In delay discounting experiments on rats, the animals are exposed to a choice between a small, immediate reinforcer and a larger, delayed reinforcer. A preference for the larger reinforcer tends to negatively correlate with response-reinforcer delay. While we do not dispute the effect of the delay function, we argue that recent learning history, i.e. order of exposure, also influences choice. We conducted delay discounting experiments using an animal model of ADHD, the Spontaneously Hypertensive Rat (SHR). One group of rats experienced systematic, ascending delays from zero to 24 seconds for the large reinforcer, while another group experienced this order in reverse. Results showed that the rats in the descending delay condition were profoundly more impulsive than the rats in the standard, ascending delay condition. The descending rats almost exclusively preferred the smaller reinforcer, regardless of the response-reinforcer delay in the second experiment. These results suggests that the rats’ previous experience influences their choice in subsequent trials: previous exposure to long delays increases the chance that the next choice will be the small reinforcer, even if the response-reinforcer delay decreases in the next trial. The results are discussed in the context of the dual-component model of Attention-Deficit/Hyperactivity Disorder (ADHD).
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