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Joseph Carroll, Jay Neitz, Maureen Neitz; Estimates of L:M cone ratio from ERG flicker photometry and genetics. Journal of Vision 2002;2(8):1. doi: 10.1167/2.8.1.
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© ARVO (1962-2015); The Authors (2016-present)
Estimates of L:M cone ratio for males with normal color vision were derived using the flicker-photometric electroretinogram (ERG). These were obtained by best fitting ERG spectral sensitivity functions to a weighted sum of long (L)- and middle (M)-wavelength-sensitive cone spectral absorption curves. Using the ERG, measurements can be made with extremely high precision, which leaves variation in the wavelength of maximal sensitivity (λmax) of the cone photopigments as the major remaining source of inaccuracy in determining the ratio of cone contributions. Here that source of inaccuracy was largely eliminated through the use of individualized L-cone spectral absorption curves deduced from L-pigment gene sequences. The method was used on 62 normal males as part of an effort to obtain a true picture of how normal variations in L:M cone ratio are distributed. The percentage of L cones in the average eye was 65%L [where %L = 100 X L / (L+M)]. There were huge individual differences ranging from 28%–93%L, corresponding to more than a 30-fold range in L:M ratio (0.4–13). However, the most extreme values were relatively rare; 80% of the subjects fell within ±15 %L of the mean, corresponding to a 4-fold range in L:M ratio (1–4). The method remedies major weaknesses inherent in earlier applications of flicker photometry to estimate cone ratio; however, it continues to depend on the assumption that the average L cone produces a response with an identical amplitude to that of the average M cone. A comparison of the ERG results with the distribution of cone ratios estimated from cone pigment messenger RNA in cadaver eyes indicates that the assumption generally holds true. However, there may be a small number of exceptions in which individuals have normally occurring (but relatively rare) amino acid substitutions in one of their pigments that significantly affect the physiology of the cone class containing that pigment, so as to reduce the amplitude of its contribution to the ERG. Consistent with this possibility, extreme cone contribution ratios were found to be associated with atypical L-pigment amino acid combinations.
Each subject’s L-cone spectral sensitivity function was based on the sequence of his L pigment (amino acid positions encoded by: exon 2 – 65, 111, 116; exon 3 – 153, 171, 174, 178, 180; exon 4 – 230, 233, 236). Some subjects had gene variants not seen in any of our deuteranopes; their L-pigment λmax values were estimated using known spectral tuning differences (see text). a. Color matching range from Nagel anomaloscope. b. Estimated peak based on the spectral tuning effect of exon 4 specified substitutions TSV. c. Estimated peak based on the spectral tuning effect of exon 2 specified substitutions VY. d. Estimated peak based on polymorphisms with no known influence on the photopigment spectrum. Single letter amino acid code is T=threonine, I=isoleucine, S=serine, L=leucine, V=valine, M=methionine, A=alanine, and Y=tyrosine.
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