Analysis of red/green color discrimination in subjects with a single X-linked photopigment gene

Many subjects despite having only a single X-linked pigment gene (single-L/ M-gene subjects) are able to make chromatic discriminations by Rayleigh mat ching, especially when large fields are used. We used a combination of psyc hophysics (Rayleigh match), electroretinograms (ERG), and molecular genetic techniques to rule out several possible explanations of this phenomenon. U se of rods for chromatic discrimination was unlikely since strong adapting fields were employed and the large-held match results were not consistent w ith rod participation. A putative mid- to long-wavelength photopigment that escapes detection by current molecular genetic analysis was ruled out by f inding only a single L/M photopigment in flicker ERGs from 16 single-L/M-ge ne subjects. Large-field match results were not consistent with participati on of S cones. Amino acid sequence polymorphisms in the S-pigment gene that might have shifted the S cone spectrum towards longer wavelengths were not found on sequencing. The mechanism of chromatic discrimination in the pres ence of a single photopigment therefore remains unknown. Further possible e xplanations such as variations in cone pigment density and retinal inhomoge neities are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.