Visual evoked potentials for reversals of red-green gratings with different chromatic contrasts: Asymmetries with respect to isoluminance

Human visual evoked potentials (VEPs) were recorded for abrupt 6.25-Hz reversals of 2 c/deg square-wave gratings combining red-green contrast with different levels of luminance contrast. Response characteristics-amplitudes and peak latencies as a function of luminance contrast-were compared for four different pairs of red-green colors and an isochromatic yellow grating. For each of the red-green color pairs, the plots of VEP amplitudes and latencies were nonsymmetrical with respect to isoluminance. The amplitude dropped to a minimum within a region of rapid phase change, at a different contrast for each color pair but always at a luminance contrast for which the greener color had the higher luminance. When the contrast-response curve for each of the four red-green pairs was modeled by a simple \C-L - C-M\ opponency of L- and M-cone contrast using a fixed C-L/C-M weighting ratio of about two, there was a close correspondence between the contrast giving a null in the modeled response and that giving a minimum in the VEP amplitude. So for the stimulus parameters applied here, the reversal VEP appeared to be dominated by L/M-opponent response contributions for which the signed C-L/C-M-cone weighting ratio was close to a value of minus two rather than to a value of minus one, which is characteristic of the psychophysical red-green detection mechanism and representative of C-L/C-M weighting ratios for precortical cells in the parvocellular pathway.