Cone-specific mediation of rod sensitivity in trichromatic observers.

PURPOSE The slope of the rod threshold versus the illuminance (TVI) function changes with the wavelength of the background light. This study was conducted to determine whether the changes in slope are due to the stimulation of specific cone classes. METHODS An eight-channel optical system was used to generate lights that differed in cone and rod photoreceptor illuminance. Rod flicker TVI functions were measured in normal trichromatic observers at mesopic light levels. The independent variables were (1) the relative contribution of the short (S)- and long (L)- wavelength cones to the background light (i.e., the background lights varied along S-only and L-only lines), and (2) the temporal frequency of the flickering lights (4, 7.5, and 15 Hz). RESULTS The 4-Hz rod flicker TVI function had a slope of 0.87 when measured near W (MacLeod-Boynton chromaticity of 0.66, 1.0). At 4 and 7.5 Hz, an increase in the relative L-cone illuminance steepened the slope of the rod-only TVI curve, but an increase in the relative S-cone illuminance had no effect. The slope of the 7.5-Hz TVI function decreased at higher illuminance levels. At 15 Hz, the thresholds could be measured over only a limited range. CONCLUSIONS The L-cone system contributes to the desensitization of the rod system at mesopic light levels, whereas, in the range of lights used in these experiments, the S-cone system apparently does not. The possibility that S-cone stimulation desensitizes the response to rod signals at higher levels of S-cone illumination cannot be eliminated.