Effects of delayed reinforcers on the behavior of an animal model of attention-deficit/hyperactivity disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD), affecting 3-5% of grade-school children, is a behavioral disorder characterized by developmentally inappropriate levels of inattention, hyperactivity, and impulsivity. It has been suggested that the symptoms are caused by altered reinforcement and extinction processes, behaviorally described as an abnormally short and steep delay-of-reinforcement gradient in ADHD. The present study tested predictions from the suggested shortened and steepened delay gradient in ADHD in an animal model, the spontaneously hypertensive rats (SHRs). It was predicted that SHR responding during baseline would mainly consist of responses with short inter-response times, and that responding would be more rapidly reduced in the SHR than in the controls by the introduction of a time interval between the response and reinforcer delivery. Effects of a resetting delay of reinforcement procedure with water as the reinforcer were tested on two baseline reinforcement schedules: variable interval 30 s (VI 30 s) and conjoint variable interval 60 s differential reinforcement of high rate 1s (VI 60 s DRH 1 s). The results showed a higher rate of responses in the SHR than in the controls during baseline, mainly consisting of responses with short inter-response times. The statistical analyses showed that response rates decreased more rapidly as a function of reinforcer delay in the SHR than in the controls. The analyses of the estimates of the reinforcer decay parameter showed no strain differences during the VI 30 s schedule but showed a significant strain difference at the end, but not at the start, of the sessions during the VI 60 s DRH 1 s schedule. In general, the results support predictions from the suggested steepened delay gradient in SHR. However, the predictions were only partly confirmed by the analyses of the decay parameter.