Fig. 5. Illustration of the scotopic portion of the increment sensitivity function. Three phases of the scotopic are indicated: the eigengrau or dark light portion in which intrinsic noise limits the threshold, the DeVries-Rose portion, in which noise intrinsic to quantal nature of the stimulus dominates, and the Weber portion, in which adaptation mechanisms control sensitivity. (Barlow HB: Optic nerve impulses and Weber's Law. Cold Spring Harbor Symp Quant Biol 30:539, 1965.)