Temporal discrimination of alternate days in rats.

The objective was to determine whether rats could learn to time a 48-h interval. Rats (n = 6) were continuously housed in operant chambers in constant darkness. The feeding cycle consisted of unlimited access to food for 6 h, followed by 42 h without access to food (i.e., meals were available on alternate days, contingent on breaking a photobeam in the food trough). Head entries into the trough increased as a function of time prior to the meal; this increase was higher, relative to the increase that occurred at the same time of day on alternate (i.e., nonfood) days. These data suggest that rats discriminated alternate days. Next, two meals were omitted to dissociate mechanisms of a self-sustained endogenous rhythm, interval timing, and alternation. Response rate increased periodically every 24 h, which suggests that the rats anticipated alternate days by discriminating the status of the previous day as a meal or a nonmeal day.