Evidence for the stochastic independence of the blue-yellow, red-green and luminance detection mechanisms revealed by subthreshold summation

We investigated the manner in which the outputs of the three postreceptoral mechanisms (red-green, blue-yellow and luminance) combine to determine contrast threshold. We used a subthreshold summation paradigm to test whether the combination of the postreceptoral mechanism outputs could be described by a probability summation model which assumes stochastic independence of the mechanisms, and determined the best fitting summation exponent. Stimuli were Gaussian enveloped 1 c/d sinusoidal gratings represented in a 3D cardinal space transformed from cone contrast axes, and normalized to detection threshold. The use of this space avoids the presence of elongated threshold contours, allowing a reliable model fit to include the less sensitive blue-yellow and luminance mechanisms. Our results were well fitted by the probability summation model and hence support the underlying stochastic independence of the three postreceptoral mechanisms.