Scotopic Spectral Sensitivity and the Absorbance of Visual Pigments in Isolated Visual Cells and in the Intact Retina

Abstract

In the study of scotopic spectral sensitivity and the absorbance of visual pigments, it has been generally assumed that the absorption properties of the visual pigments in the intact retina are the same as the absorption properties of the visual pigments when extracted into aqueous solution. Most data regarding the absorbance of visual pigments have been taken from extracts and these findings have been found to be in general agreement with scotopic function (Granit, 1947; Dartnall, 1953; Crescitelli & Dartnall, 1953). However, with more accurate determinations of the absorbance of visual pigments, a discrepancy between scotopic spectral sensitivities and the absorbance of the visual pigments in solution has become apparent, in that the scotopic spectral sensitivity is often found to be greater at longer wavelengths than would be suggested by the absorbance of the visual pigment. For example, in the human, the scotopic spectral sensitivity has a maximum at 497 nm (Crawford, 1949) whereas the visual pigment, extracted in digitonin, has a maximum absorbance at 493 nm (Wald & Brown, 1958). These discrepancies raise the question as to whether the assumption that the absorbance properties of visual pigments in solution are the same as the absorbance properties of the visual pigments in the intact visual cell is justified.