The adaptation properties of a compartment system

The fact that our senses adapt is one of the most important characteristics of our sensory system. Since ambient light varies over such a large dynamic range, adaptation is an important aspect of vision in all species. In the vertebrate retina, Weber-Fechner adaptation can be observed at the level of the photoreceptor itself. Using the ergodicity theorem from statistical mechanics, it has been possible to analyze the adaptation properties of the compartment model of Lamb et al. (1981) and to predict the value of Io, the background intensity at which the sensitivity is reduced by one-half. It is simply equal to N, the number of compartments in the outer segment of rod and cones, divided by T, the time constant of biochemical process limiting the recovery of the response. For the cones, the number of compartments is set equal to the number of folds (disks) in the outer segment, for the rods, an empirical rule has been found. The decay of the photocurrent after a small flash has been used to estimate the time constant of the limiting process. The predictions for Io of rods and cones are generally in good agreement with experimental values obtained over the years.