The idea that the first stage in normal daylight vision is characterized by three processes having different spectral sensitivities was established early in the 19th century by Young(1) and linked clearly to experimental data on color matching by Maxwell.(2) Hering's(3) notion of opponent processes was grounded in facts of color appearance. A number of "opponent process" theories were developed to account for color appearance, in which the signals from the receptors were combined to form two channels conveying chromatic information and a third carrying achromatic information.(4) More recently psychophysical, as opposed to phenomenological, evidence has been advanced in support of the existence of opponent mechanisms.(5-12) While these articles provided evidence for the interaction of cone signals, they did not determine in detail the weights of the contributions of the different classes of cones to the second stage (and higher-order) mechanisms. The work discussed here was intended to determine the weights of the second-stage mechanisms by psychophysical means.(13) Quantitative reanalysis of the results of habituation experiments demonstrated the existence of higher-order mechanisms that respond to the sums of rite signals from second-stage mechanisms.(14) Diverse evidence from other psychophysical experiments including detection and discrimination of chromatic pulses,(14) and coherence of plaid patterns,(15) confirmed the existence of higher-order mechanisms. Single unit recording from striate cortex in monkey revealed cells having the spectral sensitivities implied by the psychophysical results.(16) Electrical recording of responses in monkey lateral geniculate nucleus (LGN) revealed single units with weights corresponding to the cardinal directions psychophysically determined.(17) LGN cells show no sign of fatigue but units in VI do.(18) They also exhibit the wide range of spectral sensitivities required of higher-order mechanisms. Studies of the influence of the chromatic consent of stimuli on vernier acuity and motion thresholds and on temporal modulation sensitivity support the conjecture that, while chromatic stimuli are processed solely by the parocellular system, luminance stimuli are processed by both magno- and parvocellular neurons. The variation of the relative sensitivity of the L and M cones with temporal frequency, together with analysis of the statistics of color appearance of small, brief near threshold monochromatic stimuli and data fr-om Stiles's increment threshold experiment(19) suggest that the numerosity ratio of L:M cones in the normal human fovea is more likely to be 1:1 than 2:1. (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S2-S11, 2001.