Contrast effects of the three primary colors on human visual evoked potentials

In this study we evaluated in humans the question of whether contrast effects with patterned color stimuli varied in the same way as is known to occur with black-and-white stimuli. Using a counterphasing checkerboard pattern, we evaluated the steady-state visual evoked potential (VEP) in 10 subjects for the response to different contrast levels in each of the 3 primary colors. Overall mean luminance of each color was photometrically equated and kept constant during all trials. The VEP was computer-averaged for 90 consecutive 1 sec epochs of stimulation, and the power at the appropriate frequency was calculated. For each color, the contrast-response curves revealed small power values at low contrast (0.1) and larger power at the 3 high-contrast settings (0.3-0.5). Power varied markedly by color and by subject. The shape of the curves, depending on color and subject, often indicated a saturation of response. A given subject commonly had a physiologically 'preferred' color in that the power with that color was consistently larger. Most subjects had definite subjective color 'preferences,' believing that they perceived the contrast better for one or two colors. However, these impressions were often not validated by the VEP responses to the various colors. These results indicate that white-light VEP responses may not necessarily reflect the response characteristics of specific colors, nor do they necessarily reflect the large inter- and intra-subject differences in color responses noted in this study.