The activation of cones in scotopic and rods in photopic vision.

Over the whole course of dark adaptation, light- and color detection thresholds were measured. At 6 degrees in the nasal visual field, the effect of wavelength was examined. After the rod-cone break (Kohlrausch kink), the color threshold began to rise while the light threshold was progressively decreasing. The rise of the color threshold was more remarkable as the test wavelength was more close to the peak wavelength for rod sensitivity. With green test light, the sensitivity difference of retinal position was investigated. The rise of the color threshold was more prominent at the retinal position where rods were more densely packed. Finally, the Stiles-Crawford effect was examined with a green test light at 12 degrees in the nasal visual field. This effect appeared for the color threshold over the whole course of dark adaptation but not for the light threshold even before the rod-cone break. It was concluded that the rod activation interferes with cone sensitivity and the interference effect becomes stronger as rods become progressively more dark adapted, and that the rod system is also acting even before the rod-cone break.