Defining impulsivity: a human trait

Impulsivity is typically classified as a predisposition for premature, poorly planned, unduly risky or inappropriate actions (Daruma and Barnes, 1993). This behavioural construct consists of a heterogeneous repertoire of factors, each with independent but potentially overlapping neurobiological substrates (Evenden, 1999). Impulsivity in its simplest form consists of at least two major components: motor disinhibition (impulsive action) and impulsive decision-making (impulsive choice). Theorists, however, suggest that impulsivity encompasses a more complex array of behavioural processes, including urgency, risk-taking, sensation-seeking, nonplanning, lack of premeditation, a disregard for future consequences and insensitivity to punishment (Barratt, 1985; Evenden, 1999; Moeller et al., 2001; Monterosso and Ainslie, 1999; Whiteside and Lynam, 2003). Although still a matter of considerable debate, impulsivity has been postulated to originate from complex, dynamic ecological variables. In this sense, impulsivity might offer a biological advantage in certain social or environmental settings (Williams and Taylor, 2006) and, as such, ‘functional’ impulsivity favours advantageous outcomes and indeed is an important aspect of human behaviour without which individuals would fail to take acceptable risks or pursue unexpected opportunities. In contrast, ‘maladaptive’ impulsivity is akin to the largely accepted definition of impulsivity as excessively risky, premature and inappropriate actions associated with negative outcomes. Maladaptive impulsivity has been shown to predict antisocial behaviour, unlike functional indices of impulsivity, which facilitate extraversion and sociability (Cale and Lilienfeld, 2006). Indeed, in its extreme form, maladaptive impulsivity has been associated with a wide range of neuropsychiatric morbidities, including personality (Perry and Körner, 2011) and mood (Lombardo et al., 2012) disorder, drug abuse and addiction (Ersche et al., 2010), suicide (Dougherty et al., 2004), and attention deficit hyperactivity disorder (ADHD) (Avila et al., 2004). The diverse taxonomies of impulsivity have led to the development of a variety of self-report questionnaires evaluating these factors [e.g. Eysenck personality questionnaire (Eysenck and Eysenck, 1984), Barratt (Barratt, 1985), UPPS (Whiteside and Lynam, 2003) and Dickman Impulsiveness Scales (Monterosso and Ainslie, 1999)]. There are, however, a number of inherent limitations associated with self-report measures (Wilson and Dunn, 2004), which can be exacerbated in impulsive individuals, who might lack introspection and the ability to appropriately perceive their behaviour. Indeed, it has been suggested that questionnaire-based methods have contributed to the heterogeneity of findings relating to the biological basis of impulsivity (Eisenberg et al., 2007). Computer-based clinical psychometric behavioural tests, which subjectively assess aspects of impulsive choice or impulsive action, provide a more objective measure of impulsivity (reviewed by Chamberlain and Sahakian, 2007; Kertzman et al., 2006). Among these tests, delay-discounting is the most commonly employed measure of impulsive choice and involves individuals making a series of choices between small, immediate rewards versus larger rewards received after a longer delay. Impulsive individuals prefer immediate rewards even if they are smaller than those offered at a later period of time (Richards et al., 1999). The go/no-go task is used to assess deficits in response initiation/inhibition (impulsive REVIEW