Sites of sensitivity control within a long-wavelength cone pathway.

A flashed-field increment threshold paradigm was used to examine sites of sensitivity control within a long-wavelength cone pathway. The data were fit with a model containing two static nonlinearities, one at the receptors and the second at a red/green opponent stage. The nonlinearities are modified by multiplicative and subtractive processes of adaptation. Comparisons of the model's parameters with physiological measures of the long-wavelength cones suggest that, in the dark, sensitivity is controlled by the opponent site. At high adapting intensities, receptor nonlinearities may limit sensitivity under some conditions. The data also suggest that the spectral tuning of the opponent site varies with adapting intensity.