A novel, associative process modulating photic resetting of the circadian clock

Appropriate timing of physiological and behavioral processes requires that the circadian clock be reset daily by salient cues in the environment, part icularly light. It is known that the ability of light to reset the clock de pends both on its intensity and on the circadian time when it is applied (D aan and Pittendrigh, 1976; Moore-Ede et al,, 1982), Here we show that the a bility of a weak light stimulus to reset the clock is dramatically enhanced when it is presented daily at the same circadian time, Equivalent daily pr esentations of this light stimulus, but at different circadian times each d ay, do not lead to such enhancement. These findings suggest that the abilit y of light to reset the clock can be modified through a novel, and previous ly unrecognized, conditioning-like associative process in which circadian t ime serves as the conditioned stimulus and light as the unconditioned stimu lus. The idea that circadian time can serve as a conditioned stimulus to modulat e the effectiveness of light provides a new perspective on the lasting impa ct that light schedules have on the circadian clock and, thus, may have imp lications for existing models of photic entrainment (Pittendrigh and Daan, 1976; Moore-Ede et al,, 1982), (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.