Tuning of cortical color mechanism revealed using steady-state visually evoked potentials
Color information is thought to be received by the primary visual cortex via two dominant retinogeniculate pathways, one signals color variation between teal and red, and the other signals color variation between violet and lime. This representation is thought to be transformed in the cortex so that there are a number of different cell populations representing a greater variety of hues. However, the properties of cortical color mechanisms are not well understood. In four experiments, we characterized the tuning functions of cortical color mechanisms by measuring the intermodulation of steady-state visually evoked potentials (SSVEPs). Stimuli were isoluminant chromatic checkerboards where odd and even checks flickered at different frequencies. As hue dissimilarity between the odd and even checks increased, the amplitude of an intermodulation component (I1) at the sum of the two stimulus frequencies decreased, revealing cortical color tuning functions. In Experiment 1 we found similar broad tuning functions for cardinal and intermediate color axes, implying that the cortex has intermediately tuned color mechanisms. In Experiment 2 we found similar broad tuning functions for checkerboards with no perceptible edges because the checks were formed from single pixels ([~]0.096{degrees}), implying that the underlying neural populations do not rely on spatial chromatic edges. In Experiment 3 we manipulated check size and found that color tuning functions were consistent across check sizes used. In Experiment 4 we measured full 360{degrees} tuning functions for a cardinal cortical color mechanism and found evidence for opponent color responses. The observed cortical color tuning functions were consistent with those measured using psychophysics and electrophysiology, implying that tracking intermodulation using SSVEPs provides a useful method for measuring them.