Predicting chromatic sensitivity in normal trichromats and in subjects with congenital deficiency

Abstract

AbstractPurpose Differences between M‐ and L‐class variant pigments arise largely because amino acid substitutions in M‐class pigments contribute less to the corresponding shifts in spectral responsivity. Other factors such as the relative numbers of L and M cones, their optical density and the midpoint between their spectral peaks can also contribute to the subject’s overall, red/green (RG) chromatic sensitivity. The purpose of this study was to examine how these differences affect chromatic sensitivity in normal trichromats and in subjects with deutan‐ and protan‐like deficiency.Methods RG thresholds were measured in 269 deutans, 132 protans and 330 normal trichromats using the CAD test. The colour vision of every subject was also examined using the Nagel anomaloscope. Classification into normal, deutan and protan classes was based on the results obtained on CAD and anomaloscope tests.Results RG thresholds measured within each group were ranked in increasing order. Samples equal to the number of subjects within each group were taken from a single Gaussian distribution, in the case of normal trichromats, or from more than one distribution in the case of deutan and protan groups. The distribution parameters were optimised to predict the rank order of the measured RG thresholds in each group.Conclusion The rank order of thresholds measured in normal trichromats can be predicted by a single Gaussian distributions. Deutans produced the most complex rank order which could only be predicted adequately with a minimum of four Gaussians. In contrast, the rank order for the protan group was much simpler and could be predicted well with only two or at most three Gaussian distributions.