Operant tempo varies with reinforcement rate: implications for measurement of reward efficacy

Herrnstein's melioration theory has been used to account for the hyperbolic form of the single operant matching law and to scale the effectiveness of reinforcing brain stimulation. Underlying this scaling method is the assumption that the mean rate of responding during operant bouts (the response 'tempo') is fixed and does not vary with the rate of reinforcement. The validity of this account was assessed by testing the constant-tempo assumption via a survivor analysis of the distributions of inter-response times at different variable-intervals (VIs) in rats responding for rewarding electrical stimulation of the lateral hypothalamus. Contrary to the constant-tempo assumption, response tempo was not fixed but rather decreased as the VI was lengthened. This demonstration challenges Herrnstein's account of single-operant matching and suggests that the reinforcement rate that supports a half-maximal rate of responding on a single VI schedule may not provide a valid scale for the value of brain stimulation. Possible remedies are discussed. Although the conclusions of the study are restricted to experiments on brain stimulation reward in rats, the inter-response time analysis employed can provide the basis for testing the validity of the constant-tempo assumption in other species and for other reinforcers.