Abstract
In three experiments, access to wheel running was contingent on lever pressing. In each experiment, the duration of access to running was reduced gradually to 4, 5, or 6 s, and the schedule parameters were expanded gradually. The sessions lasted 2 hr. In Experiment 1, a fixed-ratio 20 schedule controlled a typical break-and-run pattern of lever pressing that was maintained throughout the session for 3 rats. In Experiment 2, a fixed-interval schedule of 6 min maintained lever pressing throughout the session for 3 rats, and for 1 rat, the rate of lever pressing was positively accelerated between reinforcements. In Experiment 3, a variable-ratio schedule of 20 or 35 was in effect and maintained lever pressing at a very stable pace throughout the session for 2 of 3 rats; for 1 rat, lever pressing was maintained at an irregular rate. When the session duration was extended to successive 24-hr periods, with food and water accessible in Experiment 3, lever pressing settled into a periodic pattern occurring at a high rate at approximately the same time each day. In each experiment, the rats that developed the highest local rates of running during wheel access also maintained the most stable and highest rates of lever pressing.