Changes in the human visually evoked cortical potential in response to chromatic modulation of a sinusoidal grating

Human visually evoked cortical potentials (VECPs) were recorded from 4 subjects in response to the counterphase alternation of an equal brightness chromatic grating pattern. The pattern was constructed from 2 monochromatic sinusoidal gratings registered 180 deg out of phase. Eleven wavelengths from 450 to 650 nm (at 20 nm intervals) were used. Each wavelength was paired in the grating stimulus with every other to produce a total of 55 different chromatic gratings. The chromatic modulation depth (contrast) of each grating was varied and resulting VECPs were recorded. VECP amplitude was found to vary linearly with log chromatic modulation depth. VECP threshold values were inferred by extrapolation of linear regression lines to zero VECP amplitude. Chromatic modulation sensitivity functions were derived and a multidimensional scaling analysis of the data for each subject was performed. The data were adequately described by a two-dimensional geometric configuration of the 11 wavelengths used. The configurations were similar in shape to those obtained psychophysically by Butler and Riggs (1978) Vision Res. 18, 1407-1416, who used a similar stimulus pattern. Their shapes are consistent with an opponent-color model of color vision.