Dynamics of chromatic adaptation in cones of freshwater turtle.

The closer the wavelength of a steady background of monochromatic light is to the peak sensitivity of a cone that is being illuminated, the stronger is the desensitization of that cone; this is chromatic adaptation. A model of the freshwater turtle retina with the neural components of chromatic adaptation via negative feedback circuits is used to simulate and study various aspects of chromatic adaptation. An internal negative feedback circuit resides solely within the cone pedicle and thereby, its adaptive effects are relatively specific, so that univariance is maintained. The cone-L-horizontal cell circuit is an external negative feedback circuit and its adaptive effects are less specific since all 3 chromatic cone types are involved, so that univariance is violated. Chromatic adaptation is the result of the decrease in the cone gain due to the dependency of the gains of the negative feedback circuits on the mean illuminance level. The results of the model are consistent with von Kries law, but the changes in gains of the cones due to chromatic adaptation are dependent on wavelength, intensity of the adapting light and size.