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 |CL − CM| opponency of L- and M-cone contrast using a fixed CL/CM 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 CL/CM-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 CL/CM weighting ratios for precortical cells in the parvocellular pathway.