Spectral sensitivity of the foveal cone photopigments between 400 and 500 nm

Abstract

A comparison was made between the shape of the iodopsin absorption spectrum calculated for appropriate optical density to (1) a set of König-type fundamentals in which the tritanopic copunctal point was set on the alychne and (2) data obtained from red-green dichromats using high intensity heterochromatic flicker procedures which eliminated participation by the short-wavelength sensitive mechanism. The transformation of normal color mixture data resulted in two fundamentals which gave a reasonable prediction of the tritanopic coefficients. The dichromatic HFP data corrected individually to average macular pigment agreed with their respective fundamental above 430 nm. The HFP data and transformation were converted to a retinal level, quantized and plotted as a function of wavenumber. For the middle-wavelength-sensitive mechanism, the protanopic HFP data and its König-type fundamental agreed with the predicted absorption spectrum above 460 nm. The deviations below 460 nm had the shape of the lens absorbance curve. For the long-wavelength sensitive mechanism, the deuteranopic data and its König-type fundamental agreed with the predicted absorption spectrum above 520 nm. The deviations below 520 nm could not be fit solely by the lens absorbance factor used above, but needed in addition, added macular pigment of optical density at 460 nm of ca. 0.12. This result was checked by calculating predicted tritanopic coefficients for the two predicted absorption spectra, when the long-wavelength sensitive spectrum was screened by a slight amount (o.d. of 0.12 at 460 nm) of macular pigment. These predicted coefficients agreed with the Wright tritanopic coefficients. We conclude (a) that the shape of the iodopsin absorption spectrum provides a reasonable basis for computation of absorption spectra of the middleand long-wavelength sensitive cone pigments and (b) that long-wavelength sensitive cones of deuteranopes. tritanopes, and normal trichromats are subject to a selective screening filter of optical density at 460 nm of 0.12 and spectral shape similar to macular pigment.