HUMAN CONES APPEAR TO ADAPT AT LOW-LIGHT LEVELS - MEASUREMENTS ON THERED-GREEN DETECTION MECHANISM

Recent physiological evidence suggests that cones do not light adapt a t low light levels, To assess whether adaptation is cone-selective at low light levels, the red-green detection mechanism was isolated, Thre sholds were measured with a large test flash, which stimulated the L a nd M cones in different fixed amplitude ratios, on different colored a dapting fields, Thresholds were plotted in L and M cone contrast coord inates, The red-green mechanism responded to an equally-weighted diffe rence of L and M cone contrast on each colored field, demonstrating eq uivalent, Weberian adaptation of the L and hi cone signals, The L and M cone signals independently adapted for illuminance levels as low as 60 effective trolands (e.g, M-cone trolands). Since this adaptation is entirely selective to cone type, it suggests that the cones themselve s light-adapt, The red-green detection contour on reddish fields was d isplaced further out from the origin of the cone contrast coordinates, revealing an additional sensitivity loss at a subsequent, spectrally- opponent site, This second-site effect may arise from a net ''red'' or ''green'' signal that represents the degree to which the L and M cone s are differently hyperpolarized by the steady, colored adapting field . Such differential hyperpolarization is compatible with equivalent, W eberian adaptation of the L and M cones.