SINGLE-PHOTON DETECTION BY ROD CELLS OF THE RETINA

At low light levels, the visual system detects and counts photon absor ptions with a reliability close to limits set by statistical fluctuati ons in the number of absorbed photons. Thus the rod photoreceptors tha t provide the input signals to the dark-adapted visual system act as n early perfect photon counters. This elegant performance is possible be cause light detection in the rods satisfies four functional requiremen ts: high quantum efficiency, sufficient amplification to produce a mea surable response, low dark noise, and low trial-to-trial variability i n the elementary response. The rod meets these requirements using bioc hemical reactions rather than the solid-state reactions of silicon det ectors, yet its performance equals or exceeds that of man-made detecto rs in several ways. [S0034-6861(98)00903-9].