“Paradoxical” effects of psychomotor stimulant drugs in hyperactive children from the standpoint of behavioural pharmacology

Abstract

Following a brief review of the effectiveness of psychomotor stimulant drugs in treating childhood hyperkinesis, the hypothesis of a “paradoxical” drug response is examined. The importance of three main factors, besides dosage, derived from consideration of studies in behavioural pharmacology. is assessed: rate-dependency; attention and stereotypy; and subjective and social effects. Rate-dependent effects of drugs in animals occur when the control rate of responding is related to the drug effect. A dose-dependent inverse relationship between control rate and drug effect is generally found within individual cases responding in operant situations, after treatment with stimulant drugs. This rate-dependency also occurs between organisms, such that those organisms with high control rates (for example, of operant behaviour, or spontaneous motor activity) show proportionately smaller increases in rate, or reductions in rate with amphetamine-like drugs. This article extends this form of analysis to hyperactive children in two populations receiving methylphenidate. In a third population, of normal adults and normal children, as well as hyperactive children, an inverse relationship between control rate of activity and the effects of 0.5 mg kg of d-amphetamine is found, accounting for about 38% of the variability. Therefore, the rate-reducing effects of psychomotor stimulants might arise in part as a consequence of the high rates of control responding in the hyperactive child, rather than from a unique “paradoxical” response. The effects of stimulants on “attention” in hyperactive children similarly might be predicted from studies on normal adults. Some evidence of an “over-focusing” of attention in hyperactive children treated with methylphenidate is presented in a test of cognitive flexibility. This lack of cognitive flexibility might be related to behavioural stereotypies found in animals and Man following stimulants. Finally, many of the subjective and social responses to stimulants in hyperactive children are not necessarily anomalous. The implications for clinical practice and for animal models of the hyperkinetic syndrome are discussed.