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Winocur, G., & Hasher, L. (1999). Aging and time-of-day effects on cognition in rats. Behavioral Neuroscience, 113, 991-997.


Old age is accompanied by a disruption of diurnal biological rhythms that affects a wide range of behavioral processes. In humans, age differences in cognitive function are often greatest in the afternoon when the activity cycle of old people is at a low point. The present study investigated the importance of the time at which testing occurs on for age differences in learning and memory in an animal model. Groups of old and young rats were entrained to a 12-hour, light-dark schedule and administered tests of delayed alternation and inhibitory avoidance conditioning, at the beginning or end of their high-activity cycle. Measures of water-intake confirmed differences in circadian rhythmicity between the young and aged rats. Apart from normal age differences in test performance, the behavioral results demonstrated that old, but not young, rats were affected by the time of testing. In both tasks, old rats tested late in the activity cycle performed significantly worse than old rats tested early in the cycle, under conditions that challenged memory processes that are known to involve the hippocampus. The results indicate that circadian disription in old age can adversely affect memory and related cognitive function with important implications for inhibitory control.

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