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

Recent physiological evidence suggests that cones do not light adapt at low light levels, To assess whether adaptation is cone-selective at low light levels, the red-green detection mechanism was isolated, Thresholds were measured with a large test flash, which stimulated the L and M cones in different fixed amplitude ratios, on different colored adapting fields, Thresholds were plotted in L and M cone contrast coordinates, The red-green mechanism responded to an equally-weighted difference of L and M cone contrast on each colored field, demonstrating equivalent, 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 themselves light-adapt, The red-green detection contour on reddish fields was displaced 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 cones are differently hyperpolarized by the steady, colored adapting field. Such differential hyperpolarization is compatible with equivalent, Weberian adaptation of the L and M cones.