The Neural Control of Vision

I. Adaptation and Afterimages

Solar illumination on earth ranges over about ten log units. To contend with this enormous range, several solutions have evolved to maintain high sensitivity over this entire range. It is often thought that the pupil is very important for keeping the amount of overall light entering the eye relatively constant. However, the pupil can control light over a range of only 16 to 1. The second mechanism for maintaining high sensitivity is light adaptation that for the most part occurs at the level of the photoreceptors. The third solution to this problem was the creation of two types of photoreceptors, the rods that are for night vision and the cones that are for day vision. The importance of rods is made evident by the fact that humans that do not have them cannot see at night and hence have to get special drivers licenses that do not permit night driving. Animals that lack rods, like squirrels, have to retire to bed early.