One of the most remarkable features of the vertebrate retina is its ability to adapt to ambient illumination. One can read from this page by candlelight or one can read from it on a sunny afternoon, but the page always appears white and the letters black. It is the molecular trickery of light adaptation that enables photoreceptors to evade saturation and accurately report relative contrast even under extreme illumination conditions. Photoreceptors reduce their sensitivity in direct proportion to the level of background illumination, a relationship known as Weber's Law (for review see Dowling, 1987). Without adaptation, even the black ink in these letters would reflect enough sunlight to drive photoreceptors into saturation. Outdoor activities, such as reading on a park bench, would be impossible.
Thirty years of thorough investigations into the biochemistry and physiology of phototransduction have shown how flashes or step increments of...
