Rod–cone-interactions in deuteranopic observers: models and dynamics

We studied the interactions between rods and L-cones in deuteranopic human observers by stimulating the photoreceptors independently. Thresholds were determined using a PEST procedure for different ratios of rod to L-cone modulation without modulating the S-cones. Modulation frequency was either 2 or 10 Hz and the retinal illuminance ranged from 4.7 to 470 td (10.9-1090 scot td). We measured at 2, 7.5 and 20 degrees retinal eccentricity. The threshold data could be described by a model based on a vector addition of responses originating in the rods and the L-cones. The relative strength of rod signals relative to the L-cone signals increased with increasing retinal eccentricity and decreasing retinal illuminance. At 20 degrees eccentricity, rod and cone signals were of about equal magnitude at retinal illuminances as high as 470 td. Temporal frequency did not have a large effect on the ratio of rod to L-cone signal strength.