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.

BACKGROUND: Evidence of an inverse relationship between central serotonergic (serotonin [5-hydroxytryptamine]) system function and impulsive aggressive behavior has been accumulating for more than 2 decades. If so, pharmacological enhancement of serotonin activity should be expected to reduce impulsive aggressive behavior in subjects in whom this behavior is prominent. METHODS: A double-blind, placebo-controlled trial of the selective serotonin-uptake inhibitor fluoxetine hydrochloride was conducted in 40 nonmajor-depressed, nonbipolar or schizophrenic, DSM-III-R personality-disordered individuals with current histories of impulsive aggressive behavior and irritability. Measures included the Overt Aggression Scale-Modified for Outpatients, Clinical Global Impression Rating of Improvement, and several secondary measures of aggression, depression, and anxiety. RESULTS: Fluoxetine, but not placebo, treatment resulted in a sustained reduction in scores on the Irritability and Aggression subscales of the Overt Aggression Scale-Modified for Outpatients that was first apparent during months 2 and 3 of treatment, respectively. Fluoxetine was superior to placebo in the proportion of "responders" on the Clinical Global Impression Rating of Improvement: first at the end of month 1, and then finally demonstrating a sustained drug-placebo difference from the end of month 2 through the end of month 3 of treatment. These results were not influenced by secondary measures of depression, anxiety, or alcohol use. CONCLUSION: Fluoxetine treatment has an antiaggressive effect on impulsive aggressive individuals with DSM-III-R personality disorder.

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.

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.

RATIONALE Impulsive individuals exhibit an exaggerated preference for immediate rewards over delayed but larger rewards, perhaps because they value the delayed rewards less. Dopamine (DA) has been postulated to mediate the incentive value of rewards, thus it may also mediate the exaggerated preference for immediate rewards in impulsive individuals. In this paper, we investigate the effects of DA agonists and antagonists on the value of delayed versus immediate rewards. OBJECTIVES The study had three main objectives:(1) to determine the effects of D1 and D2 type DA antagonists on the value of delayed rewards,(2) to determine the effect of the indirect DA agonist d-amphetamine on the value of delayed rewards,(3) to determine the sensitivity of the adjusting amount (AdjAmt) procedure to acute one-day changes in delay to reward, amount of reward, deprivation level and starting amount. METHODS In the AdjAmt procedure, rats choose between an adjusting amount of water given immediately (adjusting alternative) and a constant 150 microliters water delayed by 4 s (standard alternative). The immediate amount of water is adjusted across trials until the rat chooses both alternatives with equal frequency (indifference point). The final adjusted amount is an indicator of the subjective value of the standard alternative. RESULTS The D1/D2 antagonist flupenthixol (25, 50, and 100 micrograms/kg) and the D2 antagonist raclopride (40, 80, and 120 micrograms/kg), decreased the indifference points, whereas the D1 antagonist SCH 23390 (5, 10, and 20 micrograms/kg) did not affect the indifference point. All three DA antagonists slowed responding. The indirect DA agonist amphetamine (0.5, and 1.0 mg/kg) had effects opposite to those of the DA antagonists, it decreased choice latency, increased the number of trials completed and increased the indifference point. Decreasing the water deprivation level (6, 24, and 48 h) had no effect on the indifference points but slowed responding. Increasing the delay to reward (2, 4, and 8 s) and decreasing the amount of water available for choosing the standard alternative (300, 150, 75 microliters) decreased the indifference point. CONCLUSIONS The results indicate that amphetamine increased the value of delayed rewards (decreased impulsivity) and that D2 but not D1 receptor antagonists decreased the value of delayed rewards (increased impulsivity). The procedure was sensitive to acute 1-day changes in delay and magnitude of reward.

This study investigated the acute behavioral effects of d-amphetamine on several behavioral indices of impulsivity. Impulsivity has been defined, variously, as difficulty in inhibiting inappropriate behaviors, inability to wait, insensitivity to delayed consequences or an alteration in the perception of time; standardized procedures have been developed to measure these behavioral dimensions. However, it is not known how drugs affect these measures, and few studies have examined more than one measure in a single study. In this study, 36 healthy men and women participated in three sessions, in which they received placebo, 10 mg, or 20 mg d-amphetamine in randomized order. On each session they performed the following five tasks: the Stop Task, which measures behavioral inhibition, a delay discounting task, which measures the relative value of immediate vs. delayed rewards, a delay of gratification task, a Go/No-Go task, and a time estimation task. Subjects also completed mood questionnaires. Amphetamine produced its expected subjective, mood-altering effects, including increases in POMS Friendliness and Elation scales, and ARCI Euphoria and Stimulant scales. On the measures of impulsivity, amphetamine decreased impulsive responding on three of the tasks: on the Stop Task it decreased Stop reaction times without affecting Go reaction time, on the Go/No-Go task, it decreased the number of false alarms, and on the delay discounting measure, amphetamine (20 mg) decreased k values indicating less discounting of delayed reward. Other measures of impulsive behavior were unaffected. These results suggest that acute doses of amphetamine decrease several forms of impulsive behavior. These findings extend and confirm previous findings in humans and laboratory animals.

RATIONALE Moderate doses of d-amphetamine (given both acutely and chronically) have been shown to decrease impulsivity in children with attention deficit hyperactivity disorder (ADHD) and to improve attention and learning in normal adults. In contrast, chronic doses of methamphetamine (METH) in drug abusers have been associated with increased impulsivity, and impairments in learning and attention. OBJECTIVES We report the effects of METH on an animal model of impulsive behavior. METHODS Rats were tested using the adjusting amount (AdjAmt) procedure in which the animals choose between a delayed fixed (large) amount of water and an immediate adjusting (small) amount of water. In the acute METH study, rats were given a single dose of 0.5, 1.0, 2.0, and 4.0 mg/kg METH or saline 30 min before testing. In the chronic METH study, we determined the effects of the 4.0 mg/kg dose of METH injected chronically 1 h after behavioral testing for 14 days. Thus the rats were tested using the AdjAmt procedure 22 h after injections of METH or saline. RESULTS After 0.5, 1.0 and 2.0 mg/kg METH, the rats valued the delayed large rewards more than after saline, indicating that the METH decreased impulsiveness. At the 4.0 mg/kg dose, the rats failed to respond. Rats treated repeatedly with the post-session large behaviorally disruptive dose of METH valued the delayed large rewards less than the saline-treated rats, indicating that this dosing regimen of METH increased impulsiveness. CONCLUSIONS In these experiments, the rats became less impulsive after acute non-disruptive doses of pre-session METH, whereas they became more impulsive after receiving repeated post-session injections of a dose that was behaviorally disruptive when administered acutely.

A randomized, double-blind, placebo-controlled trial was conducted to assess the acute effects of placebo and three doses of methylphenidate (MPH)(0.3, 0.6, 0.9 mg/kg) on cognitive flexibility and overt behavior in 28 children with a confirmed diagnosis of attention deficit-hyperactivity disorder. Two underlying cognitive processes (response inhibition and response reengagement) were assessed by measuring the probability and speed with which subjects could inhibit responses to a primary task (forced-choice letter discrimination) and immediately execute a response to a secondary task (simple reaction time) when given a signal to do so. Results indicated that MPH enhanced cognitive flexibility, although the high dose was less effective than lower doses in enhancing response inhibition. Dissociations of dose effects on cognitive function and behavior were demonstrated: Dose-response functions for changes in behavior were linear, whereas the function for response inhibition was U-shaped. Findings argue against the typical clinical practice of determining the response to stimulant treatment from a single measure such as parent report of child behavior.

This special issue represents an attempt to answer fundamental brain and behaviour issues in attention-deficit hyperactivity disorder (ADHD). The European network on hyperkinetic disorders (Eunethydis) is trying to develop a novel, testable theory of ADHD, giving an account of its causes, its development from brain dysfunctions to behavioural symptoms and co-morbidity and explaining why no current therapy produces long-lasting improvements. The combined insights of the articles presented here suggest that there is no brain damage in ADHD, but hypo-efficient dopamine systems which give rise to neurochemical imbalances. These cause behavioural problems: deficits in sustained attention, overactivity and impulsiveness. Impulsiveness is increasingly being seen as a key characteristic of the disorder. None of these symptoms are necessarily primary, but may be secondary to an underlying deficit in reinforcement processes seen particularly in a greater than normal sensitivity to variations in the timing of stimulus presentation. Other symptoms can also be seen: altered effects of reinforcers, increased behavioural variance and motor co-ordination problems. Medication produces temporary, plastic changes in cellular components like receptors and transduction mechanisms normalising dopamine functions and behaviour. reserved.

Many autistic children have associated problems of inattention, impulsivity, and hyperactivity that limit the effectiveness of educational and behavioral interventions. Few controlled psychopharmacologic trials have been conducted in autistic children to determine which agents may be effective for these associated features. Eight male children (8.1 +/- 2.8 years) with autistic disorder, diagnosed by DSM-III-R criteria, completed a placebo-controlled, double-blind crossover trial of clonidine. Subjects were included in the study if they had inattention, impulsivity, and hyperactivity that was excessive for their developmental level. Subjects had not tolerated or responded to other psychopharmacologic treatments (neuroleptics, methylphenidate, or desipramine). Teacher ratings on the Aberrant Behavior Checklist irritability, stereotypy, hyperactivity, and inappropriate speech factors were lower during treatment with clonidine than during treatment with placebo. Attention deficit disorder with hyperactivity: Comprehensive Teacher's Rating Scale ratings were not significantly improved during the study, except for oppositional behavior. Parent Conners Abbreviated Parent-Teacher Questionnaire ratings significantly improved during clonidine treatment. Clonidine led to increased ratings of the side effects of drowsiness and decreased activity. Clinician ratings (Children's Psychiatric Rating Scale Autism, Hyperactivity, Anger and Speech Deviance factors; Children's Global Assessment Scale; Clinical Global Impressions efficacy) of videotaped sessions were not significantly different between clonidine and placebo. Clonidine was modestly effective in the short-term treatment of irritability and hyperactivity in some children with autistic disorder.