Interocular suppression in chromoluminance patterns measured with SSVEP

Many cells in the early human visual system respond to either chromatic or luminance contrast or a combination of both. In addition, depending on their location in the visual hierarchy, these cells may receive input from either one eye or both eyes. It is well understood that spatial luminance contrast patterns undergo binocular normalization: Inputs from each eye mutually suppress each other so that monocular and binocular percepts appear similar. Recent reports suggest that interocular normalization computations may depend on spatial and temporal frequency. Here, we examined the effect of chromaticity and spatial frequency on binocular normalization computations using a dichoptic frequency-tagged, steady-state visually evoked potential (SSVEP) paradigm. We found that normalization as indexed by changes in eye-tagged input SSVEP frequencies and intermodulation terms depends significantly on both spatial frequency and color. We also found that binocular combination must occur in neurons that carry half-wave rectified signals due to 1F combination frequencies being present. Overall, our results are not well explained by a model in which neurons that code low spatial frequency color are segregated anatomically in the centers of ocular dominance columns. Significant levels of binocular interaction must occur in neurons that code both color and luminance and in neurons sensitive to both low and high spatial frequencies.