Systematics of human photopic spectral sensitivity

Chromatic contrast sensitivity: the role of absolute threshold and gain constant in differences between the fovea and the periphery

“…The loss of isolation of the chromatic mechanisms in the periphery at retinal illuminances high enough for isolation of chromatic mecha-nisms in the fovea may explain why some previous studies have failed to maintain isolation of chromatic mechanisms in some peripheral locations. 35 A number of previous studies 19,36,47 have suggested that in the fovea the isolation of the chromatic mechanisms from the luminance mechanism is a consequence of differences in the gain constants of the mechanisms rather than differences in the absolute thresholds of the mechanisms. Our data confirm this finding in the fovea and extend the finding to the periphery.…”

“…This finding is consistent with studies that have reported that in the fovea the isolation of the chromatic mechanisms from the luminance mechanism declines as retinal illuminance decreases. 5,15,16,19,47 In the periphery, chromatic thresholds are lower than achromatic thresholds at retinal illuminances greater than 0.5 log td. Thus the retinal illuminance necessary for isolation of chromatic mechanisms is higher in the periphery than at the fovea, and the loss of isolation of the chromatic mechanisms reported in the fovea 5,15,16,19,47 will occur more rapidly as retinal illuminance declines in the periphery.…”

“…5,15,16,19,47 In the periphery, chromatic thresholds are lower than achromatic thresholds at retinal illuminances greater than 0.5 log td. Thus the retinal illuminance necessary for isolation of chromatic mechanisms is higher in the periphery than at the fovea, and the loss of isolation of the chromatic mechanisms reported in the fovea 5,15,16,19,47 will occur more rapidly as retinal illuminance declines in the periphery. The loss of isolation of the chromatic mechanisms in the periphery at retinal illuminances high enough for isolation of chromatic mecha-nisms in the fovea may explain why some previous studies have failed to maintain isolation of chromatic mechanisms in some peripheral locations.…”

“…This decline in sensitivity, which is sometimes referred to as Sloan's notch, 63 is often considered to represent the neutral point for the red-green mechanism. 20,62 A number of studies have shown that the depth of Sloan's notch decreases in peripheral locations, 23,35,47,56 indicating that the relative sensitivity of the photopic mechanisms varies in the periphery. The larger decline in the sensitivity with eccentricity for the red-green mechanism relative to the luminance mechanism is consistent with the suggestion that the depth of Sloan's notch is a measure of the differential sensitivity of the red-green and luminance mechanisms.…”