Variability in Cone Populations and Implications

Abstract

There have been a number of techniques directed toward estimating the relative numbers of the three cone types in the retinae of living humans. For the S-cones, psychophysical measurements which are dependent on the spatial distribution of S-cones (Stiles, 1949; Wald, 1967; Williams et al., 1981) give data which are consistent with the relative sparcity observed anatomically (Ahnelt et al., 1987; deMonasterio et al., 1985; Marc and Sperling, 1977). For typical color-normal observers, the L-and M-cones are both in relatively high spatial density, and the direct psychophysical techniques which are effective for characterizing the retinal distribution of S-cones do not yield interpretable information (Brindley, 1954; Green, 1968; Kelly, 1974). Additional techniques in the literature which have been suggested as capable of revealing L/M cone ratios include Weber fractions for mechanisms isolated by chromatic adaptation (Vos and Walraven, 1971) and retinal densitometry with chromatic bleaching lights (Rushton and Baker, 1964). In this paper we wish to describe our work on two techniques we have used to estimate the L/M cone ratio, spectral sensitivity from heterochromatic flicker photometry and point source detection for lights of varying wavelength. We then consider the perceptual consequences of individual variation in L/M cone populations.