Dependence of contrast detection and independence of AM and FM detection on retinal illuminance.

Thresholds for the detection of contrast differences ("amplitude modulation", AM) and spatial frequency differences ("frequency modulation", FM) present in suprathreshold sine-wave gratings were measured as a function of mean retinal illuminance. These thresholds are found to be virtually independent of mean retinal illuminance, despite the fact that the threshold contrasts of the gratings do depend significantly on illuminance.