What limits chromatic sensitivity in normal and colour deficient observers?

Abstract

Abstract Purpose Variability in chromatic sensitivity was assessed in normal trichromats (n=133) and in subjects with deutan (n= 106) and protan (n=48) deficiency. The purpose of this study was to quantify variability within normal colour vision and to determine how this impacts the assessment of minimum colour vision requirements within occupational environments. Methods Rayleigh matches were modelled with the aim of explaining the locations of match midpoints and ranges. Model parameters included the wavelength of peak sensitivity of cone photopigments, the photopigment optical density and the noise amplitude in the red/green colour channel. In order to avoid the suprathreshold, perceptual effects of extreme L:M cone ratios on colour vision, selective postreceptoral amplification of cone signals is needed. The noise is also amplified and this causes corresponding changes in red/green threshold sensitivity. Results We measured midpoint and matching range in anomaloscope matches and red/green and yellow/blue thresholds under conditions that isolate the use of colour signals. A subset of subjects with deutan deficiency that exhibited less common Nagel matches were selected for genetic analysis of their cone pigment genes to predict the corresponding peak wavelength separation of their two, long‐wavelength cone pigments. Conclusion The model predicts accurately the midpoint and the range for the spectral differences specified by the genes. The model also predicts the “normal” matches made by some subjects that rely on two hybrid genes and therefore exhibit red/green thresholds outside the normal range, typical of mild deuteranomaly. The anomaloscope is therefore not by itself sufficient to classify with certainty the colour deficiency involved.