The effect of delay, utility, and magnitude on delay discounting in an animal model of ADHD

Abstract

The delay discounting paradigm involves choosing between a small, immediate reinforcer (SS) or larger, delayed reinforcer (LL). Children with ADHD tend to choose the SS reward more often than controls, which is interpreted as a display of impulsivity. Studies on an animal model of ADHD, the Spontaneously Hypertensive Rat (SHR), show the same pattern, with SHR showing a preference for the SS reinforcer. However, it is not entirely clear why this pattern exists. It has been proposed that ADHD children tend to be delay averse, i.e. that the time between response and reinforcer is something they opt to avoid. An alternative hypothesis is that ADHD children struggle to see the long-term utility of their choices. We reviewed data from eight SHR studies on delay discounting and investigated which hypothesis was the best predictor of LL preference. Results found that SHRs and controls do not differ in overall performance on the delay discounting task, regardless of whether the dependent variable is delay between response and reinforcer, magnitude of the reinforcer, or utility of the large reinforcer. However, if utility is held constant while the response-reinforcer delay is manipulated, SHRs show a steeper discounting curve than controls. The evidence suggests that SHRs may be delay averse, which adds validity to the model.