Operant Wheel Running on a Fixed Interval Schedule: Does Contingent Sucrose Reinforcement Increase Running through a Schedule-Induction or Reinforcement Effect?

Sucrose reinforcement increases operant wheel running, but it is not certain that the increase is due to a reinforcement rather than a schedule-induction effect. To address this issue, eight female Long Evans rats were exposed to a fixed-interval (FI) 60-s schedule with wheel running as the operant. Across three conditions, water, sucrose, and water were delivered as outcomes. In addition, rats also ran freely for 20 min prior to, and subsequent to, exposure to the FI 60-s schedule. Results showed that operant wheel running on the FI 60-s schedule increased when sucrose was delivered. At the level of revolutions, over the reinforcement interval more rats showed increased revolutions in the sucrose condition in the latter half of the interval, consistent with a reinforcement effect, than an inverted U-shaped pattern in the initial half of the interval. Running during the pre- and postoperant running period was not substantially affected by the removal of sucrose reinforcement on the FI 60-s schedule. Sucrose reinforcement, but not water, increased running relative to the pre/post running periods. Results suggest that although both schedule-induction and reinforcement effects occur, the effect that dominates appears to be a function of the duration of the reinforcement interval with a reinforcement effect more prevalent with shorter intervals.