The problem with the inter-trial-interval in an animal model of ADHD

Does the length of the inter-trial-interval affect animal’s responses in an experiment? In an animal model of ADHD (the Spontaneously Hypertensive Rat), rats are subjected to two choices: a small, immediate reinforcer and a larger, delayed reinforcer. Following the delivery of the reinforcer, an inter-trial-interval occurs. Despite its delay, the large reinforcer is optimal as it produces the highest amount of rewards, and expressing a preference for the small reinforcer is therefore considered a measure of impulsivity. We tested two groups of rats using a modified version of this approach: in the Delay group, the delay between response and reinforcer was increased for each session, while in the ITI group the inter-trial-interval increased instead, corresponding in length to the delay in the other group. We found that the rats in the Delay group showed a decreased preference for the larger reinforcer as the trial length increased, but the rats in the ITI group did not show this pattern. This suggests that the rats were sensitive to response-reinforcer delays, but not the length of the inter-trial-interval or the trial length as a whole.