Phototransduction: Adaptation in rods

The ability of our eye to function over the enormous range of light intensities that we experience throughout the cycle of night and day is due, in part, to our possession of a duplex system of rod and cone photoreceptors, but primarily to the ability of each of these systems to adapt its performance to the ambient light intensity. The scotopic (rod) system operates in photon-counting mode at starlight intensities. At moonlight intensities, light adaptation occurs at a postreceptoral level, while at twilight intensities, the rods themselves adapt. At any higher intensities, the rods saturate, and vision is mediated exclusively by cones.