Matching rod percepts with cone stimuli.

Traditional methods for studying the effects of rod activity on color vision make it hard to assess the underlying physiological mechanisms. In this study, rod-mediated changes in color appearance were assessed by matching them with cone-mediated color changes. A four-primary photostimulator allowed independent control of rod and cone stimulation and identification of the cone types that generate color sensations equivalent to rod color sensations. The results showed that increases in rod stimulation required matches with cone stimuli that excited M-cones more than L-cones for all conditions. Matches for low-luminance conditions also required some S-cone stimulation. A subsidiary experiment showed that increases in rod modulation of an inducing field produced chromatic contrast effects like those produced by the M-cone system. The data are consistent with a hypothesis of perceptual normalization of scotopic vision to the chromatic appearance of objects under photopic conditions.