Drug addiction involves elements of both impulsivity and compulsivity. Impulsivity is the tendency to act prematurely without foresight (Dalley, Everitt, & Robbins, 20). It can be an individual endophenotype which may predispose some individuals to addiction and can be a key component in the early stages of the addiction process. Another theory is that drug consumption may result in structural changes in the brain, which further enhances impulsive behaviour and intensifies the addiction cycle (Koob & Le Moal, 200). Compulsivity can be defined as persistent maladaptive behaviour, which has no obvious relationship to the overall goal and often results in negative consequences (Dalley, Everitt, & Robbins, 20). Although sometimes confused, the two behaviours can be distinguished by their involvement in different aspects of response control during decision making processes, which is largely mediated by distinct (although still related) neural circuitry. Impulsivity is thought to facilitate the development of compulsivity and drug addiction is likely to involve a transi
tion from impulsive to compulsive behaviour (Belin et al., 2008).
Experimentally, behaviours underlying addiction can be investigated by reversal learning, which measures behavioural flexibility. This is the ability of the individual to rapidly change between different tasks, mentally, in order to produce appropriate behavioural responses to the environment (Zhukovsky et al., 209). The individual must actively suppress responses to previously rewarded behaviour, which is biologically and psychologically related to impulsivity and compulsivity. Thus, reversal learning can be used to indicate vulnerability for disorders characterised by impulsivity, such as drug addiction (Izquierdo & Jentsch, 202). Rats are often used to model reversal learning and drug addiction because they provide excellent face validity and predictive validity (Spanagel, 207). Furthermore, there is considerable overlap between rats and humans in the neuroanatomical substrates involved in drug addiction. This means that findings from experiments in rats can, with caution, be translated to humans.