The influence of cones on rod saturation with flashed backgrounds

The increment threshold for a middle-wavelength test flash was measured at the onset of a concentric long-wavelength background flash under conditions that have previously been shown to result in rod system saturation. The influence of the cone system on rod saturation under these conditions was assessed using the Stiles-Crawford effect in normal subjects and by measuring rod thresholds in protanopes, who are deficient in long-wavelength cones. When the background flash is made less effective for cones through the Stiles-Crawford effect, the onset of rod saturation occurs at a higher luminance of background flash than normal. Similarly, protanopes do not show the characteristics of rod saturation until a much higher-than-normal luminance of background flash. The results suggest that rod system saturation with flashed backgrounds is strongly influenced by cones.