Search

The purpose of this review is to introduce the reader to the concepts of delay and probability discounting as well as the major empirical findings to emerge from research with humans on these concepts. First, we review a seminal discounting study by Rachlin, Raineri, and Cross (1991) as well as an influential extension of this study by Madden, Petry, Badger, and Bickel (1997). In doing so, we describe the general procedure used for assessing discounting and the resulting form of the obtained discounting functions. With this as background, we review additional empirical findings that have emerged over the last 20 years, by addressing issues such as time consistency and preference reversals, magnitude effects, and the gain-loss asymmetry. Finally, we conclude with a discussion of more recent developments and their implications for the future of this growing area. (Contains 3 figures and 9 footnotes.)

Delay discounting describes the tendency for organisms to devalue outcomes because they are delayed. Robust, positive correlations exist between excessive delay discounting and many maladaptive behaviors (e.g., substance abuse, obesity). Several studies have demonstrated that delay discounting can be reduced and this may hold promise for improving treatment outcomes. One method of reducing delay discounting provides rats with extended training with delayed reinforcement (i.e., delay-exposure training) and this significantly reduces impulsive choices, relative to rats trained with an equal number of immediate-reinforcement sessions (i.e., immediate-exposure training). To evaluate the stability of this effect, 12 weanling male Wistar rats were randomly assigned to receive either delay-exposure or immediate-exposure training for 120 sessions. Impulsive choice was assessed using an increasing-delay procedure immediately following training and 120 days after completion of the initial assessment. Delay-exposed rats discounted delayed food rewards significantly less than immediate-exposed rats in the initial assessment and the reassessment conducted 120 days later. These results are encouraging as they suggest that the effects of delay-exposure training are robust to the passage of time and intervening experience. [ABSTRACT FROM AUTHOR]

Background Prior human research indicates robust, positive relations between impulsive choice (i.e., preference for smaller, immediate over larger, delayed rewards) and alcohol use disorders. However, varied findings in the nonhuman literature reveal a relatively ambiguous relation between impulsive choice and alcohol consumption in rodents. In addition, few rodent studies have investigated potential relations between impulsive choice and common covariates of alcohol consumption (e.g., avidity for sweet substances or anxiety-like behavior). Methods Ninety-two male Long-Evans rats completed an impulsive-choice task. From this larger sample, extreme high- and low-impulsive groups ( n = 30 each) were retained for further testing. In separate tests, subsequent open-field behavior and consumption of oral alcohol (12% w/v) and isocaloric sucrose were examined. Impulsive choice was then retested to examine whether behavior remained stable over the course of the experiment. Results No significant relations emerged between impulsive choice and either alcohol or sucrose consumption. However, impulsive choice predicted greater anxiety-like behavior (avoidance of the center field, defecation) in the open-field test. In turn, greater anxiety predicted lower alcohol and sucrose consumption. Finally, choice remained generally stable across the experiment, although high-impulsive rats tended toward less impulsive choice in the retest. Conclusions Although impulsive choice and alcohol consumption appear to share some variance with anxiety-like behavior, the present data offer no support for a relation between impulsive choice and alcohol consumption in Long-Evans rats. Together with mixed rodent data from prior reports, these findings attenuate cross-species comparisons to human relations between impulsive choice and alcohol use disorders. [ABSTRACT FROM AUTHOR]

In a prior study (Stein et al., 2013), we reported that rats pre-exposed to delayed rewards made fewer impulsive choices, but consumed more alcohol (12% wt/vol), than rats pre-exposed to immediate rewards. To understand the mechanisms that produced these findings, we again pre-exposed rats to either delayed (17.5 s; n = 32) or immediate ( n = 30) rewards. In posttests, delay-exposed rats made significantly fewer impulsive choices at 15- and 30-s delays to a larger, later food reward than the immediacy-exposed comparison group. Behavior in an open-field test provided little evidence of differential stress exposure between groups. Further, consumption of either 12% alcohol or isocaloric sucrose in subsequent tests did not differ between groups. Because Stein et al. introduced alcohol concentration gradually (3-12%), we speculate that their group differences in 12% alcohol consumption were not determined by alcohol's pharmacological effects, but by another variable (e.g., taste) that was preserved as an artifact from lower concentrations. We conclude that pre-exposure to delayed rewards generalizes beyond the pre-exposure delay; however, this same experimental variable does not robustly influence alcohol consumption. [ABSTRACT FROM AUTHOR]

Delay discounting describes the devaluation of a reward as the delay to the receipt of the reward increases. Because steep delay discounting is robustly correlated with a number of behavioral problems (e.g., substance dependence, gambling) and some evidence suggests steep discounting precedes and predicts drug-taking in humans and rats, this study sought to experimentally reduce rats' delay discounting. Human stimulant-dependent participants given working-memory training reportedly decreased their rates of discounting relative to a sham-training group (Bickel, Yi, Landes, Hill, & Baxter, 2011). To evaluate the cross-species generality of this effect, 38 male Long-Evans rats, matched on pretraining delay-discounting rates, were randomly assigned to receive 140 sessions of working-memory training or sham training (which required no memory of the sample stimulus). Large between-group differences in working memory were observed after training; however, posttraining delay-discounting rates were undifferentiated across groups. Potential explanations for these findings are discussed. [ABSTRACT FROM AUTHOR]

A number of maladaptive behaviors and poor health outcomes (e.g., substance abuse, obesity) correlate with impulsive choice, which describes the tendency to prefer smaller, immediate rewards in lieu of larger, delayed rewards. Working memory deficits are often reported in those diagnosed with the same maladaptive behaviors. Human studies suggest that impulsive choice is associated with working memory ability but, to date, only one study has explored the association between working memory and impulsive choice in rats and no relation was reported. The current study reevaluated the association between working memory and impulsive choice in 19 male Long-Evans rats. Psychophysical adjusting procedures were used to quantify working memory (titrating-delay match-to-position procedure) and impulsive choice (adjusting delay procedure). Rats were partitioned into low- and high-impulsive groups based on performance in the impulsive choice task. Low-impulsive rats performed significantly better in the working memory assessment. Across all rats, impulsive choice was negatively correlated with working memory performance. These findings support the hypothesis that prefrontal cortex function, specifically, working memory, is related to impulsive choice. Future research might profitably examine the experimental variables designed to influence working memory to evaluate the effects of these variables on impulsive choice and maladaptive behaviors with which it is correlated. [ABSTRACT FROM AUTHOR]