The irrelevancy of the inter-trial interval in delay-discounting experiments on an animal model of ADHD

• Published in: Behavioural brain research 2021-06, Vol.408, p.113236-113236, Article 113236
• Main Authors: Sjoberg, Espen A., Ramos, Sergio, López-Tolsa, Gabriela E., Johansen, Espen Borgå, Pellón, Ricardo
• Format: Article
• Language: English
• Subjects: ADHD; Animal testing; Animals; Attention Deficit Disorder with Hyperactivity - physiopathology; Behavior, Animal - physiology; Delay discounting; Delay Discounting - physiology; Disease Models, Animal; Impulsive Behavior - physiology; Impulsivity; Male; Patience; Rats; Rats, Inbred SHR; Rats, Inbred WKY; Reward; Time Factors; Trial length
• ISSN: 0166-4328; 1872-7549
• DOI: 10.1016/j.bbr.2021.113236

•The duration of the ITI had no effect on choice between large and small rewards.•Rats were unaffected by an auditory cue meant to indicate the length of the ITI.•Delays between choice and reward have a profound effect on choice.•The efficiency of the compensating design in delay discounting is limited in rats.•No main effects of strain differences between SHR and WKY in the delay function. Delay discounting involves choosing between a small, immediate reward, and a larger but delayed one. As the delay between choice and large reward gets longer, people with ADHD tend to become impulsive faster than controls, indicated by a switch in preference from the large to the smaller reward. Choosing the smaller reward when the larger is considered reward maximizing is labeled impulsive behaviour. It is well documented that increased delays between choice and reward affects choice preference in both humans and other animals. Other variables such as the inter-trial interval or trial length are observed to have an effect on human discounting, but their effect on discounting in other animals is largely assumed rather than tested. In the current experiment, we tested this assumption. One group of rats was exposed to increasing delays between choosing the large reward and receiving it, while another group experienced longer inter-trial intervals that were equal in length to the delays in the other group. This ensured that trial length was controlled for in delay discounting, but that the delay function and inter-trial intervals could be manipulated and measured separately. Results showed that while the delay between choice and reward caused impulsive behaviour in rats, the length of the inter-trial interval (and by extension trial length) had no impact on choice behaviour. A follow-up experiment found this to be the case even if the length of the inter-trial interval was signaled with audio cues. These results suggest that rats, and possibly animals in general, are insensitive to time between trials, and therefore cannot easily represent human counterparts on the task.