Chromatic Mechanisms Beyond Linear Opponency
The current picture of the human color system consists of a series of stages, each with a number of types of mechanisms functioning in parallel. Successive stages are composed of mechanisms that combine the outputs of the previous stage. An extensive account of the empirical and theoretical foundations of the early stages of the color system is given in Zaidi (1990). In brief, the first stage consists of three independent cone mechanisms, L, M and S, that form the substrate for trichromatic color matches (Maxwell, 1860), and whose spectral sensitivities have been derived by psychophysical (Smith and Pokorny, 1975) and electro-physiological (Schnapf et al, 1987) methods. The second stage consists of two chromatic mechanisms RG and YV, and an achromatic mechanism LD. The RG mechanism signals the difference between the responses of the L and M cones, and the YV mechanism signals the response of the S cones minus a sum of the responses of the L and M cones. The independence of the chromatic signals from these cardinal mechanisms are reflected in habituation (Krauskopf et al, 1982) and motion (Krauskopf and Farrell, 1990) experiments. In addition, parvo-neurons of primate lateral geniculate nucleus cluster into two classes with chromatic properties similar to the two chromatic cardinal mechanisms (Derrington et al, 1984).
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