Color Discrimination in Carriers of Color Deficiency

¶ … Color Discrimination in carriers of color deficiency

Hood, S.M., Mollon, J.D., Purves, L., Jordan, G. (2006) Color discrimination in carriers of color deficiency. Vision Research 46: 2894-2900 is an article on chromatic discrimination in carriers of the X-inactivated chromosome. The investigators examine the recurrent claim that discrimination is impaired in some carriers of color deficiency and argue that carriers of deficiencies of the long-wave-sensitive cones (protan carriers) must be considered separately from carriers of deficiencies of the middle-wave-sensitive cones (deutan carriers). The authors cite references to indicate that the photopigments of the long-wave-sensitive (L) cones and the middle-wave-sensitive (M) cones are encoded by a small array of genes on the q arm of the X-chromosome.

According to Hood et al., to understand why a difference between protan and deutan carriers might be expected, it is necessary to consider two independent and stochastic events that influence the ratio of L. And M. cones in the retina of a female subject. First, random X-chromosome inactivation determines which X-chromosome will be expressed in a given cone cell A second stochastic event later determines which type of photopigment gene (L or M) is actually expressed by the favored X-chromosome: this latter event is thought to be the probabilistic binding of an upstream locus control region to the promoter region of one of the genes in the array. From this arise differences in the ratios in L. And M. cones between protan and deutan carriers. This led the authors to hypothesize that there are differences in red-green color discrimination between protan and deutan carriers.

They used the MacLeod -- Boynton chromaticity diagram on normal male and female controls and proton and deutan carriers and discovered that protan carriers had a finer degree of discrimination matched to normal controls over deutan carriers, thus confirming their hypothesis. They conclude that this does not indicate pathological vision on the part of deutan carriers, just impaired color discrimination. They also speculate on whether women have more delicacy in color discrimination and answer tentatively in the negative to this question.

The article was well-written and understandable, and given the context provided by the authors, the research question was appropriately addressed. The materials and methods were adequately explained so that I could understand what exactly they did and the data was interpreted in sound fashion. The graphs showed clearly how deutan carriers differed in chromatic discrimination from protan carriers and normal controls. The results were discussed well and referred back to the hypothesis. The significance of the study was also addressed in the results section with reference to vision of deutan carriers and color deficiency in X-inactivated chromosome carriers.