OBJECTIVE: The authors discuss the relationship of impulsivity to psychiatric disorders and present selected hypotheses regarding the reasons for these relationships. METHOD: Previous research has shown significantly higher levels of impulsivity among patients with conduct disorder, personality disorders, substance use disorders, and bipolar disorder, compared to other psychiatric patients or healthy comparison subjects. A literature review of the theoretical bases of the relationship between these disorders and impulsivity is presented. Measurements of impulsivity and treatment options are discussed in relation to the physiology of impulsivity and the disorders in which it is a prominent feature. RESULTS: Impulsivity, as defined on the basis of a biopsychosocial approach, is a key feature of several psychiatric disorders. Behavioral and pharmacological interventions that are effective for treating impulsivity should be incorporated into treatment plans for these disorders. CONCLUSIONS: The high comorbidi...

Psychiatric aspects of impulsivity

The concept of impulsivity covers a wide range of ”actions that are poorly conceived, prematurely expressed, unduly risky, or inappropriate to the situation and that often result in undesirable outcomes”. As such it plays an important role in normal behaviour, as well as, in a pathological form, in many kinds of mental illness such as mania, personality disorders, substance abuse disorders and attention deficit/hyperactivity disorder. Although evidence from psychological studies of human personality suggests that impulsivity may be made up of several independent factors, this has not made a major impact on biological studies of impulsivity. This may be because there is little unanimity as to which these factors are. The present review summarises evidence for varieties of impulsivity from several different areas of research: human psychology, psychiatry and animal behaviour. Recently, a series of psychopharmacological studies has been carried out by the present author and colleagues using methods proposed to measure selectively different aspects of impulsivity. The results of these studies suggest that several neurochemical mechanisms can influence impulsivity, and that impulsive behaviour has no unique neurobiological basis. Consideration of impulsivity as the result of several different, independent factors which interact to modulate behaviour may provide better insight into the pathology than current hypotheses based on serotonergic underactivity.

Varieties of impulsivity.

On a gambling task that models real-life decisions, patients with bilateral lesions of the ventromedial prefrontal cortex (VM) opt for choices that yield high immediate gains in spite of higher future losses. In this study, we addressed three possibilities that may account for this behaviour: (i) hypersensitivity to reward; (ii) insensitivity to punishment; and (iii) insensitivity to future consequences, such that behaviour is always guided by immediate prospects. For this purpose, we designed a variant of the original gambling task in which the advantageous decks yielded high immediate punishment but even higher future reward. The disadvantageous decks yielded low immediate punishment but even lower future reward. We measured the skin conductance responses (SCRs) of subjects after they had received a reward or punishment. Patients with VM lesions opted for the disadvantageous decks in both the original and variant versions of the gambling task. The SCRs of VM lesion patients after they had received a reward or punishment were not significantly different from those of controls. In a second experiment, we investigated whether increasing the delayed punishment in the disadvantageous decks of the original task or decreasing the delayed reward in the disadvantageous decks of the variant task would shift the behaviour of VM lesion patients towards an advantageous strategy. Both manipulations failed to shift the behaviour of VM lesion patients away from the disadvantageous decks. These results suggest that patients with VM lesions are insensitive to future consequences, positive or negative, and are primarily guided by immediate prospects. This 'myopia for the future' in VM lesion patients persists in the face of severe adverse consequences, i.e. rising future punishment or declining future reward.

Characterization of the decision-making deficit of patients with ventromedial prefrontal cortex lesions.

Objective: Epidemiological studies indicate that experimentation with addictive drugs and onset of addictive disorders is primarily concentrated in adolescence and young adulthood. The authors describe basic and clinical data supporting adolescent neurodevelopment as a biologically critical period of greater vulnerability for experimentation with substances and acquisition of substance use disorders. Method: The authors reviewed recent literature regarding neurocircuitry underlying motivation, impulsivity, and addiction, with a focus on studies investigating adolescent neurodevelopment. Results: Adolescent neurodevelopment occurs in brain regions associated with motivation, impulsivity, and addiction. Adolescent impulsivity and/or novelty seeking as a transitional trait behavior can be explained in part by maturational changes in frontal cortical and subcortical monoaminergic systems. These developmental processes may advantageously promote learning drives for adaptation to adult roles but may also confer greater vulnerability to the addictive actions of drugs. Conclusions: An exploration of developmental changes in neurocircuitry involved in impulse control has significant implications for understanding adolescent behavior, addiction vulnerability, and the prevention of addiction in adolescence and adulthood.

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Developmental Neurocircuitry of Motivation in Adolescence: A Critical Period of Addiction Vulnerability

The author explores the viability of a comparative approach to personality research. A review of the diverse animal-personality literature suggests that (a) most research uses trait constructs, focuses on variation within (vs. across) species, and uses either behavioral codings or trait ratings; (b) ratings are generally reliable and show some validity (7 parameters that could influence reliability and 4 challenges to validation are discussed); and (c) some dimensions emerge across species, but summaries are hindered by a lack of standard descriptors. Arguments for and against cross-species comparisons are discussed, and research guidelines are suggested. Finally, a research agenda guided by evolutionary and ecological principles is proposed. It is concluded that animal studies provide unique opportunities to examine biological, genetic, and environmental bases of personality and to study personality change, personality-health links, and personality perception.

From mice to men: what can we learn about personality from animal research?

Impulsive behaviour is an important component of many psychiatric syndromes. It is often expressed as aggressive or violent behaviour, but may also be non-violent. One important factor which might lead to aggression or violence is an inability to tolerate a delay of gratification, leading to frustration and aggressive outbursts. In animals and in man, tolerance to delay of gratification can be studied using delay of reinforcement (also known as self-control) procedures. Experiments with delay of reinforcement in rats show that serotonergic mechanisms may be involved in this form of impulsive behaviour, which seems to support clinical findings in this area. The present experiment examined the effects of a series of psychoactive drugs on delays of reinforcement using a steady state operant procedure involving lever-pressing by rats. Rats were trained to choose between one food pellet delivered immediately and three or five pellets delivered after varying delays which increased during the course of the session. Using this procedure the rats remained sensitive to within- and between-session variations in delay of reinforcement even after long periods of testing. It was used to demonstrate an increase in impulsivity (after d-amphetamine) and a reduction in impulsivity (after diazepam and metergoline), indicated by shifts in the choice/delay function. The other drugs tested, imipramine, citalopram, haloperidol and carbamazepine, had no consistent effects although tested at doses which are active in other procedures. This method has proved to be a useful way of examining the effects of drugs on choice of delay of reinforcement in the rat. Although the behavioural basis of tolerance to delay of reinforcement (self-control) has received considerable attention, little is yet known about the biological basis. The present data indicate that the procedure described here can be used to elucidate the pharmacological basis of this aspect of impulsive behaviour.

The pharmacology of impulsive behaviour in rats: the effects of drugs on response choice with varying delays of reinforcement.

Impulsivity can often be an important clinical problem in psychiatry and neurology. In psychiatry, the manifestation of impulsive behaviour in syndromes such as personality disorders, attention deficit hyperactivity disorder and in substance abuse may be different, and this has led to conflicting definitions. There has also been a tendency to concentrate on the nature of the behavioural manifestation (problems with the law, aggression, drug use, behavioural problems in school) rather than shared psychological processes, and to ignore the fact that impulsivity can also have positive aspects. In a normal population, the personality trait of impulsivity has been analysed using personality inventory questionnaires. Analysis of these data lead to the suggestion that impulsivity as commonly defined and understood may be made up of several independent factors, which may have separate biological bases. These self-rating questionnaires have been complemented by objective tests that are now often computerized, and which have been used in man (e.g. with criminal offenders, children, or patients who have undergone brain surgery). Some of these tests, such as the differential reinforcement of low rates procedure or the delay of reinforcement procedure, have also been used to study impulsivity in animals. Analysis of the behavioural principles of these tests suggests that they too may reflect different aspects of impulsivity. Many different biological systems have been proposed to contribute to the neurobiological basis of impulsivity. The serotonergic neurotransmitter system has recently received the most attention, with evidence of its involvement coming from animal studies as well as from studies in psychiatric patients. The frontal lobes have been proposed to play an important role in regulating impulsivity, although it unclear how specific this is. None of this biological knowledge has yet led to reliable pharmacotherapy for excessive impulsivity and, as yet, there is little understanding of the mechanisms by which those drugs, which have been found empirically to have some efficacy (e.g. the psychomotor stimulants in attention deficit hyperactivity disorder), exert their therapeutic effect. By bringing together knowledge from different areas of research it is hoped that a cross fertilization will be achieved, which will lead to a sharpening of concepts, an improvement in methodology and the stimulation of biological studies.

Impulsivity: a discussion of clinical and experimental findings:

Rationale: Tolerance to delay of reinforcement has been proposed as an important facet of self-control in both animals and man. Poor self-control, leading to impulsive behaviour, can be a major problem if it reaches pathological levels. Objectives: The effects of five serotonergic drugs were compared to those of ethanol on a procedure for measuring tolerance to delay of reinforcement in rats in order to elucidate further the role of the serotonin systems in the regulation of impulsive behaviour. Methods: Rats were trained to choose between a single food pellet (small reinforcer) delivered immediately or five food pellets (large reinforcer) delivered after programmed delays. At the start of each session, there was no delay between the response and delivery of the large reinforcer, but this was increased stepwise during the session to delays of 10, 20, 40 and 60 s. Results: The rats showed consistent preference for the larger reinforcer when it was not delayed but showed a shift in preference as the session continued, so that they preferred the small reinforcer when the large was delayed by 40 or 60 s. Ethanol at a dose of 1.0 g/kg produced a significance increase in preference for the small, immediate reinforcer throughout the session, although there were marked individual differences in the size of the effect. A similar, but somewhat smaller effect was seen with the 5-HT2 agonist, DOI, at a dose of 1.0 mg/kg. In contrast, the 5-HT1A agonist, 8-OH-DPAT (0.3 mg/kg) reduced preference for the large reinforcer at the start of the session, and reduced preference for the small reinforcer at the end of the session, i.e. produced a regression to indifference. Lower doses of these three drugs, and treatment with the 5-HT receptor subtype selective antagonists WAY-100635 (5-HT1A: 0.01–0.1 mg/kg), ritanserin (5-HT2: 0.1 and 0.3 mg/kg) and MDL-72222 (5-HT3: 1.0 and 3.0 mg/kg) had no significant effects on reinforcer choice. Conclusion: These data show that ethanol and DOI increase preference for the immediate reinforcer, which can be construed as evidence of an increase in impulsive behaviour (reduction in self control), whereas selective blockade of the 5-HT1A, 5-HT2 or 5-HT3 receptors using selective antagonists does not affect self-control.

The pharmacology of impulsive behaviour in rats VI: the effects of ethanol and selective serotonergic drugs on response choice with varying delays of reinforcement.

The effects of 8-OH-DPAT on locomotor activity have not yet been clearly defined. Tricklebank et al. (1984) and Dourish et al. (1985) provide evidence that 8-OH-DPAT increases activity, whereas Mittman and Geyer (1989), Hillegaart et al. (1989) and Carli et al. (1989) suggest that it is reduced by the drug. In the present study, the effects of 8-OH-DPAT on locomotor activity and rearing were examined in habituated and unhabituated mice and rats. The effects of the drug were followed for up to 2 h in the mouse and up to 4 h in the rat. In unhabituated mice and rats, doses of 0.1 mg/kg or more of 8-OH-DPAT blocked activity during the period post-injection when control levels of activity were highest. However, after about 60 min in mice and 150 min in the rat a marked hyperactivity was observed, which was followed by a period of increased rearing. In habituated mice this biphasic effect on locomotor activity was also observed, but there was no increase in rearing. In habituated rats there was no decrease in locomotor activity, rather a biphasic increase was observed. The effects of 8-OH-DPAT on locomotor activity immediately post-injection are interpreted as being a result of the stereotyped, uncoordinated “ambulation” which forms a part of the 5-HT syndrome, and which results in a level of activity intermediate between that of unhabituated and habituated rats. The mechanism by which 8-OH-DPAT produces elevated locomotor activity and increased rearing seen 60 min or more post-injection is not yet known, but may be a result of brain concentrations of the drug falling to a low, but still effective level.

J. L. Evenden

Papers

Varieties of impulsivity.

The pharmacology of impulsive behaviour in rats: the effects of drugs on response choice with varying delays of reinforcement.

Impulsivity: a discussion of clinical and experimental findings:

The pharmacology of impulsive behaviour in rats VI: the effects of ethanol and selective serotonergic drugs on response choice with varying delays of reinforcement.

Effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on locomotor activity and rearing of mice and rats