Human scotopic sensitivity is regulated postreceptorally by changing the speed of the scotopic response

Sensitivity regulation enables the visual system to function effectively from the absorption of a few photons at the lowest visual threshold to the absorption of enough photons to bleach nearly all the light-sensitive photopigment in the eye. Here, we investigate sensitivity regulation in the rod (or scotopic) range from -3.8 to -0.8 log(10) scotopic trolands. Over most of this range, the rate of photon absorption per rod is too low for sensitivity regulation to be practicable within the rod photoreceptor itself, so that regulation must occur postreceptorally. We measured adaptation-dependent changes in visual sensitivity and visual delay, which together provide a much more complete characterization of the effects of light adaptation than the usual method of measuring sensitivity changes alone. Our results demonstrate clearly that changes in scotopic sensitivity with increasing light levels are achieved in large part by a speeding up of the scotopic response and a decrease in the temporal integration time. Thus, the scotopic and the photopic systems both regulate their steady-state sensitivity using the same strategy, even though the scotopic system does it largely postreceptorally and the photopic system largely receptorally.