Drug addiction has generally been conceptualized as a disorder that involves elements of both impulsivity and compulsivity, in which impulsivity can be defined behaviorally as “a predisposition toward rapid, unplanned reactions to internal and external stimuli without regard for the negative consequences of these reactions to themselves or others” (Moeller et al. 2001). Impulsivity is measured in two domains: the choice of a smaller, immediate reward over a larger, delayed reward (Rachlin and Green 1972) or the inability to inhibit behavior by changing the course of action or to stop a response once it is initiated (Logan et al. 1997). Impulsivity is a core deficit in substance abuse disorders (Allen et al. 1998) and neuropsychiatric disorders such as attention deficit hyperactivity disorder. Operationally, delay-to-gratification tasks (delayed discounting tasks; impulsive choice) and the stop-signal or go/no-go task (behavioral impulsivity) have been used as measures of impulsivity (Fillmore and Rush 2002; Green et al. 1994). Compulsivity can be defined as elements of behavior that result in perseveration of responding in the face of adverse consequences or perseveration in the face of incorrect
