Individual variability in steady-state VEP responses for hues sweeping around cardinal color axes: Clues to cortical color coding?

We investigated how early human visual cortex processes color by analyzing individual variability in steady-state visual evoked potentials (SSVEPs). Sixteen participants viewed a flickering checkerboard that swept around the isoluminant hue circle at three chromatic contrasts. The current study analyzed the individual variability in the SSVEP data from the study to elucidate the hue-selective mechanisms in the early visual areas using a factor-analytic approach. The initial analyses of the correlations revealed that the responses to the nearby hues correlated highly, which is consistent with multiple overlapping color channels. Also, the correlational pattern showed consistent peaks and troughs at specific hue angles: 0° (+L-M), 30°, 120°, 180° (-L+M), 240°, and 300°. We further performed nonmetric multidimensional scaling, identifying four significant hue dimensions. Peaks and troughs of the dimension components were consistent with the hue angles revealed in the correlational pattern. Additional four hues also appeared in the last dimension: 90° (+S), 150°, 270° (-S), and 330°. The 10 (six plus four) hues suggested in these analyses may subserve the basis of early cortical color processing, including classical cone opponency and the mechanisms tuned to the intermediate hues.