Sensitivity and spectral tuning of the red-green chromatic pathway in heterozygous carriers of congenital colour vision defect

We measured sensitivity and spectral tuning of the red–green chromatic pathway in 20 women heterozygous for congenital colour vision deficiencies, and 13 age-matched normal individuals. Spectral increment thresholds from 510 to 660 nm were measured in 30 nm steps, for a 4.8 test superimposed on a contiguous 4000 K, 3.7 log td white pedestal. Stimulus intensity was varied with temporal 2AFC staircases, and maximum likelihood estimation was used to derive thresholds. There was no difference between normal individuals and carriers either for clinical colour vision tests or for spectral increment thresholds. The carriers appear to have normal sensitivity and spectral tuning for the red–green chromatic system, despite heterogeneous expression of cone photopigment genes. This is consistent with the assumption that the primary effect of heterogeneity in the cone mosaic on sensitivity of the midget ganglion cell mosaic is to reduce the number of ganglion cells mediating detection.