Distinguishing opponent and non-opponent detection pathways in early dark adaptation.

Desensitization of the red-green opponent pathway was demonstrated in early dark adaptation with the aid of a test chosen to isolate that pathway. Isolation was achieved by requiring the observer to adjust the intensity of a foveal test light to the threshold for flicker, when the test alternated slowly between luminance-matched red and yellow fields. The luminance matches were precise enough that only an opponent pathway could mediate the flicker thresholds. Desensitization occurred after continuous light adaptation to 626 nm fields, but did not occur after adaptation to yellow fields, or if the 626 nm field was turned on and off at 2 Hz throughout adaptation. The properties of the red-green pathway measured with the flicker thresholds resemble those of the yellow-blue pathway as shown in transient tritanopla.