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Impulsivity. Fig. 1. Indifference points for a series of delays in three hypothetical individuals, who show different levels of delay aversion.

discount function. However, there is some debate about whether discounting occurs exponentially or hyperboli-cally (Green and Myerson, 2004). The extent of discounting can be quantified using an index of the gradient, or by calculating the area under the discount curve.

Delay aversion, or delay discounting, has also been assessed in other species including pigeons, mice, rats and nonhuman primates. The studies with nonhumans use food, water, sucrose or drug rewards, and usually involve delays of up to 60 s. Notably, the animals experience the entire delay period, as well as receiving the delayed (larger) reward during the experimental session. This is in contrast to most studies with humans that involve prospective delays and rewards occurring in the future. There are two basic types of discounting tasks. One type is the ''within session'' procedure, in which the delay to large reward increases in a fixed, systematic way over trials within a session. Subjects choose between a small, sooner (often immediate) reward and a larger, later reward, e.g., three food pellets immediately and six pellets in 10 s, on a fixed number of consecutive trials. The delay to the large reward is 0 s on the first block of trials, but systematically increases over blocks within an experimental session. The within session procedure measures the percent of occasions on which the animals choose the small immediate reward or larger delayed reward at each delay. The other type of task is the ''adjusting procedure,'' which requires multiple sessions to determine a discounting function. In adjusting procedure studies, the reward size or delay on each trial is adjusted, according to the animal's behavior on the previous trial, and different amount or delay conditions are determined on different sessions. There are two main variations of the adjusting procedure. In the adjusting amount procedure, the size of the smaller, sooner reward varies across trials, while the size of the large reward and its delay is fixed throughout the session. If the animal chooses the immediate reward then on subsequent trials the immediate reward is decreased, whereas if the animal chooses the delayed reward the size of the immediate reward is increased. Delay is varied between sessions. In the adjusting delay procedure, the delay to the large reward varies while the smaller reward has a fixed size and delay. Choices of the small reward cause the delay to the large reward to decrease, and choices of the large reward cause its delay to increase. In both adjusting procedures, subjects' choices cause the adjusting alternative to change systematically until subjects become indifferent between the smaller, sooner and larger later rewards, i.e., the ► indifference point is achieved. The dependent measure in the adjusting amount procedure is the size of the immediate reward at indifference, making this procedure similar to the questionnaire method shown in Table 1. As in the human assessment, the extent to which the animal accepts smaller immediate amounts is the measure of impulsivity. The dependent measure in the adjusting delay procedure is the length of the delay to the large reward at indifference. Less impulsive individuals will tolerate a longer delay to the larger reward. All of these tasks are used to assess the effects of experimental treatments, for example, the effects of drugs hypothesized to increase or decrease delay aversion.

Several tasks measure the ability to inhibit a prepotent response, i.e., a response that the organism is strongly inclined to make. Different tasks have been developed for humans and nonhumans. One response conflict task used in humans is the Stroop task, in which participants must suppress extraneous salient information. They are required to name the color of ink of a word that names a different color. Thus, there is a conflict between the response that is required (naming the ink color) and the response that occurs when reading the color word. In other tasks, the prepotent response is created by manipulating the relative frequencies of behaviors. The Conner's Continuous Performance Task is a ► Go/No-Go task, which requires a button press whenever a new letter of the alphabet is shown on a computer screen, except when the letter is a certain specific, designated letter. Errors of commission or false alarms refer to the tendency to emit the prepotent, but incorrect, response. More false alarms are indicative of greater impulsivity. In yet other tasks, the prepotent behavior has been signaled to be appropriate but the individual is required to terminate that behavior or switch to a different behavior (Logan et al., 1997). In these tasks, the dependent measure is the time needed to stop a response. For example, in the Stop task, an individual is signaled to press a button or make a saccadic eye movement in a specific direction when a "Go" signal is presented. On a small percentage of trials, some milliseconds following the Go signal, a "Stop" signal is presented, signaling the individual to terminate the initiation of the button press or saccade. The amount of lead time needed to inhibit a Go response provides a measure of impulsivity, longer Stop Times indicate less ability to inhibit and thus more impulsivity.

The tasks used to assess conflict and response inhibition across species include some tasks that resemble the tasks used in humans and some are different. There are both human and nonhuman versions of Go/No-Go conflict procedures. In these procedures, typically the organism is rewarded for correct Go responses, and may either be rewarded for correct inhibitions or punished for not inhibiting appropriately (errors of commission). Higher numbers of false alarms, or failures to inhibit, indicate higher levels of impulsivity. One example of a widely used Go/No-Go procedure used with nonhumans to examine behavioral inhibition is the ► 5-Choice Serial Reaction Time Task (5CSRTT; Robbins, 2002). One of five response options is briefly illuminated, and the animal is required to respond at the correct location within a short time window to earn reward. This task measures the number of premature responses as the indicator of impulsivity, under conditions when premature responses are punished with a brief time out. Another example of a Go/No-Go procedure used with nonhumans but not humans is the differential reinforcement of low rates (DRL) schedule, in which the animals are required to refrain from pressing a lever until a certain minimum time (e.g., 72 s) has elapsed. In this case, the Go signal is internal, and ability to inhibit responding is viewed as largely reliant on timing processes.

Impulsivity and Substance Use Disorders

Impulsivity plays a significant role in substance use disorders. Individuals diagnosed with substance use disorders score higher on both behavioral and personality measures of impulsivity than comparison groups (Perry and Carroll, 2008). Thus, drug users score higher on personality measures of impulsivity, they exhibit a steeper devaluation of delayed rewards and they are also less able to inhibit prepotent responses. Impulsive behavior may be related to drug use in several ways. Impulsivity may affect the likelihood of initiating use, or it may affect the progression to problematic use or persistence of use in inappropriate situations. It may also affect the ability to abstain, or remain abstinent for extended periods. Drug use itself may in turn affect impulsive behavior, either through direct pharmacological effects or through mild toxicity related to chronic use. Finally, impulsivity and drug abuse may be related to a common etiological factor, such as genetic predisposition for both drug use and impulsive behavior. These various hypotheses are not mutually exclusive and indeed data support each point of view (see Perry and Carroll, 2008). It remains a challenge for behavioral scientists to identify the basic underlying constructs of impulsive behaviors and their role in substance abuse.

A substantial number of controlled studies have examined the effects of drugs of abuse on measures of impulsiv-ity. Whereas in studies with humans, the measures of delay discounting have proven relatively insensitive to the acute effects of drugs, studies with nonhumans provide a rich body of data regarding the effects of drugs on delay aversion. Drugs such as ► alcohol, ► cocaine, and ► morphine reportedly increase discounting, whereas other drugs, such as ► amphetamine, may actually decrease discounting. Studies with laboratory animals have also investigated the neural basis of discounting by making selective lesions in brain regions associated with reward sensitivity and decision making. Studies with both humans and nonhu-mans demonstrate that certain drugs of abuse also impair behavioral inhibition, although again, there is evidence that certain drugs, especially stimulant drugs, may also improve inhibition at low doses. The effects of drugs on measures of impulsivity depend on many factors including the procedure, the animals, the drug doses, and the individuals' baseline levels of impulsivity. In general, however, drugs of abuse are associated with increased delay aversion as well as decreased behavioral inhibition.

Neural and Genetic Mechanisms of Impulsivity

Researchers are actively investigating the neural and genetic mechanisms underlying impulsivity, and several brain areas and neurotransmitter systems have been implicated (see Cardinal, 2006; Pattij and Vanderschuren, 2008). There is evidence that impulsive behavior is related to dysfunction in the ► nucleus accumbens and ► prefrontal cortex. Lesions of the nucleus accumbens and of the orbi-tofrontal cortex, a part of prefrontal cortex, lead to higher levels of delay aversion and impairments in behavioral inhibition. In humans, ► functional magnetic resonance imaging (fMRI) studies indicate that choosing between immediate and delayed rewards activates the caudate-putamen and prefrontal cortical areas, whereas tasks involving behavioral inhibition activate the rostromedial prefrontal cortex. Other brain areas implicated in impulsive behaviors include the mesial prefrontal cortex, the orbitofrontal cortex, the dorsolateral striatum, the insula, and the ► amygdala. Research has also implicated both dopamine and serotonin in the performance of impulsive behaviors. For example, D2 receptor antagonists increase delay discounting while D1 receptor antagonists have little effect. Lesions of brain areas rich in ► dopamine result in increases in impulsive behaviors. Correlational studies assessing ► serotonin function, including the measurement of serotonin metabolites, have linked low levels of serotonin to higher impulsivity. Advances have also been made in identifying genetic determinants of impulsive behaviors (Verdejo-Garcia et al., 2008). For example, genes related to dopaminergic function such as DAT1, D2DR, COMT and MAO-A have been associated with impulse control disorders including ► substance use disorders, ► attention-deficit/hyperactivity disorder and other externalizing disorders.

In sum, impulsivity is a heterogenous construct, consisting of behaviors relating to poor inhibitory control, bias for immediate versus delayed rewards, and an insen-sitivity to negative consequences. Impulsivity is a characteristic of a number of psychiatric diagnoses such as externalizing disorder and drug use, and it also varies in the normal populations. The underlying constructs of impulsive behaviors are an active subject of research, as are efforts to identify the neural circuitry and genetic predispositions to the behavior.

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