Color vision in two observers with highly biased LWS/MWS cone ratios

Two sisters, heterozygous carriers for congenital X-linked protanopia, were diagnosed as normal trichromats by the Rayleigh match on the anomaloscope. The heterozygous state was established by molecular analysis of their visual pigment genes. The normal color match establishes that the spectral sensitivities of their long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cone visual photopigments are within normal variability. Their FM 100-hue test error scores were low, demonstrating superior chromatic discrimination. Heterochromatic flicker photometric (HEP) spectral sensitivities were like those of protanopes. The estimated LWS/MWS cone ratios from the HFP data were 0.09/1 and 0.03/1, compared with ratios in the range of 0.6/1 to 10/1 for typical normal trichromats. Measurements of chromatic grating acuity on chromatically selective backgrounds were performed to study the cone mosaic. The data were consistent with a sparsity of LWS cones. Both protan carriers showed normal spectral sensitivities for all three cone types under cone isolating chromatic adaptation and normal three-peaked curves for increment thresholds on a white pedestal. Hue estimation, run on one carrier was normal. The equilibrium yellow locus was measured in the other carrier and was in the range of normal trichromats. The data indicate that normal color vision can occur even when the LWS/MWS cone ratio is quite abnormal.