Evidence for the stochastic independence of the blue-yellow, red-green andluminance 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 con trast 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 t he mechanisms, and determined the best fitting summation exponent. Stimuli were Gaussian enveloped I c/d sinusoidal gratings represented in a 3D cardi nal 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 proba bility summation model and hence support the underlying stochastic independ ence of the three postreceptoral mechanisms. (C) 1998 Elsevier Science Ltd. All rights reserved.