The Spectral Sensitivity of the Goldfish and the Clawed Toad Tadpole Under Photopic Conditions*

Abstract

The visual pigments of the vertebrate retina fall into two major classes, depending on whether they are based on vitamin A₁ or vitamin A₂. The rod pigment in the former case is rhodopsin, and in the latter case porphyropsin, and the scotopic sensitivities of the animals agree well with the absorption spectra of the relevant pigment. Later work led to the view that the photopic sensitivities of animals also fall into the same two classes: those animals with visual pigments based on vitamin A₁ are maximally sensitive at about 560 mμ, and those with pigments based on vitamin A₂ at about 620 mμ (Granit, 1941, 1947). The first type of photopic sensitivity can be roughly fitted by the absorption spectrum of the pigment iodopsin, the second by the absorption spectrum of the pigment cyanopsin (Wald, Brown & Smith, 1953, 1955). The difference between the rods and cones within each group is due to the opsin part of the molecule.