Rod-cone interaction in the dark-adapted fovea.

The foveal cone threshold was significantly lower after 45 min of dark adaptation than it was near the start of the cone plateau of the dark-adaptation curve. A concentric rod background subsequently raised the threshold by an amount correlated wit the difference between the cone plateau and the dark-adapted thresholds. Paradoxically, the rod background also lowered the cone threshold by an amount that differed from subject to subject. This sensitizing effect was identifiable by its relatively small variability across sessions. These results show that adaptation of parafoveal rods by either real light or dark light can change foveal cone thresholds.