Photoreceptor Light Adaptation : Untangling Desensitization and Sensitization

The illumination at the earth's surface varies by >10 orders of magnitude during the normal day–night cycle, and the vertebrate visual system covers this entire range of light intensities with two neuronal subsystems that rely on the activity of two types of photoreceptor cells, rods and cones. Human rod vision operates over approximately seven decimal orders of illumination. The cone visual system operates over an even wider range (Rodieck 1998). Light adaptation occurs at all levels of the visual system, from photoreceptors to central neurons. Yet, the function of the entire visual system depends on the ability of the photoreceptors themselves to adjust their sensitivity to the ambient lighting situation. Thus, photoreceptors must generate reliable signals at night when single photons are captured between long intervals of darkness, and must also continue to signal at the very...