Comparison of hue differences measured by perception versus visual evoked potentials

Despite the continuous variation in wavelength or cone excitations, colors tend to group into a small number of categories, and the ranges of these categories can vary widely. For example, “yellow” occupies a relatively narrow range of wavelengths or stimulus angles in a linear cone-opponent space compared to “green” or “blue.” The nature of the size differences between color categories and where they arise in visual processing remains poorly understood. We compared the magnitude of hue differences as measured behaviorally or with steady-state Visual Evoked Potentials (VEPs), recorded from a single electrode placed on Oz. The stimuli were hue angles of equal chromatic contrast in a scaled LvsM or SvsLM cone-opponent plane, and differed over a range from 0 to 50 deg (corresponding to hues ranging roughly from blue-green to yellow-green for the green set and yellow-green to red for the yellow set). Psychophysical measurements confirmed that the difference between two hue angles that straddled an observer’s unique green is perceptually smaller than the same angular difference around their unique yellow – i.e. equivalent stimulus differences in the green region appear more similar. For VEPs the hue angles were presented as 1 c/deg gratings (from gray to the maximum chromaticity) in a tapered 5 deg window. The two angles forming a pair were alternated at a rate of 6 images/sec, with the response amplitude extracted from the 6 Hz component. Responses increased with increasing difference in the alternated angles, but unlike the perceptual responses, the amplitudes were similar for the differences in the green and yellow regions. These results are consistent with the perceptual range of different hue categories arising at later stages of cortical color processing, beyond primary visual cortex.