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K. Mancuso, M. Rodriguez–Carmona, J. Neitz, J.L. Barbur, M. Neitz; The Resilience of Chromatic Sensitivity to Reduction in the Spectral Separation of L and M Pigments . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2664.
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© ARVO (1962-2015); The Authors (2016-present)
Red–green color vision loss is normally attributed to a reduction in the spectral separation of L and M photopigments. Many unexplained examples remain in which color vision is superior to what might be predicted from information about the underlying pigments. Individuals with severely abnormal Rayleigh matches often exhibit only mild color deficiency in other tests. Similarly, some single–gene dichromats, predicted to have no red–green color vision, can exhibit remarkable residual chromatic sensitivity. In an effort to resolve apparent discrepancies, the relationship between chromatic sensitivity and photopigment complement was examined in normal and color–deficient subjects.
Nagel matches were determined and chromatic sensitivity was measured using the Colour Assessment and Diagnosis (CAD) test in 225 normal trichromats and 250 color–deficient observers. Twenty–three subjects from this group were selected for genetic analysis to determine the spectral separation between the L and M pigments.
There was a predictable, non–linear relationship between chromatic sensitivity and the separation of L and M pigments. Color thresholds changed little over most of the range of L to M spectral separations. Compared to the average spectral difference between normal L and M pigments, significant chromatic sensitivity was observed even for large reductions in L to M separation. For example, anomalous trichromats with a spectral separation of 10 nm (> 66% reduction) exhibited red–green chromatic thresholds that were only twice as large as an average normal trichromat’s. Large loss of chromatic sensitivity was only observed when the L to M separation was less than 3 nm. Residual red–green vision exhibited by individuals with separations close to zero was not correlated with photopigment complement. Presumably, small differences in optical density, post–receptoral gain, and other factors predominate in determining residual color discrimination when middle–to–long wavelength sensitive cones do not measurably differ in peak wavelength.
Red–green chromatic sensitivity exhibits a nonlinear relationship with photopigment proximity; observers do not exhibit color deficiency until L to M spectral separations are markedly reduced. Except for spectral separations near zero, the observed relationship can be predicted by a simple calculation of the differences between two lights required to achieve a threshold difference in pigment absorptions.
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