Motivational shift: Effects of unrestricted and restricted feeding on wheel running as an operant and reinforcing consequence on a multiple schedule of reinforcement

Rats were exposed to a multiple schedule on which they pressed a lever on a variable-ratio schedule for wheel-running reinforcement in one component and ran a fixed number of revolutions for 15% sucrose solution reinforcement in the other component. Feeding was varied from restricted to unrestricted (ad-libitum) and then returned to restricted. Across these changes in feeding, average body weight varied from 262 g to 339 g to 258 g. Under ad-lib feeding, wheel-running and lever-pressing rates decreased while postreinforcement pause duration increased in the wheel-running reinforcement component. In the operant wheel-running component, wheel-running rates also decreased and postreinforcement pause duration increased. Notably, wheel-running rates in the operant running component were higher than in the wheel-running reinforcement component when rats were food restricted, but did not differ when rats were freely fed, indicating a reduced effectiveness of sucrose reinforcement. This reduction in response strengthening of operant wheel running by sucrose under ad-libitum feeding is consistent with a shift from food-related to intrinsic motivation of running. From a response deprivation perspective (Allison & Timberlake, 1974), this reduced effectiveness of sucrose reinforcement would occur if wheel running in freely fed rats became the more constricted behavior in the contingency between instrumental wheel running and contingent sucrose consumption.