Operant tempo varies with reinforcement rate: implications for measurementof reward efficacy

Herrstein's melioration theory has been used to account for the hyperbolic form of the single operant matching law and to scale the effectiveness of r einforcing brain stimulation. Underlying this scaling method is the assumpt ion that the mean rate of responding during operant bouts (the response 'te rnpo') is fixed and does not vary with the rate of reinforcement. The valid ity of this account was assessed by testing the constant-tempo assumption v ia a survivor analysis of the distributions of inter-response times at diff erent variable-intervals (VIs) in rats responding for rewarding electrical stimulation of the lateral hypothalamus. Contrary to the constant-tempo ass umption, response tempo was not fixed but rather decreased as the VI was le ngthened. This demonstration challenges Herrnstein's account of single-oper ant 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 brai n stimulation reward in rats, the inter-response time analysis employed can provide the basis for testing the validity of the constant-tempo assumptio n in other species and for other reinforcers. (C) 2001 Elsevier Science BN. All rights reserved.