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The inter-trial-interval in delay discounting experiments on an animal model of ADHD
In a delay discounting experiment, two choices are available: a small, immediate reinforcer and a larger, delayed reinforcer. The large reinforcer produces the highest amount of rewards. As the delay increases for the large reinforcer, a preference switch occurs. The inter-trial-interval (ITI) that follows the delivery of the reinforcer has not received much attention. We tested the Spontaneously Hypertensive Rat, an animal model of ADHD, on a modified version of this approach in an experiment. The rats were split into two groups: in one group, the response-reinforcer delay was increased for each session, while in another group the ITI increased, corresponding in length to the delay in the first group. We found that the rats in the Delay group showed a decreased preference for the larger reinforcer as the delay increased, eventually showing a preference switch. The rats in the ITI group were unaffected by changes in the ITI. This suggests that the rats were sensitive to response-reinforcer delays, but not the length of the ITI or the trial length as a whole, and that using compensating designs on animals in delay discounting is problematic.