Abstract
Spontaneously hypertensive rats (SHR) have been promoted as animal models of attention-deficit hyperactivity disorder (AD/HD). Free-operant experiments have compared the effects of simple schedules, extinction, delayed reinforcers, and non-contingent reinforcement on the behaviour of SHR and Wistar Kyoto (WKY) controls. Differences between SHR and WKY performance have been paralleled with the overactivity, impulsiveness, and inattention that characterize AD/HD. Re-examination of selected studies suggests that differences in these strains’ overall response rates can exaggerate other differences between SHR and WKY performance. Removing this confounding factor can reduce differences between these two strains’ behaviours.