There is strong empirical evidence that, under adaptation to another achromatic color stimulus, the lightness of an achromatic color stimulus depends anthe-ratinof the luminances oftha two stimuli. In the present study, the suitability ofthis ratio~principleis~testedfar two chromatic
postreceptoral opponent channels. A Hering redlgreen channel and a non-Hering yellow/blue channel are specified as chromaticchannels. The yellow/blue channel is definedby extrapolating the plane corresponding to unique green-white linearly to the reddish part of color space, using the
plane’s surface as the channel’s equilibria. The experiment was run on an isoluminant plane, measured individually for each observer. Moving along an observer’s measured opponent axes, eight adaptation stimuli were selected for each channel and spanriedthe whole-rangeofth~fnannel’s coordinates. Redlgreen equilibria or yellow/blue equilibria were measured as excursions along the adaptation axes. Forbothpresumedchannels, the ratios ofthe equilibrium coordinatea of test and adaptation stimuli were essentially constant. This supports the principle’s suitability.
However, small asymmetries were found with respect to each channel’s opponent hues. The status of the proposed yellow/blue channel is discussed, as are conditions that might have favored the present findings.