Increment-threshold spectral-sensitivity (ITSS) functions and threshold-versus-intensity (tvi) curves were measured under white-light adaptation in rhesus monkeys. The tvi curves showed shape and test wavelength invariance, implying that three cone mechanisms were mediating detection. In general, the results were in agreement with the differential adaptation hypothesis proposed by Stiles that predicted spectral shape invariance of the cone mechanisms but overall changes in the shape of the spectral-sensitivity function with increases in the intensity of the adapting field. The principal changes occurring in the ITSS function as the level of adaptation increased involved a smaller loss in sensitivity of the short-wavelength and the long-wavelength peaks compared with the corresponding loss in sensitivity of the middle-wavelength peak. A three-channel model with an opponent L–M mechanism and a nonopponent L–M mechanism (both with S-cone input) and an independent S-cone mechanism described the ITSS data as well as other increment-threshold and suprathreshold data. The model values for the ITSS functions, along with parameters derived from the transformation of these data to cone-contrast coordinates, permitted the factoring out of first-site adaptation, second-site adaptation, and the relative strengths of contribution of the L and M cones within the opponent and nonopponent L–M channels.
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