A ratio principle for a red/green and a yellow/blue channel?

There is strong empirical evidence that, under adaptation to another achromatic color stimulus, the lightness of an achromatic color stimulus depends on the ratio of the luminances of the two stimuli. In the present study, the suitability of this ratio principle is tested for two chromatic postreceptoral opponent channels. A Hering red/green channel and a non-Hering yellow/blue channel are specified as chromatic channels. The yellow/blue channel is defined by 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 spanned the whole range of the channel's coordinates. Red/green equilibria or yellow/blue equilibria were measured as excursions along the adaptation axes. For both presumed channels, the ratios of the equilibrium coordinates 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.