Peripheral chromatic sensitivity for flashes: a post-receptoral red-green asymmetry

Thresholds of luminance and red-green chromatic flashes (200 msec) were measured on a yellow adapting field in the fovea and periphery (up to 12 degrees eccentricity for 1 degree flashes and 21 degrees eccentricity for 2 degrees flashes). Chromatic sensitivity (in cone contrast coordinates) is about 7 times higher than luminance sensitivity in the fovea but falls faster with eccentricity, so that luminance and chromatic sensitivities are similar at eccentricities of 20 degrees or less. At eccentricities greater than about 14 degrees, there is a clear asymmetry wherein green chromatic flashes are considerably less detectable than red ones. By measuring complete detection contours for many ratios of incremental and decremental red and green flashes, we isolated the red and green chromatic detection mechanisms, and demonstrated that the red-green asymmetry is not a property of the L- or M-cone response per se, but rather is a property of the post-receptoral, chromatic mechanisms. The peripheral luminance and chromatic mechanisms could be further separated with a suprathreshold luminance flash (a pedestal), since an intense pedestal masks coincident luminance test flashes but facilitates the chromatic flashes. The luminance pedestal approximately linearizes the chromatic detection function (the psychometric function).