Detection of quantum flux modulation by single photopigments in human observers

Abstract

The action spectra of single cone photopigments of trichromatic human observers were measured by a modification of the exchange threshold technique. The quantum flux of two superimposed 2° fields, 540 and 640 nm. was adjusted to the inverse ratio of the sensitivity of either chlorolabe or erythrolabe at these wavelengths, so that one of the photopigments made a constant quantum catch. The 540 and 640 nm lights were flickered sinusoidally in counterphase above the flicker fusion frequency of the blue cones so that when the quantum fluxes were equated for chlorolabe. only erythrolabe could detect the alternation, and vice versa. This flickering field was superimposed upon a 12° background which was variable in both quantum flux and wavelength. At any wavelength between 410 and 690 nm the log of the reciprocal of the number of quanta required to extinguish the flicker of the 540/640 field described the log sensitivity at that wavelength of the only photopigment capable of detecting the flicker. The log sensitivity curves thus obtained were consistent with other measures of the cone photopigments of dichromatic and trichromatic human observers.