Tuning of cortical color mechanisms revealed using steady-state visually evoked potentials.

Color information is thought to enter the cortex via two dominant retinogeniculate pathways, one signaling teal to red, and the other violet to lime color variation. The cortex is thought to transform this representation, but the properties of human cortical color mechanisms are not very well understood. In four experiments, we characterized the tuning of cortical color mechanisms by measuring the intermodulation of steady-state visually evoked potentials (SSVEPs), thought to index the extent to which shared neural resources process stimuli flickering at different frequencies. Stimuli were isoluminant chromatic checkerboards where odd and even checks flickered at different frequencies. As hue dissimilarity between odd and even checks increased, the amplitude of an intermodulation component (I₁) at the sum of the two stimulus frequencies decreased, revealing color tuning functions. In Experiment 1, we found similar broad tuning functions for "cardinal" and intermediate color axes, implying the action of intermediately tuned cortical color mechanisms. In Experiment 2 we found similar tuning functions for "checkerboards" without perceptible edges because the checks were formed from single pixels (~0.096°), implying that the underlying neural populations do not rely on spatial chromatic edges. In Experiment 3 we found consistent color tuning functions across check sizes. In Experiment 4 we measured full 360° tuning functions and found results compatible with opponent color responses. The observed cortical color tuning functions are consistent with those measured using psychophysics and electrophysiology, implying that the method is useful for investigating color representations in the brain.