Cocaine exposure causes long-term increases in impulsive choice.

In this study, the authors examined the long-term effects of prior exposure to cocaine on a delay-discounting task commonly used to measure impulsive choice. Male Long-Evans rats received daily intraperitoneal injections of 30 mg/kg cocaine HCl or saline for 14 days. Following 3 weeks of withdrawal, rats began training. On each trial, rats were given a choice between 2 levers. A press on 1 lever resulted in immediate delivery of a single 45-mg food pellet, and a press on the other resulted in delivery of 4 pellets after a delay period. Impulsive choice was defined as preference for the small immediate over the large delayed reward. Three months after treatment, cocaine-exposed rats displayed increased impulsive choice behavior. They also showed less anticipatory responding (entries into the food trough) during the delays prior to reward delivery, indicating that the enhanced impulsive choice in these rats may be related to deficits in bridging the delay between response and reward. These data demonstrate that cocaine exposure can cause enduring increases in impulsive choice behavior, consistent with observations in human subjects with drug addictions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Reward discounting as a measure of impulsive behavior in a psychiatric outpatient population.

Impulsivity has been operationalized as a choice of an immediate smaller reward over a larger delayed or uncertain reward. This study examined a procedure that measures reward preference under these contingencies in psychiatric outpatients considered either at a high or low risk for engaging in impulsive behavior depending on their psychiatric diagnoses. The participants' rates of delay and uncertainty reward discounting were compared with their performances on a behavioral inhibition task and responses on a self-report personality impulsivity measure. The high-risk participants discounted delayed rewards more sharply and scored higher on the self-report impulsivity measure relative to the low-risk participants. Delay and uncertainty discounting were modestly correlated, but no other relationships were found between the other measures. Results from this study indicate that delay-discounting tasks may be sensitive to at least one form of impulsive behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Interactions between the prefrontal cortex and amygdala during delay discounting and reversal.

Interactions between the prefrontal cortex and amygdala are thought to be critical for reward anticipation. Alterations in reward anticipation that lead to an inability to wait for rewards or a diminished capacity to change behavior when doing so would be optimal are often termed impulsivity and compulsivity, respectively. Distinct regions of the prefrontal cortex may support decreased impulsivity through self-control and decreased compulsivity through flexibility. However, both self-control and flexibility appear to involve the amygdala. Using a delay discounting paradigm, the current investigation found that inactivation and disconnection of the medial prefrontal cortex and basolateral amygdala led rats to become more impulsive by affecting preference for smaller immediate over larger delayed rewards. Conversely, inactivation and disconnection of the orbitofrontal cortex and amygdala led rats to become more compulsive as demonstrated by an inability to flexibly reverse stimulus–reward relationships in an odor reversal task. The current findings support a double dissociation between orbitofrontal cortex–amygdala interactions for odor reversal and medial prefrontal cortex–amygdala interactions for delay discounting. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Positive mood effects on delay discounting.

Delay discounting is the process by which the value of an expected reward decreases as the delay to obtaining that reward increases. Individuals with higher discounting rates tend to prefer smaller immediate rewards over larger delayed rewards. Previous research has indicated that personality can influence an individual's discounting rates, with higher levels of Extraversion predicting a preference for immediate gratification. The current study examined how this relationship would be influenced by situational mood inductions. While main effects were observed for both Extraversion and cognitive ability in the prediction of discounting rates, a significant interaction was also observed between Extraversion and positive affect. Extraverted individuals were more likely to prefer an immediate reward when first put in a positive mood. Extraverts thus appear particularly sensitive to impulsive, incentive-reward-driven behavior by temperament and by situational factors heightening positive affect. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Differential effects on effort discounting induced by inactivations of the nucleus accumbens core or shell.

The authors investigated the contribution of the nucleus accumbens (NAc) core and shell to effort-based decision making using a discounting procedure. Selection of 1 lever delivered a smaller, 2-pellet reward immediately, whereas the other lever delivered a 4-pellet reward after a fixed ratio of presses (2, 5, 10, or 20) that increased over 4 blocks of 10 discrete choice trials. Subsequent testing employed an equivalent delays procedure, whereby the relative delay to reward delivery after selection of either option was equalized. In well-trained rats, inactivation of the core, but not the shell, via infusion of GABA A/B agonists muscimol/baclofen reduced preference for the high-effort option under standard conditions and also when rats were tested using an equivalent delays procedure. However, inactivation of the core did not alter preference for 4-pellet versus 2-pellet rewards when the relative costs of each option were the same (1 press). Thus, the NAc core, but not the shell, appears to be part of a neural circuit that biases choice toward larger rewards associated with a greater effort cost. Furthermore, the contributions by the NAc core to this form of decision making can be dissociated from its role in delay discounting. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Delay discounting of cocaine by rhesus monkeys.

The present, subjective value of a reinforcer typically decreases as a function of the delay to its receipt, a phenomenon termed delay discounting. Delay discounting, which is assumed to reflect impulsivity, is hypothesized to play an important role in drug abuse. The present study examined delay discounting of cocaine injections by rhesus monkeys. Subjects were studied on a discrete-trials task in which they chose between 2 doses of cocaine: a smaller, immediate dose and a larger, delayed dose. The immediate dose varied between 0.012 and 0.4 mg/kg/injection, whereas the delayed dose was always 0.2 mg/kg/injection and was delivered after a delay that varied between 0 and 300 s in different conditions. At each delay, the point at which a monkey chose the immediate and delayed doses equally often (i.e., the ED50) provided a measure of the present, subjective value of the delayed dose. Dose-response functions for the immediate dose shifted to the left as delay increased. The amount of the immediate dose predicted to be equal in subjective value to the delayed dose decreased as a function of the delay, and hyperbolic discounting functions provided good fits to the data (median R2 = .86). The current approach may provide the basis for an animal model of the effect of delay on the subjective value of drugs of abuse. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Observing response acquisition: Preference for unpredictable appetitive rewards obtained under conditions predicted by DMOD.

To investigate the preference for unpredictable rewards predicted by the present author and J. T. Daly's (see record 1983-20275-001) modification, known as the DMOD model, of R. A. Rescorla and A. R. Wagner's (1972) previous model of reinforcement, the present author conducted 5 E-maze experiments with 144 male Holtzman rats. In Exps I–V, Ss were given a choice between receiving reward and nonreward in a situation in which stimuli were correlated with reward outcome (predictable situation) vs a situation in which the stimuli were uncorrelated with reward outcome (unpredictable situation). Preference for the unpredictable situation occurred under the following conditions: small (1 37-mg pellet) immediate rewards, small delayed (15-sec) rewards when the cues correlated with reward outcome were absent during the delay interval, large (15 pellets) immediate rewards when a difficult discrimination was required, and when the stimulus predicting nonreward was present at the choice point. Preference for the predictable situation was strongest if reinforcement was delayed and large or if the stimulus predicting reward was present at the choice point. A weaker preference for the predictable situation occurred if reinforcement was immediate, large, and required a simple discrimination or if reinforcement was large and delayed and the cues that correlated with reward outcome were absent during the delay interval. Findings support the predictions of the DMOD model of appetitive learning. (33 ref) (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Impulsive choice as a predictor of acquisition of IV cocaine self- administration and reinstatement of cocaine-seeking behavior in male and female rats.

Drug abuse and impulsive choice are related in humans. In female rats, impulsive choice predicted the rate of acquisition of IV cocaine self-administration. The objectives of the present experiments were to: (a) compare impulsive choice in males and females, (b) extend previous research on impulsive choice and acquisition of cocaine self-administration to males, and (c) compare males and females during maintenance, extinction, and reinstatement of cocaine-seeking behavior. Male and female rats were trained on an adjusting delay task in which a response on one of two levers yielded one food pellet immediately, and a response on the other resulted in three pellets after an adjusting delay that decreased after responses on the immediate lever and increased after responses on the delay lever. A mean adjusted delay (MAD) was used as the quantitative measure of impulsivity. In Experiment 1, MADs were analyzed for sex differences. In Experiment 2, acquisition of cocaine self-administration was examined in rats selected for high (HiI; MADs ≤9 seconds) or low (LoI; MADs ≥13 seconds) impulsivity. In Experiment 3, HiI and LoI groups were compared on maintenance and extinction of cocaine self-administration and cocaine-primed reinstatement of drug-seeking behavior. There were no sex differences in impulsive choice; however, HiI male and female rats acquired cocaine self-administration faster than their LoI counterparts. LoI females responded more on a cocaine-associated lever during maintenance and extinction than HiI females, but HiI females showed greater reinstatement of cocaine-seeking behavior than all other groups at the highest dose tested (15 mg/kg). Thus, individual differences in impulsive choice were associated with differences in cocaine-seeking behavior. Impulsive choice and sex may be additive vulnerability factors in certain phases of drug abuse. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of the nucleus accumbens core in impulsive choice, timing, and reward processing.

The present series of experiments aimed to pinpoint the source of nucleus accumbens core (AcbC) effects on delay discounting. Rats were trained with an impulsive choice procedure between an adjusting smaller sooner reward and a fixed larger later reward. The AcbC-lesioned rats produced appropriate choice behavior when the reward magnitude was equal. An increase in reward magnitude resulted in a failure to increase preference for the larger later reward in the AcbC-lesioned rats, whereas a decrease in the larger later reward duration resulted in normal alterations in choice behavior in AcbC-lesioned rats. Subsequent experiments with a peak timing (Experiments 2 and 3) and a behavioral contrast (Experiment 4) indicated that the AcbC-lesioned rats suffered from decreased incentive motivation during changes in reward magnitude (Experiments 2 and 4) and when expected rewards were omitted (Experiments 2 and 3), but displayed intact anticipatory timing of reward delays (Experiments 2 and 3). The results indicate that the nucleus accumbens core is critical for determining the incentive value of rewards, but does not participate in the timing of reward delays. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Impulsivity and rapid discounting of delayed hypothetical rewards in cocaine-dependent individuals.

In this study, crack/cocaine-dependent (CD) and non-drug-using matched control (MC) participants were presented with hypothetical immediate and delayed rewards, with 16 delay conditions ranging from 5 min to 25 years. All participants were presented with hypothetical monetary rewards; however, the CD group was also presented with hypothetical crack/ cocaine rewards. The objective value of the rewards ranged from $1 to $1,000. Hyperbolic discounting functions provided a good fit of the data. The CD group discounted monetary rewards at a higher rate than the MC group did, and the CD group discounted crack/cocaine rewards at a higher rate than it did monetary rewards. Moreover, scores on self-report measures indicated greater impulsivity in the CD group when compared with the MC group. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal horizons in decision making.

The perception of duration is crucial when we make choices between immediate and delayed rewards. Immediate rewards are valued more than the same rewards if they are delayed. Preferences for earlier rewards become even stronger when the reward can be received within a subjectively proximate time frame. A “rational” decision to wait for a delayed reward may be overruled by an “impulsive” choice when the option for the closer reward falls within this proximate time range. Based on findings on circadian and circannual physiological rhythms, we suggest that there are 2 time units that are both biologically and culturally determined and have an impact on human experience and behavior: the day and the year. We highlight results of a neuroimaging study showing that rewards with delays up to 1 year are discounted differently than reward delays longer than 1 year. This duration-dependent discounting is associated with specific brain activation in the striatum. We present various conceptualizations of subjective time incorporated in parametric models of intertemporal decisions that may lead to a better understanding of human choice behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Disinhibitory psychopathology and delay discounting in alcohol dependence: Personality and cognitive correlates.

Increased discounting of delayed rewards may reflect a decision bias that contributes to excessive use of alcohol and more generally, to an impulsive, disinhibitory predisposition that is characterized by a preference for immediate over long-term rewards. The current study examined the association between delay discounting of rewards and the covariation among several types of disinhibitory problems that are often comorbid with alcohol dependence (AD). Lifetime problems with alcohol, marijuana, other drugs, childhood conduct disorder, and adult antisocial behavior were assessed in a sample of 426 young adults, 257 of whom had a lifetime diagnosis of AD. Higher delay discounting rates were associated with the covariation among all domains of disinhibitory problems and were not uniquely associated with any one domain. Higher delay discounting rates also were associated with lower intelligence, lower working memory capacity, and higher trait impulsivity. The results suggest that increased delay discounting of rewards may reflect aspects of a general vulnerability to externalizing, disinhibitory disorders. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal Discounting When the Choice Is Between Two Delayed Rewards.

The present experiments extend the temporal discounting paradigm from choice between an immediate and a delayed reward to choice between 2 delayed rewards: a smaller amount of money available sooner and a larger amount available later. Across different amounts and delays, the data were consistently well described by a hyperbola-like discounting function, and the degree of discounting decreased systematically as the delay to the sooner reward increased. Three theoretical models (the elimination-by-aspects, present-value comparison, and common-aspect attenuation hypotheses) were evaluated. The best account of the data was provided by the common-aspect attenuation hypothesis, according to which the common aspect of the choice alternatives (i.e., the time until the sooner reward is available) receives less weight in the decision-making process. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Alcohol demand, delayed reward discounting, and craving in relation to drinking and alcohol use disorders.

A behavioral economic approach to alcohol use disorders (AUDs) emphasizes both individual and environmental determinants of alcohol use. The current study examined individual differences in alcohol demand (i.e., motivation for alcohol under escalating conditions of price) and delayed reward discounting (i.e., preference for immediate small rewards compared to delayed larger rewards) in 61 heavy drinkers (62% with an AUD). In addition, based on theoretical accounts that emphasize the role of craving in reward valuation and preferences for immediate rewards, craving for alcohol was also examined in relation to these behavioral economic variables and the alcohol-related variables. Intensity of alcohol demand and delayed reward discounting were significantly associated with AUD symptoms, but not with quantitative measures of alcohol use, and were also moderately correlated with each other. Likewise, craving was significantly associated with AUD symptoms, but not with alcohol use, and was also significantly correlated with both intensity of demand and delayed reward discounting. These findings further emphasize the relevance of behavioral economic indices of motivation to AUDs and the potential importance of craving for alcohol in this relationship. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Competition between novelty and cocaine conditioned reward is sensitive to drug dose and retention interval.

The conditioned rewarding effects of novelty compete with those of cocaine for control over choice behavior using a place conditioning task. The purpose of the present study was to use multiple doses of cocaine to determine the extent of this competition and to determine whether novelty’s impact on cocaine reward was maintained over an abstinence period. In Experiment 1, rats were conditioned with cocaine (7.5, 20, or 30 mg/kg ip) to prefer one side of an unbiased place conditioning apparatus relative to the other. In a subsequent phase, all rats received alternating daily confinements to the previously cocaine paired and unpaired sides of the apparatus. During this phase, half the rats had access to a novel object on their initially unpaired side; the remaining rats did not receive objects. The ability of novelty to compete with cocaine in a drug free and cocaine challenge test was sensitive to cocaine dose. In Experiment 2, a place preference was established with 10 mg/kg cocaine and testing occurred after 1, 14, or 28 day retention intervals. Findings indicate that choice behaviors mediated by cocaine conditioning are reduced with the passing of time. Taken together, competition between cocaine and novelty conditioned rewards are sensitive to drug dose and retention interval. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Discounting of delayed rewards in opioid-dependent outpatients: Exponential or hyperbolic discounting functions?

Recent theories of substance abuse have used value discounting of delayed rewards to partly explain the decision to take drugs. Normative-economic theory holds that an exponential function describes the effects of delay on discounting, whereas the matching law posits a hyperbolic discounting function. The ability of these functions to describe 18 human heroin-dependent individuals' monetary- and heroin-reward delay-discounting functions was assessed. In the 1st condition, participants chose between immediate and delayed hypothetical monetary rewards. Delayed rewards were $1,000, and the immediate reward amount was adjusted until choices reflected indifference. In the 2nd condition, participants chose between immediate and delayed heroin (the delayed amount was that which each participant reported he or she could purchase with $1,000). The hyperbolic function produced significantly higher R2 values and significantly lower sums of squared error values. Consistent with previous findings, delayed heroin rewards were discounted at a significantly higher rate than were delayed monetary rewards. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Amount of reward has opposite effects on the discounting of delayed and probabilistic outcomes.

Previous research has shown that the value of large future rewards is discounted less steeply than is the value of small future rewards. These experiments extended this line of research to probabilistic rewards. Two experiments replicated the standard findings for delayed rewards but demonstrated that amount has an opposite effect on the discounting of probabilistic rewards. That is, large probabilistic amounts were discounted at the same or higher rates than small amounts. Although amount had opposite effects on the discounting of delayed and probabilistic rewards, nevertheless, the same form of mathematical function accurately described discounting of both types of reward. The findings suggest that fundamentally similar, but not identical, processes are involved in decision making regarding delayed and probabilistic rewards. The implications of these findings for impulsivity and self-control are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Delay discounting in C57BL/6J and DBA/2J mice: Adolescent-limited and life-persistent patterns of impulsivity.

Impulsivity is a defining characteristic of adolescence. Compared to adults, for example, adolescents engage in higher rates of drug and alcohol experimentation, risky sexual practices, and criminal activity. Such behavior may reflect reduced sensitivity to long-term consequences of behavior during adolescence. Recently, our lab has attempted to refine mouse procedures to study developmental trends in decision making in the laboratory. In the present experiment, we examined sensitivity to delayed rewards in C57BL/6J (B6) and DBA/2J (D2) mice during adolescence and adulthood using an adaptation of a 2-week delay discounting procedure developed by Adriani and Laviola (2003). During training, mice could choose between a 20- or 100-μl drop of milk delivered after a 1-s delay. During testing, the delay to the large drop of milk was increased from 1 to 100 seconds. As the delay to the larger volume increased, preference shifted to the smaller, more immediate option. In adolescence, both strains showed similar shifts in preference. In contrast, adult B6 mice were less sensitive to increasing delays than were adult D2 mice, who continued to perform much as their adolescent counterparts. A subsequent resistance-to-extinction test ruled out the possibility that the slower change in the adult B6 mice was due to perseverative responding. The present findings suggest that B6 and D2 strains may be differentially suited to uncovering the biological mechanism of short-term and long-term patterns of impulsive behavior. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Pathological gamblers, with and without substance abuse disorders, discount delayed rewards at high rates.

Pathological gambling is classified as a disorder of impulse control, yet little research has evaluated behavioral indices of impulsivity in gamblers. The rates at which rewards delayed in time are subjectively devalued may be a behavioral marker of impulsivity. This study evaluated delay discounting in 60 pathological gamblers and 26 control participants. Gamblers were divided into those with (n?=?21) and without (n?=?39) substance use disorders. A hypothetical $1,000 reward was delayed at intervals ranging from 6 hr to 25 years, and immediate rewards varied from $1 to $999. Pathological gamblers discounted delayed rewards at higher rates than control participants, and gamblers with substance use disorders discounted delayed rewards at higher rates than non-substance-abusing gamblers. These data provide further evidence that rapid discounting of delayed rewards may be a feature central to impulse control and addictive disorders, including pathological gambling. (PsycINFO Database Record (c) 2010 APA, all rights reserved)