Behavioral models of impulsivity in relation to ADHD: translation between clinical and preclinical studies

Abstract

Impulsivity, broadly defined as action without foresight, is a component of numerous psychiatric illnesses including attention deficit/hyperactivity disorder (ADHD), mania and substance abuse. In order to investigate the mechanisms underpinning impulsive behavior, the nature of impulsivity itself needs to be defined in operational terms that can be used as the basis for empirical investigation. Due to the range of behaviors that the term impulsivity describes, it has been suggested that impulsivity is not a unitary construct, but encompasses a variety of related phenomena that may differ in their biological basis. Through fractionating impulsivity into these component parts, it has proved possible to devise different behavioral paradigms to measure various aspects of impulsivity in both humans and laboratory animals. This review describes and evaluates some of the current behavioral models of impulsivity developed for use with rodents based on human neuropsychological tests, focusing on the five-choice serial reaction time task, the stop-signal reaction time task and delay-discounting paradigms. Furthermore, the contributions made by preclinical studies using such methodology to improve our understanding of the neural and neurochemical basis of impulsivity and ADHD are discussed, with particular reference to the involvement of both the serotonergic and dopaminergic systems, and frontostriatal circuitry.

Fig. 1

Impulsive choice and preference reversal as predicted by hyperbolic discounting functions (adapted from Ainslie, 1975). Hyperbolic discounting models predict that a larger but more delayed reward will be valued more highly when the choice is made far in advance of its delivery (towards the left of the graph). However, as time advances, and the difference between the delay to the small reward is judged as being considerably less than the delay to the large reward, preference switches so that the small reward is deemed to be more valuable.