A review of delay-discounting research with humans: relations to drug use and gambling

Advances in animal models of drug addiction

Drug addiction is a syndrome of impaired response inhibition and salience attribution, which involves a complex neurocircuitry underlying drug reinforcement, drug craving, and compulsive drug-seeking and drug-taking behaviors despite adverse consequences. The concept of disease stages with transitions from acute rewarding effects to early- and end-stage addiction has had an important impact on the design of nonclinical animal models. This chapter reviews the main advances in nonclinical paradigms that aim to at model (1) positive and negative reinforcing effects of addictive drugs; (2) relapse to drug-seeking behavior; (3) reconsolidation of drug cue memories, and (4) compulsive/impulsive drug intake. In addition, recent small animal neuroimaging studies and invertebrate models will be briefly discussed (see also Bifone and Gozzi, Animal models of ADHD, 2011). Continuous improvement in modeling drug intake, craving, withdrawal symptoms, relapse, and comorbid psychiatric associations is a necessary step to better understand the etiology of the disease and to ultimately foster the discovery, validation and optimization of new efficacious pharmacotherapeutic approaches. The modeling of specific subprocesses or constructs that address clinically defined criteria will ultimately increase our understanding of the disease as a whole. Future research will have to address the questions of whether some of these constructs can be reliably used as outcome measures to assess the effects of a treatment in clinical settings, whether changes in those measures can be a target of therapeutic efforts, and whether they relate to biological markers of traits such as impulsivity, which contribute to increased drug-seeking and may predict binge-like patterns of drug intake.

Within-subject comparison of degree of delay discounting using titrating and fixed sequence procedures

Different procedures are often used across experiments to estimate the degree of delay discounting, a common measure of impulsivity. In all procedures, participants indicate their choice between a reward available immediately and one available after a delay. The present experiment determined whether there are differences in the degree of discounting for a hypothetical $100 produced by a procedure that titrates the immediate amount (titrating sequence procedure) versus a procedure that presents a fixed sequence of immediate amounts (fixed sequence procedure) using a within-subject design. The adult human participants showed no significant differences in degree of discounting between procedures as assessed by a hyperboloid model and the Area Under the Curve. Furthermore, the Area Under the Curve values from the two procedures showed a strong positive correlation. These findings suggest there may be no systematic difference between the degree of delay discounting as estimated by the titrating sequence and fixed sequence procedures. Given the apparent similarities in the results, it appears researchers may be justified in basing their choice of which procedure to use on convenience.

A visual aid to decision-making for people with intellectual disabilities

Previous studies have shown that people with mild intellectual disabilities have difficulty in 'weighing-up' information, defined as integrating information from two different sources for the purpose of reaching a decision. This was demonstrated in two very different procedures, temporal discounting and a scenario-based financial decision-making task. In the present study, both tasks were presented to 24 participants who attended day services for people with learning disabilities (mean Full-Scale IQ = 59.8), half of whom were trained to use a visual aid to support decision-making. Performance of control participants did not change over repeated testing, but use of the visual aid substantially improved the quality of decision-making on both tasks: temporal discounting performance became more orderly, and participants were able to provide more information to justify their decisions in the financial decision-making task. The visual aid also substantially improved participants' ability to justify decisions they made about their own lives. We suggest that, while the visual aid was designed and evaluated as a means of increasing the quality of reasoning that supports a decision, it may also have potential as an aid to therapeutic interventions aimed at encouraging wiser decision-making.

Delay-discounting among homeless, out-of-treatment, substance-dependent men who have sex with men

BACKGROUND

Impulsivity is associated with substance use; however, to date, impulsivity has not been characterized among a sample of homeless, non-treatment seeking, substance-dependent men who have sex with men (MSM).

OBJECTIVES

The aim of this study was to utilize the delay-discounting instrument to assess impulsive behaviors among a subsample of homeless, non-treatment seeking, substance-dependent men who have sex with men (S-D MSM) enrolled in a randomized, controlled, contingency management (CM) trial.

METHODS

Twenty S-D MSM participants from the CM parent study were matched on age and ethnicity to 20 non-substance-dependent, non-homeless control participants using propensity scores (N=40) and were administered the delay-discounting procedure.

RESULTS

Although discounting values decreased rapidly with time in both groups, the S-D MSM participants consistently discounted rewards more steeply than controls (p=.05), particularly at all intermediate measured timeframes. The S-D MSM participants also presented greater median discounting rates (k values) compared with the control group (m(S-D MSM)=2.39 (SD=3.72) vs. m(ctrl)=1.27 (SD=3.71), p≤.01).

CONCLUSION

This work extends existing findings of increased delay-discounting among substance-dependent individuals to homeless, substance-dependent, non-treatment seeking MSM.

SCIENTIFIC SIGNIFICANCE

A better understanding of the prevalence of delay-discounting type behaviors among homeless, substance-dependent MSM can be used to inform the development of tailored substance abuse interventions for this high-risk population.

Impulsive choice and altruistic punishment are correlated and increase in tandem with serotonin depletion

Human cooperation may partly depend on the presence of individuals willing to incur personal costs to punish noncooperators. The psychological factors that motivate such 'altruistic punishment' are not fully understood; some have argued that altruistic punishment is a deliberate act of norm enforcement that requires self-control, while others claim that it is an impulsive act driven primarily by emotion. In the current study, we addressed this question by examining the relationship between impulsive choice and altruistic punishment in the ultimatum game. As the neurotransmitter serotonin has been implicated in both impulsive choice and altruistic punishment, we investigated the effects of manipulating serotonin on both measures. Across individuals, impulsive choice and altruistic punishment were correlated and increased following serotonin depletion. These findings imply that altruistic punishment reflects the absence rather than the presence of self control, and suggest that impulsive choice and altruistic punishment share common neural mechanisms.

DAT1 and COMT effects on delay discounting and trait impulsivity in male adolescents with attention deficit/hyperactivity disorder and healthy controls

Choice impulsivity has been linked to dopamine function and is consistently observed in attention deficit/hyperactivity disorder (ADHD) as a preference for smaller-immediate over larger-delayed rewards using choice-delay paradigms. More sophisticated delay discounting paradigms have yielded inconsistent results. Context and sample characteristics may have contributed to these variations. In this study we examine the effect of type (real vs hypothetical) and magnitude of reward as well as of variation in dopamine genes on choice impulsivity. We selected 36 male adolescents with ADHD-combined subtype (ADHD-CT) and 32 controls (mean age=15.42, SD=2.05) to form four roughly equally sized subgroups on the basis of DAT1(10/6) haplotype dosage (2 copies and <2 copies). Participants, who were also genotyped for the COMT(val158met) and DRD4(48bp-VNTR) polymorphisms, performed a hypothetical and a real-time discounting task and provided self-ratings of trait impulsivity. The ADHD-CT group discounted rewards more steeply than controls only in the hypothetical task, with delay, but not reward magnitude, influencing choices. They also rated themselves as more impulsive compared with controls. DAT1(10/6) dosage and the COMT(Val158Met) genotype predicted trait impulsivity and discounting rates in the hypothetical task, but not in the real-time task. Our results directly link variation in genes putatively influencing dopamine signaling in the prefrontal cortex (COMT(Val158Met)) and the striatum (DAT1(10/6)) with discounting rates in a hypothetical task (but not a real-time task) and self-ratings of trait impulsivity in ADHD-CT and healthy controls. The lack of magnitude effects in the hypothetical task suggests that discounting in this task may be influenced by different processes in ADHD-CT than in healthy controls.

Diminished error processing in smokers during smoking cue exposure

Deficits in error processing may contribute to the continuation of impulsive behaviors such as smoking. Previous studies show deficits in error processing among substance abuse patients. However, these studies were all conducted during affectively neutral conditions. Deficits in error processing in smokers may become more pronounced under affectively challenging conditions, such as during smoking cue exposure. The aim of the present study was to investigate whether smokers showed initial error processing deficits, as measured with the error-related negativity (ERN), and decreased motivational significance attributed to an error, as measured with the error positivity (Pe) when exposed to smoking cues. Additionally, we examined the nature of the ERN and Pe amplitudes in more detail by investigating their associations with trait impulsivity, nicotine dependence levels and cigarette craving. Event-related potentials were measured during a modified Erikson flanker task in both smokers and non-smoking controls. Smokers showed reduced ERN and Pe amplitudes after making an error, accompanied by diminished post-error slowing of reaction times. These results suggest that initial error processing and motivational significance attributed to an error are affected in smokers during smoking cue exposure. Furthermore, individual variation in impulsivity and nicotine dependence was associated with reduced ERN amplitudes.

Does traumatic brain injury increase risk for substance abuse?

Wars in Afghanistan and Iraq have resulted in thousands of military personnel suffering traumatic brain injury (TBI), including closed-head injuries. Of interest is whether these individuals and other TBI survivors are at increased risk for substance use disorder (SUD). While it has been well established that drug or alcohol intoxication itself increases probability of suffering a TBI in accidents or acts of violence, little is known about whether the brain insult itself increases the likelihood that a previously non-drug-abusing individual would develop SUD. Might TBI survivors be unusually vulnerable to addiction to opiate analgesics compared to other pain patients? Similarly, it is not known if TBI increases the likelihood of relapse among persons with SUD in remission. We highlight challenges in answering these questions, and review neurochemical and behavioral evidence that supports a causal relationship between TBI and SUD. In this review, we conclude that little is known regarding the directionality of TBI increasing drug abuse, and that collaborative research in this area is critically needed.

Studying the relation between temporal reward discounting tasks used in populations with ADHD: a factor analysis

BACKGROUND

This study aimed at investigating the relationship between tasks that have been used in attention deficit hyperactivity disorder (ADHD) to measure choices between smaller immediate and larger delayed rewards: real and hypothetical temporal discounting tasks, and single-choice paradigms.

METHODS

Participants were 55 undergraduate psychology students. Tasks included a real and hypothetical version of a temporal discounting (TD) task with choices between a large reward (10 cents) after delays up to 60 seconds, and smaller immediate rewards (2-8 cents); two versions of a hypothetical temporal discounting task with choices between a large reward ($100) after delays up to 120 months, and smaller immediate rewards ($1-$95); a Choice Delay Task with choices between one point now and two points after 30 seconds (one point is worth five cents).

RESULTS

Correlation analyses showed that the real and the hypothetical TD tasks with 10 cents were very strongly associated. However, the hypothetical TD tasks with $100 did not correlate with either the real or the hypothetical TD task with 10 cents. Principal component analysis extracted two components: one for small amounts and short delays, and a second one for large rewards and long delays.

CONCLUSIONS

Temporal reward discounting is not a uniform construct. Functional brain imaging research could shed more light on unique brain activation patterns associated with different forms of temporal reward discounting.

Can adolescents learn self-control? Delay of gratification in the development of control over risk taking

Recent findings from developmental neuroscience suggest that the adolescent brain is too immature to exert control over impulsive drives, such as sensation seeking, that increase during adolescence. Using a discounting of delayed reward paradigm, this research examines the ability to delay gratification as a potential source of control over risk-taking tendencies that increase during adolescence. In addition, it explores the role of experience resulting from risk taking as well as future time perspective as contributors to the development of this ability. In a nationally representative sample (n = 900) of young people aged 14-22, a structural equation analysis shows that risk taking as assessed by use of three popular drugs (tobacco, marijuana, and alcohol) is inversely related to the ability to delay gratification. The relation is robust across gender, age, and different levels of sensation seeking. In addition, high sensation seekers exhibit dramatic age-related increase in delay of gratification, lending support to the hypothesis that engaging in risky behavior provides experience that leads to greater patience for long-term rewards. The findings support the conclusion that a complete understanding of the development of self-control must consider individual differences not easily explained by universal trends in brain maturation.

Delay discounting and future-directed thinking in anhedonic individuals

Anhedonia (lack of reactivity to pleasurable stimuli) and a negatively skewed view of the future are important components of depression that could affect economic decisions in depressed individuals. Delay discounting paradigms might be useful for probing putative affective and cognitive underpinnings of such decisions. As a first step to evaluate whether difficulties experiencing pleasure might affect delay discounting, 36 undergraduate students with varying levels of anhedonia performed a delay discounting task in which they made choices between a small immediate and larger future monetary reward. Increasing levels of anhedonia (Snaith-Hamilton Pleasure Scale) were negatively associated with delay discounting rate, indicating that anhedonic individuals tended to choose the larger, albeit delayed reward. These correlations remained after controlling for variables previously linked to delay discounting (working memory capacity and impulsivity) and pessimistic future-directed thinking. The current findings provide preliminary evidence indicating that anhedonic individuals make less myopic decisions about their future, possibly due to their decreased responsiveness to immediate rewards.

Cognitive behavioral therapy for substance use disorders

Cognitive behavioral therapy (CBT) for substance use disorders has shown efficacy as a monotherapy and as part of combination treatment strategies. This article provides a review of the evidence supporting the use of CBT, clinical elements of its application, novel treatment strategies for improving treatment response, and dissemination efforts. Although CBT for substance abuse is characterized by heterogeneous treatment elements such as operant learning strategies, cognitive and motivational elements, and skills-building interventions, across protocols several core elements emerge that focus on overcoming the powerfully reinforcing effects of psychoactive substances. These elements, and support for their efficacy, are discussed.

Is there an impatience genotype leading to non-adherence to long-term therapies?

In chronic diseases such as diabetes, adherence to therapy aims to preserve health, which is a long-term objective, whereas non-adherence tends to present an immediate 'reward'. We propose that non-adherence, like addiction, is at least in part due to the fact that, for physiological, and maybe genetic reasons described in a new field, neuroeconomics, a number of people have a taste for the present rather than the future. Thus, for 'impatient patients' it is natural not to adhere to therapeutic prescriptions that share the characteristic of being future-oriented. This hypothesis may apply to any disease requiring long-term therapy.

Self-administered cocaine causes long-lasting increases in impulsive choice in a delay discounting task

Cocaine use is associated with high levels of impulsive choice (preference for immediate over delayed rewards), but it is not clear whether cocaine use causes elevated impulsive choice, or whether elevated impulsive choice is solely a predisposing factor for cocaine use. This study examined the effects of prior cocaine self-administration on rats performing a delay discounting task commonly used to measure impulsive choice. Male Long-Evans rats were implanted with intravenous catheters, and following recovery, were trained to self-administer 30 mg/kg/day cocaine HCl (approx. 0.5 mg/kg/infusion) for 14 consecutive days (a control group received yoked intravenous saline infusions). Following three weeks of withdrawal, all rats were food-restricted and began training on the delay discounting task in standard operant chambers. On each trial, rats were given a choice between two levers. A press on one lever delivered a small food reward immediately, and a press on the other delivered a large food reward after a variable delay period. Rats that self-administered cocaine displayed greater impulsive choice (enhanced preference for the small immediate over the large delayed reward, as reflected by shorter indifference points) compared to controls, but were no different from controls on a "probabilistic discounting" task in which they chose between small certain and large uncertain rewards. These data suggest that self-administered cocaine can cause lasting elevations in impulsive choice, and that the high levels of impulsive choice observed in human cocaine users may be due in part to long-term effects of cocaine on brain function.

Human behavioral pharmacology, past, present, and future: symposium presented at the 50th annual meeting of the Behavioral Pharmacology Society

A symposium held at the 50th annual meeting of the Behavioral Pharmacology Society in May 2007 reviewed progress in the human behavioral pharmacology of drug abuse. Studies on drug self-administration in humans are reviewed that assessed reinforcing and subjective effects of drugs of abuse. The close parallels observed between studies in humans and laboratory animals using similar behavioral techniques have broadened our understanding of the complex nature of the pharmacological and behavioral factors controlling drug self-administration. The symposium also addressed the role that individual differences, such as sex, personality, and genotype play in determining the extent of self-administration of illicit drugs in human populations. Knowledge of how these factors influence human drug self-administration has helped validate similar differences observed in laboratory animals. In recognition that drug self-administration is but one of many choices available in the lives of humans, the symposium addressed the ways in which choice behavior can be studied in humans. These choice studies in human drug abusers have opened up new and exciting avenues of research in laboratory animals. Finally, the symposium reviewed behavioral pharmacology studies conducted in drug abuse treatment settings and the therapeutic benefits that have emerged from these studies.

Non-substance-addictive behaviors in youth: pathological gambling and problematic Internet use

Adolescence is characterized by participation in multiple novel and potentially risky behaviors. Amongst these behaviors are gambling and use of the Internet, and excessive engagement in these activities (as seen in pathological gambling and problematic Internet use) may be accompanied by serious impairments in school, mental health, and social functioning. This article reviews the potential impact of pathological gambling and problematic Internet use in youth, the relevance of subsyndromal levels of participation, and how prevention and treatment strategies may be considered and tested within a developmental framework.

Treatment of anabolic-androgenic steroid dependence: Emerging evidence and its implications

Currently, few users of anabolic-androgenic steroids (AAS) seek substance abuse treatment. But this picture may soon change substantially, because illicit AAS use did not become widespread until the 1980s, and consequently the older members of this AAS-using population - those who initiated AAS as youths in the 1980s - are only now reaching middle age. Members of this group, especially those who have developed AAS dependence, may therefore be entering the age of risk for cardiac and psychoneuroendocrine complications sufficient to motivate them for substance abuse treatment. We suggest that this treatment should address at least three etiologic mechanisms by which AAS dependence might develop. First, individuals with body image disorders such as "muscle dysmorphia" may become dependent on AAS for their anabolic effects; these body image disorders may respond to psychological therapies or pharmacological treatments. Second, AAS suppress the male hypothalamic-pituitary-gonadal axis via their androgenic effects, potentially causing hypogonadism during AAS withdrawal. Men experiencing prolonged dysphoric effects or frank major depression from hypogonadism may desire to resume AAS, thus contributing to AAS dependence. AAS-induced hypogonadism may require treatment with human chorionic gonadotropin or clomiphene to reactivate neuroendocrine function, and may necessitate antidepressant treatments in cases of depression inadequately responsive to endocrine therapies alone. Third, human and animal evidence indicates that AAS also possess hedonic effects, which likely promote dependence via mechanisms shared with classical addictive drugs, especially opioids. Indeed, the opioid antagonist naltrexone blocks AAS dependence in animals. By inference, pharmacological and psychosocial treatments for human opioid dependence might also benefit AAS-dependent individuals.

Evaluation of the ability of people with intellectual disabilities to 'weigh up' information in two tests of financial reasoning

BACKGROUND

An assessment of mental capacity includes an evaluation of the ability to 'weigh up' information, but how to do this is uncertain. We have previously used a laboratory decision-making task, temporal discounting, which involves a trade-off between the value and the delay of expected rewards. Participants with intellectual disabilities (ID) showed very little evidence of 'weighing up' of information: only a third of participants showed consistent temporal discounting performance, and when present, consistent performance was usually impulsive; and the ability to perform consistently was more strongly related to executive functioning than to IQ. The aim of the present study was to replicate these observations and extend them to a more realistic financial decision-making task.

METHODS

We administered a temporal discounting task and a financial decision-making task, as well as tests of executive functioning and IQ, to 20 participants who attended day services for people with learning disabilities (mean Full-Scale IQ = 59), and to 10 staff members.

RESULTS

Performance in both decision-making tasks was related more strongly to executive functioning than to IQ. In both tasks, decisions by service users were made largely on the basis of a single item of information: there was very little evidence in either task that information from two sources was being 'weighed'.

CONCLUSIONS

The results suggest that difficulty in 'weighing up' information may be a general problem for people with ID, pointing to a need for psycho-educational remediation strategies to address this issue. The importance of executive functioning in decision-making by people with ID is not recognized in the legal test for mental capacity, which in practice includes a possibly irrelevant IQ criterion.

Adolescent risk taking, impulsivity, and brain development: implications for prevention

Individual differences in impulsivity underlie a good deal of the risk taking that is observed during adolescence, and some of the most hazardous forms of this behavior are linked to impulsivity traits that are evident early in development. However, early interventions appear able to reduce the severity and impact of these traits by increasing control over behavior and persistence toward valued goals, such as educational achievement. One form of impulsivity, sensation seeking, rises dramatically during adolescence and increases risks to healthy development. However, a review of the evidence for the hypothesis that limitations in brain development during adolescence restrict the ability to control impulsivity suggests that any such limitations are subtle at best. Instead, it is argued that lack of experience with novel adult behavior poses a much greater risk to adolescents than structural deficits in brain maturation. Continued translational research will help to identify strategies that protect youth as they transition to adulthood.

Cognitive impulsivity in Parkinson's disease patients: assessment and pathophysiology

Impulsivity may be induced by therapeutic interventions (dopamine replacement therapies and sub-thalamic nucleus (STN) stimulation) in patients with Parkinson's disease (PD). The present review has two goals. First, to describe the most frequently encountered facets of cognitive impulsivity and to stress the links between cognitive impulsivity and aspects such as reward-related decision making, risk-taking, and time-processing in healthy population. The most widely used related cognitive impulsivity paradigms are presented. Second, to review the results of studies on cognitive impulsivity in healthy volunteers and in patients with PD, the latter support the applicability and clinical relevance of this construct in PD population. Data show that PD treatments may favor impulsivity via different mechanisms. Suggestions on the roles of dopamine and STN in the pathophysiology of cognitive impulsivity are proposed.

Performance in temporal discounting tasks by people with intellectual disabilities reveals difficulties in decision-making and impulse control

The subjective value of rewards declines as a function of the delay to receive them (temporal discounting). Three temporal discounting tasks that assessed preferences between small amounts of money (10 pence) over short delays (60 s), moderate amounts of money (10 pound) over moderate delays (2 weeks), and large amounts of money (1000 pound) over long delays (12 months) were presented to people with intellectual disabilities (Full-Scale IQ < 70) and to a comparison group (ns = 20 for each group). Measures of IQ, financial knowledge, memory, and executive functioning were also obtained. Only a third of the service users were able to perform the temporal discounting tasks consistently, and they tended to respond impulsively. The proportion of participants responding consistently increased following training. Both the initial performance and the effect of training were related to executive functioning but not IQ.

Home cage testing of delay discounting in rats

Testing rodents in their home cages has become increasingly popular. Since human intervention, handling, and transport are minimized, behavior can be recorded undisturbed and continuously. Currently existing home cage systems are too complex if only relatively simple operant-learning tests are to be carried out in rats. For that purpose, a new low-cost computer-controlled operant panel was designed, which can be placed inside the home cage. A pilot study was carried out, using an intolerance-to-delay protocol, classically developed for testing behavioral impulsivity. Male adult rats were tested in their home cages, containing the operant panel provided with nose-poking holes. Nose poking in one hole resulted in the immediate delivery of one food pellet (small-soon, SS), whereas nose poking in the other hole delivered five food pellets after a delay (large-late), which was increased progressively each day (0-150 sec). The two daily sessions, spaced 8 h apart, lasted 1 h each, and the time-out after food delivery was 90 sec. A clear-cut shift toward preference for SS, which is considered a classical index of cognitive impulsivity, was shown at the longest delay. It is noteworthy that rats shifted when the delay interval was longer than the mean intertrial interval-that is, when they experienced more than one delay-equivalent odds against discounting (see Adriani & Laviola, 2006). The shortened training (2 days) and testing (5 days) phases, as allowed by prolonged and multiple daily sessions, can be advantageous in testing rodents during selected short phases of development. Current research is focusing on further validation of this and similar protocols.

Brain activations reflect individual discount rates in intertemporal choice

Humans discount the value of future rewards following a hyperbolic function and thus may prefer a smaller immediate reward over a larger delayed reward. Marked interindividual differences in the steepness of this discounting function can be observed which can be quantified by the parameter k of the discount function. Here, we asked how differences in delay discounting behaviour are reflected by brain activation patterns. Sixteen healthy participants were studied in a slow event-related functional magnetic resonance imaging experiment at 3T. In each trial, participants had to decide between a smaller but immediately available monetary reward (ranging between 14 and 84 Euro) and a larger delayed reward (26 to 89 Euro; delay 5 to 169days) by button press. Participants had the chance to receive the reward corresponding to one of their decisions at the end of the experiment. As expected, participants differed widely with respect to the steepness of their discount function. By contrasting decisions at or near the individual participant's indifference point (as determined by parameter k) with trials either well below or well above this point two different brain networks with opposing activation patterns were revealed: Trials below or above the indifference point were associated with activation in the ventral striatum and ventromedial prefrontal cortex, whereas decisions at the indifference point gave rise to activation in medial prefrontal cortex. The opposite effects in the two systems at individual indifference point were interpreted as a reflection of response conflict.

Impulsive choice and response in dopamine agonist-related impulse control behaviors

RATIONALE

Dopaminergic medication-related impulse control disorders (ICDs) such as pathological gambling and compulsive shopping have been reported in Parkinson's disease (PD).

HYPOTHESIS

We hypothesized that dopamine agonists (DAs) would be associated with greater impulsive choice or greater discounting of delayed rewards in PD patients with ICDs (PDI).

METHODS

Fourteen PDI patients, 14 PD controls without ICDs, and 16 medication-free matched normal controls were tested on the Experiential Discounting Task (EDT), a feedback-based intertemporal choice task, spatial working memory, and attentional set shifting. The EDT was used to assess choice impulsivity (hyperbolic K value), reaction time (RT), and decision conflict RT (the RT difference between high conflict and low conflict choices). PDI patients and PD controls were tested on and off DA.

RESULTS

On the EDT, there was a group by medication interaction effect [F(1,26) = 5.62; p = 0.03] with pairwise analyses demonstrating that DA status was associated with increased impulsive choice in PDI patients (p = 0.02) but not in PD controls (p = 0.37). PDI patients also had faster RT compared to PD controls [F(1,26) = 7.51, p = 0.01]. DA status was associated with shorter RT [F(3,24) = 8.39, p = 0.001] and decision conflict RT [F(1,26) = 6.16, p = 0.02] in PDI patients but not in PD controls. There were no correlations between different measures of impulsivity. PDI patients on DA had greater spatial working memory impairments compared to PD controls on DA (t = 2.13, df = 26, p = 0.04).

CONCLUSION

Greater impulsive choice, faster RT, faster decision conflict RT, and executive dysfunction may contribute to ICDs in PD.

The behavioral economics of drug dependence: towards the consilience of economics and behavioral neuroscience

In this chapter, we review the research in this growing field by first discussing the concepts related to price and consumption (demand), its applications to the study of drug consumption and drug seeking, and the impact of other commodities on drug consumption. We then review the discounting of future commodities and events among the addicted, review the most recent research examining the neural correlates of discounting, and describe and review the new theory of addiction that results from that research. We conclude by addressing the next research steps that these advances engender.

Delay discounting as a mediator of the relationship between perceived stress and cigarette smoking status in adolescents

There has been a wealth of research providing evidence for the relationship between stress and cigarette smoking during adolescence. Despite this knowledge, little is known about possible behavioral mechanisms by which stress exerts its influence on the decision to smoke. This study sought to examine one such behavioral characteristic, delay discounting, that may mediate the relationship between stress and cigarette smoking. Delay discounting generally refers to the discounting of value for outcomes because they are delayed; and high rates of delay discounting have been linked to impulsive behavior. For the current research, adolescent smokers (n = 50) and nonsmokers (n = 50) were compared using a self-report measure of perceived stress and a laboratory assessment of delay discounting. Smokers tended to report higher levels of stress and to discount more by delay, and there was a significant association between reported stress and delay discounting. In addition, delay discounting mediated the relationship between stress and cigarette smoking status. These results suggest that discounting by delay may be a behavior through which stress exerts influence on an adolescent's decision to smoke.

Investigating the behavioral and self-report constructs of impulsivity domains using principal component analysis

Impulsivity, often defined as a human behavior characterized by the inclination of an individual to act on urge rather than thought, with diminished regard to consequences, encompasses a range of maladaptive behaviors, which are in turn affected by distinct neural systems. Congruent with the above definition, behavioral studies have consistently shown that the underlying construct of impulsivity is multidimensional in nature. However, research to date has been inconclusive regarding the different domains or constructs that constitute this behavior. In addition there is also no clear consensus as to whether self-report and laboratory based measures of impulsivity measure the same or different domains. This study aimed to: (i) characterize the underlying multidimensional construct of impulsivity using a sample with varying degrees of putative impulsivity related to substance misuse, including subjects who were at-risk of substance use or addicted (ARA), and (ii) assess relationships between self-report and laboratory measures of impulsivity, using a principal component-based factor analysis. In addition, our supplementary goal was to evaluate the structural constructs of impulsivity within each group separately (healthy and ARA). We used five self-report measures (Behavioral Inhibition System/Behavioral Activation System, Barratt Impulsivity Scale-11, Padua Inventory, Zuckerman Sensation Seeking Scale, and Sensitivity to Punishment and Sensitivity to Reward Questionnaire) and two computer-based laboratory tasks (Balloon Analog Risk Task and the Experiential Discounting Task) to measure the aspects of impulsivity in a total of 176 adult subjects. Subjects included healthy controls (n = 89), nonalcoholic subjects with family histories of alcoholism (family history positive; n = 36) and both former (n = 20) and current (n = 31) cocaine users. Subjects with a family history of alcoholism and cocaine abusers were grouped together as 'at-risk/addicted' (ARA) to evaluate our supplementary goal. Our overall results revealed the multidimensional nature of the impulsivity construct as captured optimally through a five-factor solution that accounted for nearly 70% of the total variance. The five factors/components were imputed as follows 'Self-Reported Behavioral Activation', 'Self-Reported Compulsivity and Reward/Punishment', 'Self-Reported Impulsivity', 'Behavioral Temporal Discounting', and 'Behavioral Risk-Taking'. We also found that contrary to previously published reports, there was significant overlap between certain laboratory and self-report measures, indicating that they might be measuring the same impulsivity domain. In addition, our supplemental analysis also suggested that the impulsivity constructs were largely, but not entirely the same within the healthy and ARA groups.

Extended access to amphetamine self-administration increases impulsive choice in a delay discounting task in rats

BACKGROUND

d-Amphetamine (AMPH) is a widely prescribed attention deficit hyperactivity disorder medication, but little is known about its effects on impulsive choice with escalated use.

OBJECTIVE

The current study examined the effects of short and long access to AMPH self-administration on impulsive choice in a delay discounting task in which rats chose between a small immediate reward (one sucrose pellet immediately) and a larger delayed reward (three sucrose pellets after an adjusting delay).

METHODS

Following choice stability in delay discounting, all rats received 15 1-h sessions of AMPH self-administration (0.1 or 0.03 mg/kg/infusion); self-administration sessions began 45 min after each delay discounting session. Rats were then either maintained on the short access (ShA) self-administration session or were switched to a long access (LgA) 6-h session for 21 days, followed by a 7-day withdrawal phase in which only the delay discounting task continued.

RESULTS

LgA rats in the 0.03 mg/kg/infusion dose group escalated in total number of infusions across sessions, although rats in the 0.1 mg/kg/infusion dose group did not. LgA groups at both unit doses showed decreased mean adjusted delays across sessions compared to the ShA groups, indicating that long access to AMPH increases impulsive choice. During the AMPH withdrawal phase, LgA groups returned back to baseline mean adjusted delays, indicating that the effect on impulsive choice was reversible.

CONCLUSION

These results show that extended AMPH self-administration produces a transient loss of inhibitory control, which may play a role in the escalating pattern of drug intake that characterizes the addiction process.

Anabolic-androgenic steroid dependence: an emerging disorder

AIMS

Anabolic-androgenic steroids (AAS) are widely used illicitly to gain muscle and lose body fat. Here we review the accumulating human and animal evidence showing that AAS may cause a distinct dependence syndrome, often associated with adverse psychiatric and medical effects.

METHOD

We present an illustrative case of AAS dependence, followed by a summary of the human and animal literature on this topic, based on publications known to us or obtained by searching the PubMed database.

RESULTS

About 30% of AAS users appear to develop a dependence syndrome, characterized by chronic AAS use despite adverse effects on physical, psychosocial or occupational functioning. AAS dependence shares many features with classical drug dependence. For example, hamsters will self-administer AAS, even to the point of death, and both humans and animals exhibit a well-documented AAS withdrawal syndrome, mediated by neuroendocrine and cortical neurotransmitter systems. AAS dependence may particularly involve opioidergic mechanisms. However, AAS differ from classical drugs in that they produce little immediate reward of acute intoxication, but instead a delayed effect of muscle gains. Thus standard diagnostic criteria for substance dependence, usually crafted for acutely intoxicating drugs, must be adapted slightly for cumulatively acting drugs such as AAS.

CONCLUSIONS

AAS dependence is a valid diagnostic entity, and probably a growing public health problem. AAS dependence may share brain mechanisms with other forms of substance dependence, especially opioid dependence. Future studies are needed to characterize AAS dependence more clearly, identify risk factors for this syndrome and develop treatment strategies.

The effects of real versus hypothetical reward on delay and probability discounting

Delay discounting and probability discounting tasks providing hypothetical rewards have commonly been used to measure levels of impulsivity and risk taking. Results have been inconsistent as to whether real, compared with hypothetical, rewards influence choice behaviour in delay discounting tasks. This experiment examined choice behaviour in 38 healthy individuals using delay and probability discounting tasks involving both real and hypothetical rewards (pound sterling 0.10/pound sterling 0.20), real delays, and probabilistic outcomes. It was hypothesized that choice behaviour when receiving real rewards would be less impulsive and less risky than when receiving hypothetical rewards. The tasks were designed so that a mathematical model (the multiplicative hyperbolic model of choice) could be applied to assess effects on discounting parameters. The provision of real rewards was associated with significantly decreased impulsive, but not risky, choice although the size of effect was small and was influenced by the order of presentation. Repeated presentation of both tasks increased quantity discounting but not delay or probability discounting parameters. These results indicate that the nature of the reward provided, and repetition effects, need to be taken into account when designing discounting studies.

Temporal-difference reinforcement learning with distributed representations

Temporal-difference (TD) algorithms have been proposed as models of reinforcement learning (RL). We examine two issues of distributed representation in these TD algorithms: distributed representations of belief and distributed discounting factors. Distributed representation of belief allows the believed state of the world to distribute across sets of equivalent states. Distributed exponential discounting factors produce hyperbolic discounting in the behavior of the agent itself. We examine these issues in the context of a TD RL model in which state-belief is distributed over a set of exponentially-discounting "micro-Agents", each of which has a separate discounting factor (gamma). Each microAgent maintains an independent hypothesis about the state of the world, and a separate value-estimate of taking actions within that hypothesized state. The overall agent thus instantiates a flexible representation of an evolving world-state. As with other TD models, the value-error (delta) signal within the model matches dopamine signals recorded from animals in standard conditioning reward-paradigms. The distributed representation of belief provides an explanation for the decrease in dopamine at the conditioned stimulus seen in overtrained animals, for the differences between trace and delay conditioning, and for transient bursts of dopamine seen at movement initiation. Because each microAgent also includes its own exponential discounting factor, the overall agent shows hyperbolic discounting, consistent with behavioral experiments.

Dimensions of impulsive behavior in adolescent smokers and nonsmokers

Robust associations have been identified between impulsive personality characteristics and cigarette smoking during adolescents, indicating that impulsive behavior may play an important role in the initiation of cigarette smoking. The present study extended this research by using laboratory behavioral assessments to explore relationships between three specific dimensions of impulsive behavior (impulsive decision-making, inattention, and disinhibition) and adolescent cigarette smoking. Participants were male and female adolescent smokers (n = 50) and nonsmokers (n = 50). Adolescent smokers were more impulsive on a measure of decision-making; however, there were significant smoking status by gender interaction effects for impulsive inattention and disinhibition. Male smokers were most impulsive on the measure of inattention, but male smokers were least impulsive on the measure of disinhibition. Correlations between biomarkers of smoking and impulsive inattention and disinhibition were found for females but not males. The current findings, coupled with previous findings (Reynolds et al., 2007), indicate there may be robust gender difference in associations between certain types of impulsive behavior and cigarette smoking during adolescence.

Effects of methylphenidate on discounting of delayed rewards in attention deficit/hyperactivity disorder

Impulsivity is a central component of attention deficit/hyperactivity disorder (ADHD). Delay discounting, or a preference for smaller, immediate rewards over larger, delayed rewards, is considered an important aspect of impulsivity, and delay-related impulsivity has been emphasized in etiological models of ADHD. In this study, we examined whether stimulant medication, an effective treatment for ADHD, reduced discounting of delayed experiential and hypothetical rewards among 49 children (ages 9-12 years) with ADHD. After a practice day, participants completed a 3-day double-blind placebo-controlled acute medication assessment. Active doses were long-acting methylphenidate (Concerta), with the nearest equivalents of 0.3 and 0.6 mg/kg TID immediate-release methylphenidate. On each testing day, participants completed experiential (real-world money in real time) and hypothetical discounting tasks. Relative to placebo, methylphenidate reduced discounting of delayed experiential rewards but not hypothetical rewards. Broadly consistent with etiological models that emphasize delay-related impulsivity among children with ADHD, these findings provide initial evidence that stimulant medication reduces delay discounting among those with the disorder. The results also draw attention to task parameters that may influence the sensitivity of various delay discounting measures to medication effects.

Temporal horizon: modulation by smoking status and gender

Recently, delay discounting has been argued to be conceptually consistent with the notion of temporal horizon [Bickel, W.K., Yi, R., Kowal, B.P., Gatchalian, K.M., 2008. Cigarette smokers discount past and future rewards symmetrically and more than controls: is discounting a measure of impulsivity? Drug Alcohol Depend. 96, 256-262]. Temporal horizon refers to the temporal distance over which behavioral events or objects can influence behavior. Here we examine the results on two putative measures of temporal horizon, future time perspective (FTP) and delay discounting, collected over three separate studies (n=227), to determine the influence of smoking and gender on temporal horizon. By comparing the results on these temporal horizon measures we address our population of interest: women who smoke. One of the measures of FTP indicates that smoking women have a shorter temporal horizon than their nonsmoking counterparts. Additionally, the story completion measures of FTP are positively correlated with delay discounting. In contrast, results of delay discounting measures showed no difference between smoking women and nonsmoking women, while results of delay discounting measures indicated smoking men have a shorter temporal horizon than non-smoking men. Additionally, the results of the FTP story completion measure indicated that lower third income earners had a shortened temporal horizon compared to upper third income earners. A possible explanation for these results is explored, and the implications of the modulation of temporal horizon by gender and smoking are discussed.

The subjective value of delayed and probabilistic outcomes: Outcome size matters for gains but not for losses

The subjective value of a reward (gain) is related to factors such as its size, the delay to its receipt and the probability of its receipt. We examined whether the subjective value of losses was similarly affected by these factors in 128 adults. Participants chose between immediate/certain gains or losses and larger delayed/probabilistic gains or losses. Rewards of $100 were devalued as a function of their delay ("discounted") relatively less than $10 gains while probabilistic $100 rewards were discounted relatively more than $10 rewards. However, there was no effect of outcome size on discounting of delayed or probabilistic losses. For delayed outcomes of each size, the degree to which gains were discounted was positively correlated with the degree to which losses were discounted, whereas for probabilistic outcomes, no such correlation was observed. These results suggest that the processes underlying the subjective valuation of losses are different from those underlying the subjective valuation of gains.

Percent body fat is related to delay and probability discounting for food in humans

This study describes delay and probability discounting patterns for hypothetical food and money in relation to percent body fat (PBF). Sixty university students completed four computerized discounting tasks in which they were asked to make a series of hypothetical decisions between (a) 10 dollars after one of several different delays (1, 2, 30, 180, and 365 days) or a smaller amount of money available immediately; (b) 10 bites of food after one of several delays (1, 2, 5, 10, and 20h) or a smaller number of bites available immediately; (c) $10 at one of several probabilities (0.9, 0.75, 0.5, 0.25, 0.1) or a smaller amount of money to be received for sure; and (d) 10 bites of food at one of several probabilities (0.9, 0.75, 0.5, 0.25, 0.1) or a smaller number of bites to be received for sure. Median indifference points for all participants across each task were well described using the hyperbolic discounting function. Results suggest that percent body fat predicted discounting for hypothetical food, but not money, using regression analyses with the entire sample and when comparing individuals in the high and low quartiles for PBF. None of the other dietary variables (body mass index, subjective hunger, and time since last meal or snack) were related to discounting patterns. This suggests that individuals with high PBF may exhibit heightened sensitivities to delay and probability when making decisions about food.

Impulsivity, frontal lobes and risk for addiction

Alcohol and substance abuse disorders involve continued use of substances despite negative consequences, i.e. loss of behavioral control of drug use. The frontal-cortical areas of the brain oversee behavioral control through executive functions. Executive functions include abstract thinking, motivation, planning, attention to tasks and inhibition of impulsive responses. Impulsiveness generally refers to premature, unduly risky, poorly conceived actions. Dysfunctional impulsivity includes deficits in attention, lack of reflection and/or insensitivity to consequences, all of which occur in addiction [Evenden JL. Varieties of impulsivity. Psychopharmacology (Berl) 1999;146:348-361.; de Wit H. Impulsivity as a determinant and consequence of drug use: a review of underlying processes. Addict Biol 2009;14:22-31]. Binge drinking models indicate chronic alcohol damages in the corticolimbic brain regions [Crews FT, Braun CJ, Hoplight B, Switzer III RC, Knapp DJ. Binge ethanol consumption causes differential brain damage in young adolescent rats compared with adult rats. Alcohol Clin Exp Res 2000;24:1712-1723] causing reversal learning deficits indicative of loss of executive function [Obernier JA, White AM, Swartzwelder HS, Crews FT. Cognitive deficits and CNS damage after a 4-day binge ethanol exposure in rats. Pharmacol Biochem Behav 2002b;72:521-532]. Genetics and adolescent age are risk factors for alcoholism that coincide with sensitivity to alcohol-induced neurotoxicity. Cortical degeneration from alcohol abuse may increase impulsivity contributing to the development, persistence and severity of alcohol use disorders. Interestingly, abstinence results in bursts of neurogenesis and brain regrowth [Crews FT, Nixon K. Mechanisms of neurodegeneration and regeneration in alcoholism. Alcohol Alcohol 2009;44:115-127]. Treatments for alcoholism, including naltrexone pharmacotherapy and psychotherapy may work through improving executive functions. This review will examine the relationships between impulsivity and executive function behaviors to changes in cortical structure during alcohol dependence and recovery.

Effects of chronic administration of drugs of abuse on impulsive choice (delay discounting) in animal models

Drug-addicted individuals show high levels of impulsive choice, characterized by preference for small immediate over larger but delayed rewards. Although the causal relationship between chronic drug use and elevated impulsive choice in humans has been unclear, a small but growing body of literature over the past decade has shown that chronic drug administration in animal models can cause increases in impulsive choice, suggesting that a similar causal relationship may exist in human drug users. This article reviews this literature, with a particular focus on the effects of chronic cocaine administration, which have been most thoroughly characterized. The potential mechanisms of these effects are described in terms of drug-induced neural alterations in ventral striatal and prefrontal cortical brain systems. Some implications of this research for pharmacological treatment of drug-induced increases in impulsive choice are discussed, along with suggestions for future research in this area.

White matter integrity predicts delay discounting behavior in 9- to 23-year-olds: a diffusion tensor imaging study

Healthy participants (n = 79), ages 9-23, completed a delay discounting task assessing the extent to which the value of a monetary reward declines as the delay to its receipt increases. Diffusion tensor imaging (DTI) was used to evaluate how individual differences in delay discounting relate to variation in fractional anisotropy (FA) and mean diffusivity (MD) within whole-brain white matter using voxel-based regressions. Given that rapid prefrontal lobe development is occurring during this age range and that functional imaging studies have implicated the prefrontal cortex in discounting behavior, we hypothesized that differences in FA and MD would be associated with alterations in the discounting rate. The analyses revealed a number of clusters where less impulsive performance on the delay discounting task was associated with higher FA and lower MD. The clusters were located primarily in bilateral frontal and temporal lobes and were localized within white matter tracts, including portions of the inferior and superior longitudinal fasciculi, anterior thalamic radiation, uncinate fasciculus, inferior fronto-occipital fasciculus, corticospinal tract, and splenium of the corpus callosum. FA increased and MD decreased with age in the majority of these regions. Some, but not all, of the discounting/DTI associations remained significant after controlling for age. Findings are discussed in terms of both developmental and age-independent effects of white matter organization on discounting behavior.

Why gamblers fail to win: a review of cognitive and neuroimaging findings in pathological gambling

The purpose of this review is to gain more insight in the neuropathology of pathological gambling (PG) and problem gambling, and to discuss challenges in this research area. Results from the reviewed PG studies show that PG is more than just an impulse control disorder. PG seems to fit very well with recent theoretical models of addiction, which stress the involvement of the ventral tegmental-orbito frontal cortex. Differentiating types of PG on game preferences (slot machines vs. casino games) seems to be useful because different PG groups show divergent results, suggesting different neurobiological pathways to PG. A framework for future studies is suggested, indicating the need for hypothesis driven pharmacological and functional imaging studies in PG and integration of knowledge from different research areas to further elucidate the neurobiological underpinnings of this disorder.

Impulsiveness as a timing disturbance: neurocognitive abnormalities in attention-deficit hyperactivity disorder during temporal processes and normalization with methylphenidate

We argue that impulsiveness is characterized by compromised timing functions such as premature motor timing, decreased tolerance to delays, poor temporal foresight and steeper temporal discounting. A model illustration for the association between impulsiveness and timing deficits is the impulsiveness disorder of attention-deficit hyperactivity disorder (ADHD). Children with ADHD have deficits in timing processes of several temporal domains and the neural substrates of these compromised timing functions are strikingly similar to the neuropathology of ADHD. We review our published and present novel functional magnetic resonance imaging data to demonstrate that ADHD children show dysfunctions in key timing regions of prefrontal, cingulate, striatal and cerebellar location during temporal processes of several time domains including time discrimination of milliseconds, motor timing to seconds and temporal discounting of longer time intervals. Given that impulsiveness, timing abnormalities and more specifically ADHD have been related to dopamine dysregulation, we tested for and demonstrated a normalization effect of all brain dysfunctions in ADHD children during time discrimination with the dopamine agonist and treatment of choice, methylphenidate. This review together with the new empirical findings demonstrates that neurocognitive dysfunctions in temporal processes are crucial to the impulsiveness disorder of ADHD and provides first evidence for normalization with a dopamine reuptake inhibitor.

Temporal discounting and inter-temporal choice in rhesus monkeys

Humans and animals are more likely to take an action leading to an immediate reward than actions with delayed rewards of similar magnitudes. Although such devaluation of delayed rewards has been almost universally described by hyperbolic discount functions, the rate of this temporal discounting varies substantially among different animal species. This might be in part due to the differences in how the information about reward is presented to decision makers. In previous animal studies, reward delays or magnitudes were gradually adjusted across trials, so the animals learned the properties of future rewards from the rewards they waited for and consumed previously. In contrast, verbal cues have been used commonly in human studies. In the present study, rhesus monkeys were trained in a novel inter-temporal choice task in which the magnitude and delay of reward were indicated symbolically using visual cues and varied randomly across trials. We found that monkeys could extract the information about reward delays from visual symbols regardless of the number of symbols used to indicate the delay. The rate of temporal discounting observed in the present study was comparable to the previous estimates in other mammals, and the animal's choice behavior was largely consistent with hyperbolic discounting. Our results also suggest that the rate of temporal discounting might be influenced by contextual factors, such as the novelty of the task. The flexibility furnished by this new inter-temporal choice task might be useful for future neurobiological investigations on inter-temporal choice in non-human primates.

Features of men with anabolic-androgenic steroid dependence: A comparison with nondependent AAS users and with AAS nonusers

BACKGROUND

Anabolic-androgenic steroid (AAS) dependence has been a recognized syndrome for some 20 years, but remains poorly understood.

METHODS

We evaluated three groups of experienced male weightlifters: (1) men reporting no history of AAS use (N=72); (2) nondependent AAS users reporting no history of AAS dependence (N=42); and (3) men meeting adapted DSM-IV criteria for current or past AAS dependence (N=20). We assessed demographic indices, lifetime history of psychiatric disorders by the Structured Clinical Interview for DSM-IV, variables related to AAS use, and results from drug tests of urine and hair.

RESULTS

Nondependent AAS users showed no significant differences from AAS nonusers on any variable assessed. Dependent AAS users, however, differed substantially from both other groups on many measures. Notably, they reported a more frequent history of conduct disorder than nondependent AAS users (odds ratio [95% CI]: 8.0 [1.7, 38.0]) or AAS nonusers (13.1 [2.8, 60.4]) and a much higher lifetime prevalence of opioid abuse and dependence than either comparison group (odds ratios 6.3 [1.2, 34.5] and 18.6 [3.0, 116.8], respectively).

CONCLUSIONS

Men with AAS dependence, unlike nondependent AAS users or AAS nonusers, showed a distinctive pattern of comorbid psychopathology, overlapping with that of individuals with other forms of substance dependence. AAS dependence showed a particularly strong association with opioid dependence - an observation that recalls recent animal data suggesting similarities in AAS and opioid brain reward mechanisms. Individuals with AAS dependence and individuals with "classical" substance dependence may possibly harbor similar underlying biological and neuropsychological vulnerabilities.

Strain differences in delay discounting using inbred rats

A heightened aversion to delayed rewards is associated with substance abuse and numerous other neuropsychiatric disorders. Many of these disorders are heritable, raising the possibility that delay aversion may also have a significant genetic or heritable component. To examine this possibility, we compared delay discounting in six inbred strains of rats (Brown Norway, Copenhagen, Lewis, Fischer, Noble and Wistar Furth) using the adjusting amount procedure, which provides a measure of the subjective value of delayed rewards. The subjective value of rewards decreased as the delay to receipt increased for all strains. However, a main effect of strain and a strain x delay interaction indicated that some strains were more sensitive to the imposition of delays than others. Fitting a hyperbolic discount equation showed significant strain differences in sensitivity to delay (k). These data indicate that there are significant strain differences in delay discounting. All strains strongly preferred the 10% sucrose solution (the reinforcer in the delay discounting task) over water and the amount of sucrose consumed was correlated with sensitivity to delay. Locomotor activity was not correlated with delay discounting behavior. Additional research will be required to disentangle genetic influences from maternal effects and to determine how these factors influence the underlying association between heightened delay discounting and neuropsychiatric disorders.

Delay discounting correlates with proportional lateral frontal cortex volumes

BACKGROUND

Functional neuroimaging experiments in healthy control subjects have shown that choosing between small and immediate rewards versus larger but deferred rewards in delay discounting (DD) tasks recruits mesofrontal and lateral frontal cortex. Might individual differences in frontocortical gray matter morphology be related to preference for immediate reward?

METHODS

We related DD in a laboratory decision-making task to proportional frontocortical gray matter (GM) volumes calculated from segmented magnetic resonance images in 29 healthy adults.

RESULTS

Dorsolateral and inferolateral frontal cortex GM volumes (corrected as a proportion of whole cerebral brain volume) each correlated inversely with preference for immediate gratification during decision making, as indexed by DD constant k. Conversely, neither proportional orbitofrontal or mesofrontal cortex GM volume nor cerebral brain volume (CBV) or total intracranial volume (ICV; a measure of maximal brain growth) significantly correlated with severity of DD.

CONCLUSIONS

Severity of discounting of delayed rewards correlates with proportional lateral frontocortical GM morphology but not with whole brain measures. In light of evidence of frontocortical abnormalities in substance dependence and sociopathy, future studies can assess whether reduced frontocortical volume itself is a morphological marker or risk factor for inability to delay gratification in psychiatric disorders.

Bright and dark sides of impulsivity: performance of women with high and low trait impulsivity on neuropsychological tasks

We administered a multidimensional measure of trait impulsivity (the UPPS-P impulsivity scale; Cyders et al., 2007) to a nonclinical sample of 155 individuals and selected 32 participants at the two ends of the trait impulsivity continuum: high (HI, n = 15) and low (LI, n = 17) impulsive women. We further tested these extreme groups on neuropsychological measures of motor impulsivity (go/no-go, d2), delay discounting (Now or Later Questionnaire), reflection impulsivity (Matching Familiar Figures Test), self-regulation (Revised-Strategy Application Test), and decision making (Iowa Gambling Task). High-trait-impulsivity women were found to commit more commission errors in the initial stage of the go/no-go task but also to make fewer omission errors in the d2 test than did low-trait-impulsivity women. Both effects can be accounted for by a lower response criterion in impulsive women. On the other hand, measures of delay discounting, reflection impulsivity, self-regulation, and decision making did not yield significant differences between the two groups. This pattern of results supports the idea that trait impulsivity in healthy women is linked to neurocognitive mechanisms involved in response monitoring and inhibition, but not to mechanisms involved in self-regulation or decision making. These findings temper the assumption that impulsivity is the core cause of dysfunctional risky and/or impulsive behavior in psychopathological or neuropsychological profiles.

Within-subject differences in degree of delay discounting as a function of order of presentation of hypothetical cash rewards

Procedural variants in estimating delay discounting (DD) have been shown to yield significant differences in estimated degree of DD as well as variations in individual patterns of choice. For example, a recent study found significantly different degrees of DD between groups assessed using either an ascending or descending order of presentation of the immediately available rewards. The purpose of this study was to test for within-subject effects of order of presentation of the immediate rewards in a DD task. In a single session, college students (N=29) were asked to complete two DD tasks, one with the immediate rewards presented in ascending order and one in descending order. Consistent with previous results, significantly larger mean area under the discounting curve (AUC) was observed when the descending sequence was used compared to the ascending order of presentation; and the correlation between both measurements was moderate. These results suggest that some DD assessment tasks may be sensitive to contextual variables such as order and range of the reward and delay values.

Dissociable neural representations of future reward magnitude and delay during temporal discounting

In temporal discounting, individuals often prefer smaller immediate rewards to larger delayed rewards, implying a trade off between the magnitude and delay of future rewards. While recent functional magnetic resonance imaging (fMRI) investigations of temporal discounting have generated conflicting findings, no studies have focused on whether distinct neural substrates respond to the magnitude and delay of future rewards. Combining a novel, temporally distributed discounting task with event-related fMRI, we found that while nucleus accumbens (NAcc), mesial prefrontal cortical (MPFC), and posterior cingulate cortical (PCC) activation positively correlated with future reward magnitude, dorsolateral prefrontal cortical (DLPFC) and posterior parietal cortical (PPC) activation negatively correlated with future reward delay. Further, more impulsive individuals showed diminished NAcc activation to the magnitude of future rewards and greater deactivations to delays of future rewards in the MPFC, DLPFC, and PPC. These findings suggest that while mesolimbic dopamine projection regions show greater sensitivity to the magnitude of future rewards, lateral cortical regions show greater (negative) sensitivity to the delay of future rewards, potentially reconciling different neural accounts of temporal discounting.