Abstract

The relative number of the R (or long-wavelength-sensitive) and G (or middle-wavelength-sensitive) cone types in human retina is not known, since no differential histological stain has been developed for human or any other species. A cumulation of evidence from indirect studies summarized in Vimal et al. (1989) suggests that there may be about twice as many R cones as G cones in human fovea. In particular, the additive model of Smith and Pokorny (1975) with their R and G fundamentals derived from the photometric Vλ function (solid circles) is shown in Fig. 1A. However, those studies were all conducted under light-adapted conditions, where various suprathreshold influences and nonlinearities may have contaminated the results (e.g. Boynton and Whitten, 1970; Guth, 1991). Moreover, at least one of the studies (Rushton and Baker, 1964) was based on flicker photometric matches at high frequency, which we have shown to be contaminated by different temporal properties of the R and G cone systems (Hamer and Tyler, submitted). Moreover, Guth’s (1991) current version of his "vector model" requires the initial weights of the R to G mechanisms to be 0.66:1 rather than 2:1. Although Guth disavows any implication for receptor densities, it is important to note that this rather complete model disagrees with the other evidence under the same assumptions about the relation between psychophysical sensitivity and cone densities.

© 1992 Optical Society of America