Physiology & Behavior, Vol. 35, pp. 395-403. Copyright© PergamonPress Ltd., 1985. Printedin the U.S.A. 0031-9384/85$3.00 + .00 A Microcomputer-Based Method for Physiologically Interpretable Measurement of the Rewarding Efficacy of Brain Stimulation KENNETH A. CAMPBELL, 2 GARY EVANS 3 AND C. R. GALLISTEL Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104 Received 13 August 1984 CAMPBELL, K. A., G. EVANS AND C. R. GALLISTEL. A microcomputer-based method for physiologically interpre- table measurement of the rewarding efficacy of brain stimulation. PHYSIOL BEHAV 35(3) 395-403, 1985.- Determination of the function relating rate of pressing to the number of pulses in a train of fixed duration (the rate- frequency function) yields a physiologically interpretable measure of changes in the rewarding efficacy of the stimulation, because the number of action potentials in the reward-relevant first stage axons is directly proportional to the number of pulses in the train. We describe a system, based on a low cost microcomputer, which permits determination of 16-data- point rate-frequency functions in 4--6 animals simultaneously in less than 10 minutes. We give an empirical and theoretical justification for using the curve-shift measurement procedure in drug and lesion work, where the experimental treatments must be presumed to have substantial effects on performance factors. Microcomputer Brain-stimulation-reward Quantitative methods Performance-vs.-reward IDENTIFICATION of the neural substrate for brain stimu- lation reward requires a behavioral measurement technique that satisfies two requirements: it should distinguish changes in rewarding efficacy from changes in other factors that af- fect the performance of the rewarded behavior; and it should yield quantitative estimates of the effect on rewarding effi- cacy that can be related to physiological measures. The re- warding effect of stimulation is the level of reward that it produces. By contrast, the rewarding efficacy of stimulation is its capacity to produce a criterial level of reward. Meas- ures of changes in this capacity reveal quantitative charac- teristics of the neural substrate that carries the rewarding signal from the electrode to the site where the rewarding effect is realized [10]. Behavioral measurements of elec- trophysiological, anatomical and pharmacological properties provide quantitative data about the substrate, against which to compare the results from electrophysiological, anatomical and neurochemicat measurements. It has long been recognized that changes in the rate of responding do not provide a suitable measure of changes in the rewarding efficacy of the stimulation [13,23]. Firstly, changes in the rate may occur in response to changes in performance factors---the motor, perceptual and motiva- tional processes that determine how an underlying rewarding effect maps into an observed performance. Secondly, even if a change in the rate could be ascribed to a change in reward- ing efficacy, the magnitude of the behavioral change would not measure the magnitude of the underlying physiological change, that is, the magnitude of the change in the neural signal that generates the rewarding effect. One could not conclude that a drug that reduced the rate of responding by a factor of two reduced the rewarding efficacy of stimulation by a factor of two. Most of the behavioral methods that have been suggested for distinguishing non-specific performance effects from ef- fects on reward in drug and lesion studies yield at best a qual- itative answer to the question whether the drug or lesion has altered the rewarding efficacy of the stimulation [6, 11, 21]. These methods do not attempt to measure the magnitude of the underlying physiological effect. Measurements of the self-stimulation current threshold [4] come closer to a quan- titative approach. However, any measure that looks at only a single fixed level of performance may confound changes in performance factors with changes in rewarding efficacy. An overall depression or elevation of performance will produce an artifactual depression or elevation in the amount of stimu- 1This research was supported by NSF Grant BNS 82 11972 to CRG, to whom requests for reprints, copies of the disks, circuit diagrams, and other documentation should be addressed. The authors are grateful to Gwen Freyd and Elysa Braunstein for the data used in Figs. l and 3. 2Now at Department of Physiology; Bowman Gray School of Medicine; Winston-Salem, NC 27103; supported as a postdoctoral fellow on NIMH Training Grant T32 MH15092, during the period of this work. 3Now at Simple Solutions; 4108 Pavilion Place; Durham, NC 27707. 395