Human rods are acting in the light and cones are inhibited in the dark

The light- and color detection thresholds were measured at three different states of retinal adaptation, i.e., light- and dark adapted states and the intermediate state which was obtained just before the Kohlrausch kink in the course of dark adaptation. At 20 degrees from fixation in the nasal visual field, the wavelength function was examined. The color threshold was higher in the dark adapted state than in the state just before the kink, and the amount of increase was more prominent when the wavelength of the stimulus light was closer to the peak wavelength of rod sensitivity. It was concluded that the rod activation interferes with the cone sensitivity, and that the interference effect becomes stronger as a function of the responsiveness of rods. On the other hand, the spectral sensitivity curve determined in the light adapted state could be divided into two parts. The curve for shorter wavelength than 500 nm was well fitted by the scotopic luminosity curve, suggesting that the light threshold for shorter wavelength is determined by the rod system even in the light adapted state.