The extrafoveal spectral sensitivity function was measured during dark adaptation at different intensity levels above the cone plateau of the long-term dark-adaptation curve using both flicker and heterochromatic brightness-matching techniques. During most of the cone-plateau period, the spectral sensitivity function was found to be photopic in form at all the intensity levels employed. In the dark-adapted state, the two psychophysical techniques appeared to measure different processes. Thus, the flicker technique yielded a spectral sensitivity function which was basically scotopic in form at all the intensity levels employed while the spectral sensitivity function obtained with the brightness technique was basically photopic in form. It is suggested that, in a dark-adapted state, both rods and cones contribute to the brightness response at each wavelength over the major portion of the spectrum for a long transitional intensity range when the brightness technique is used. The flicker technique, on the other hand, appears to single out the rod activity.
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