Early scotopic dark adaptation

Abstract

The human visual system can function over a broad range of light levels, from few photons to bright sunshine. I am interested in sensitivity regulation of the rod pathway in dim light, below the level at which the cones become effective. I obtained thresholds for large (1.3 deg) and tiny (5 min arc) circular test spots, either after 20-30 mins exposure to complete darkness (absolute threshold), or on uniform backgrounds of varying light intensities, or just after the background was removed to plunge the eye into darkness; Maxwellian view optics were used so that variations in pupil size would have no effect. The overall theory is that sensitivity in the rod system is adjusted by a slow process of adaptation in the retina driven by the long-term mean background light level and a rapid process sensitive to variability in the light provided by the background ("photon noise"). My data show that thresholds for tiny test spots primarily reveal the photon-driven noise effect, and those for large test spots reveal both processes. The traditional idea that all components of rod dark adaptation are slow is challenged here, since almost immediately after the background light is removed, thresholds of tiny spots drop to absolute threshold directly, and thresholds of large test spots drop half-way to absolute threshold before continuing to recover back to absolute threshold over the next 25 minutes. These initial rapid drops in threshold are accounted for by the postulated photon-variability-driven rapid process, as removing the background removes the noise.