Color and luminance detection and discrimination asymmetries and interactions.

We investigated the nature of color and luminance processes under threshold and suprathreshold conditions in normal trichromatic observers. Detection and discrimination contours as well as threshold-vs-contrast (Tvc) functions were measured in the Derrington-Krauskopf-Lennie (DKL) color space using a masking paradigm. Such contours revealed substantial threshold asymmetries along the three cardinal axes for excursions of opposite polarity along a single axis (e.g. "red" vs "green"). The detection threshold asymmetry was significant for the "blue" and "yellow" (P < 0.05) and luminance increments and decrements (P < 0.01). For suprathreshold discrimination contours the polarity of these asymmetries reversed but remained significant for "blue" and "yellow" (P < 0.001) and luminance increments and decrements (P < 0.01). No significant differences were found between the "red" and "green" cardinal axes under either condition. The discrimination contours also indicated that suprathreshold performance had variable masking along the different axes. A characteristic Tvc curve was found in all cardinal directions except "yellow". The Tvc for "yellow" differed from the other cardinal directions by showing no masking after the initial facilitation and by giving a greater saturating response as a function of contrast. We considered whether the state of retinal adaptation had any role in producing the asymmetries.