Pharmacology Biochemistry & Behavior, Vol. 23, pp. 77-83, 1985. ¢ AnkhoInternational Inc. Printed in the U.S.A. 0091-3057/85$3.00 + .00 Rate-Dependency and Hyperactivity: Methylphenidate Effects o n Operant Responding MARK D. RAPPORT, GEORGE J. DuPAUL AND NELSON F. SMITH Department of Psychology, University of Rhode Island, Kingston, RI 02881 Received 29 August 1984 RAPPORT, M. D., G. J. DuPAUL AND N. F. SMITH. Rate-dependency and hyperactivity: Methylphenidate effi,cts on operant responding. PHARMACOL BIOCHEM BEHAV 23(1) 77-83, 1985.--The two most common treatment for hyperactivity are psychopharmacological regimens and behavior therapy. Although the concurrent use of stimulant medi- cation has been purported to enhance a child's rate of responding under a behavior management program, studies examin- ing the interaction of the two treatments have been unable to confirm this hypothesis. The present investigation sought to examine the effects of differing levels of methylphenidate hydrochloride (Ritalin) upon operant responding with hyperac- tive children. After an initial drug-free training period, 10 first through fourth grade hyperactive males performed an operant key-pressing task under a mult VR 5 FI 30 sec reinforcement schedule across four randomly determined, double- blind drug conditions (placebo, 5, 10, 15 rag). Only VR responding changed significantly during medication conditions; however, rate-dependent psychostimulant effects were found within both reinforcement schedules. Discrepancies with animal rate-dependency and implications for treatment and future research are discussed. Medication effects on operant responding appear to depend upon the reinforcement schedule and dose employed. Rate-dependency Hyperactivity Methylphenidate HYPERACTIVITY is currently viewed as an Attention Deficit Disorder (ADD) characterized by impulsivity, a short attention span, and excessive (non-goal directed) activity which is pervasive across settings [1]. The relatively high prevalence rate (i.e., 3-5% of school-age children) and pro- tracted nature of the disorder have generated increasing in- terest in establishing effective treatment modalities. The two most widespread treatments for ADD children have been psychopharmacological regimens and behavior therapy. The former typically involves the prescription of stimulant medication (e.g., methylphenidate, d-amphet- amine, or pemoline), which serves to enhance the atten- tional-arousal processes of a child and results in a less im- pulsive and more focused response style [9]. Behavior therapy has been used in both school and familial settings to control attention [21], social behavior [34], and academic performance [3,26]. The combined use of behavioral techniques and stimulant drugs has become more prevalent in applied settings in an effort to maximize therapeutic outcome. Although several studies have examined the interactions of the two treatments using between and within-subject designs [ 14, 23, 25], none have directly measured the effects of psychostimulants on operant responding. The effects of CNS stimulants on the operant perform- ance of animals has been explained across species via the "rate-dependency hypothesis" [10, 12, 30]. Briefly, this hy- pothesis states that stimulants tend to increase the rate of responding under reinforcement schedules which usually engender relatively low rates of responding (e.g., a high fixed interval schedule), but have limited effects or decrease re- sponding under schedules which bring about high response rates (e.g., a rich variable ratio schedule). Further, there is an interaction between schedule and dose level. Lower doses tend to have the greatest enhancing effect under schedules which control for a low response rate, while higher doses produce greater decrements in response rate under rich reinforcement schedules. Only a few studies have directly examined this hypothesis with human subjects and have shown equivocal results [11, 20, 29, 32, 35]; primarily due to methodological problems, e.g., the failure to measure peak drug effects or the use of both reinforcing and punishing stimuli in the same schedule. Indirect investigations of rate-dependent effects in humans have been more encouraging. For example, Robbins and Sahakian [27] found an inverse relationship between pre- and post-drug activity scores in their re-analysis of several inde- pendent samples of hyperactive children following methyl- phenidate and/or d-amphetamine administration. Their re- sults were remarkably similar to the rate-dependent effects reported in animal studies, and provide a further impetus for investigating rate-dependent phenomena in humans. The purpose of the present study was to: (1) examine the effects of differing levels of methylphenidate on the operant performance of ADD children; and (2) directly test the "rate-dependency hypothesis" with humans by examining the interaction between drug levels and schedule param- eters. Our first prediction was that lower doses of methyl- phenidate (5, 10 mg) would increase responding under a rein- forcement schedule which typically produces a low response 77