Effects of varying reinforcement schedule, reward current, and pretrial priming stimulation on discrete-trial performance for brain stimulation reward

The present study determined the effects of independently varying the reward current, the fixed ratio reinforcement schedule, and the amount of pretrial priming stimulation on reward summation functions collected in a discrete-trial self-stimulation paradigm. Rats were trained to respond eight times to receive a single train of brain-stimulation reward following the pretrial delivery of 10 trains of priming stimulation. The number of pulses in the reward train was decreased after every 12 trials, and reward summation functions were derived, relating response speed to reward strength. These reward summation functions were shifted laterally by varying the reward current or the fixed-ratio reinforcement schedule. In contrast, decreasing the priming yielded a transient decrease in the asymptotic response speeds and failed to produce enduring lateral shifts in the reward summation functions. Thus, the effects of changing the reinforcement schedule were not likely due to changes in priming. Rather, the schedule of reinforcement appeared to interact with the neural signal produced by the rewarding stimulation to determine the magnitude of the rewarding effect.