pathways of those who are considered color blind. There are the varieties of specific conditions that have their specific names, but within these categories we must also account for the variation among each individual. Now that we have realized this variation among color blind individuals, however, we must take a look at the variation among those considered to have normal color vision.

Each of us has the experience of trichromacy failing us everyday. There is a complete failure when we transition from day to night, as immediate darkness causes the cones to shut off and the rods to take over in processing the images coming through our retinas. We also experience a partial reduction to dichromacy and the loss of the S-cone function when we view small objects centrally, as there are no S-cones located in the central retina. Small objects viewed in the periphery, where there are few S-cones, are also seen only with two dimensions. Another partial reduction takes place when we view a small object immediately after a strong yellow light illuminating it has been turned off, as the yellow illumination polarizes the yellow-blue subsystem.(2) Along with our incomplete trichromatic vision, we must also consider that normal color vision can vary significantly among individuals. There are variations in the densities of photopigments among individuals, along with the densities of the lens and macular screening pigments, which absorb light mostly of short wavelengths.(2) Variation also results from polymorphisms caused by crossing over in the L- and M-cone pigment gene.(2)

With so much variation among those labeled with color deficiencies and those said to have normal vision, where do we draw the line when separating those who are color blind and those who are not? Are you color blind because you have two or fewer cone photopigments? Not exactly, as individuals with anomalous trichromacy are labeled as color blind. Do you have normal vision if you have exactly three cone photopigments? That seems wrong as well, since there are individuals who have four cone photopigments who are still trichromatic. We have discovered first hand that individuals with three cone photopigments experience a failure in there trichromacy everyday. We even saw in class how our brain alters colors subconsciously depending on their illumination, as with the checkerboard example. Color blindness is a fairly rare condition, yet the variation among color blind individuals and those with normal color vision is so great that color blindness as a specific disability becomes extremely subjective. We again return to our notion of reality as the average plus or minus a standard deviation to account for all this variation. We all have a variable perception of reality when it comes to color vision, some individuals, however, just stand farther to the left or right.

References:

1Neitz, Maureen and Jay, “Color Vision Defects,” Jan. 27, 2006 http://www.mrw.interscience.wiley.com/emrw/9780470015902/els/article/a0006000/current/pdf

2Gegenfurtner, Karl R. and Lindsay T. Sharpe Ed. Color Vision from Genes to Perception, Cambridge: Cambridge University Press, 1999, pp.3-29.

Carrol, Joseph et al., “Functional Photoreceptor Loss Revealed with Adaptive Optics: An Alternate Cause of Color Blindness,” Proceedings of the National Academy of Sciences of the USA, Vol. 101, No. 22. (Jun. 1, 2004), pp. 8461-8466, http://www.jstor.org.

Foster, David H. et al., “Parallel Detection of Violations of Color Constancy,” Proceedissg of the National Academy of Sciences of the USA, Vol.98, No. 14. (Jul. 3, 2001), pp. 8151-8156, http:www.jstor.org.