Circadian variations in time perception in rats.

The hypothesis that time perception is determined by an internal clock's rate, which is sensitive to physiological circadian changes was examined. This hypothesis suggests that when body temperature and activity increase, clock rate increases causing overestimations of time and a decrease in time interval production. The following experiments examine this hypothesis with rats. Since rats are nocturnal, and body temperature is highest at night, time estimation should be longer and time production shorter than during the day. All experiments were run 24 hr a day under a 12:12 light:dark cycle. In the first experiment, rats discriminated among stimulus durations. Consistent with the hypothesis, both rats reliably judged standard durations as "longer" during the night than during the day. In Experiment 2, fixed-interval (FI) schedules of 60, 120 and 180 sec were used to examine circadian variations in time production. All seven rats produced longer postreinforcement pause (PRP) durations as FI length increased, and shorter PRP durations at night than during the day. However, absolute differences in PRP durations between day and night did not increase as FI duration increased, suggesting the PRP durations alone do not directly characterize proportional changes in internal clock rate. To more directly examine circadian variation in clock rate and the production of shorter intervals (i.e., between 1 and 16 sec) Experiment 3 used a lever holding procedure. Since this procedure requires the measured lever hold duration to meet or exceed the required hold duration, it more accurately characterizes the timing process.(ABSTRACT TRUNCATED AT 250 WORDS)