Saturation and purity of near-threshold spectral increments on achromatic backgrounds

Vector models of color vision predict that the chromatic saturation of near-threshold incremental stimuli should grow with increasing achromatic adaptation. In contrast, colorimetric models predict that saturation should decrease with increasing adaptation in correspondence with excitation purity. An experimental test disagrees with both predictions, but is consistent with a two-process model of chromatic adaptation and contrast. Foveal monochromatic test stimuli 0.6 log unit above threshold were superposed on achromatic backgrounds with retinal illuminances of up to 5.3 log Td. Saturation and purity both decreased as background illuminance increased up to 2 log Td, but the saturation decrease was relatively small. As background illuminance increased above 2 log Td, purity remained nearly constant and saturation often increased, making the overall saturation function U-shaped. Thus, a purity of 0.1 at high background illuminance could sometimes elicit a higher saturation estimate than a purity of 0.9 at low background illuminance.