Generalizability of time-based interventions: Effects of choice procedure and smaller-sooner delay

Active and passive waiting in impulsive choice: Effects of fixed-interval and fixed-time delays

Behavioral interventions to improve self-control, preference for a larger-later (LL) reward over a smaller-sooner (SS) reward, involve experience with delayed rewards. Whether they involve timing processes remains controversial. In rats, there have been inconsistent results on whether timing processes may be involved in intervention-induced improvements in self-control. Interventions that improved self-control with corresponding timing improvements used fixed-interval (FI) delays, whereas interventions that failed to find corresponding timing improvements used fixed-time (FT) delays. The FI schedule includes a response contingency (active waiting), whereas the FT schedule delivers reward automatically (passive waiting). The present study compared the effects of FI and FT schedules in interventions and impulsive choice tasks to evaluate effects on self-control and timing behavior. The impulsive choice task evaluated preference for an SS option (one pellet after 10-, 15-, 20-, 25-, and 30-s delays) versus an LL option (two pellets after a 30-s delay). The intervention task included forced-choice SS (one pellet after 10 s) and LL (two pellets after 30 s) sessions under FI or FT schedules. FI schedules produced greater sensitivity to SS delay in the impulsive choice task. Both FI and FT interventions increased LL choices. Following choice testing, temporal bisection and peak interval tasks revealed better timing precision for rats with an FI delay experience. Overall, the FI choice contingency was associated with improved temporal attention and timing precision.

Mechanisms of impulsive choice: Experiments to explore and models to map the empirical terrain

Impulsive choice is preference for a smaller-sooner (SS) outcome over a larger-later (LL) outcome when LL choices result in greater reinforcement maximization. Delay discounting is a model of impulsive choice that describes the decaying value of a reinforcer over time, with impulsive choice evident when the empirical choice-delay function is steep. Steep discounting is correlated with multiple diseases and disorders. Thus, understanding the processes underlying impulsive choice is a popular topic for investigation. Experimental research has explored the conditions that moderate impulsive choice, and quantitative models of impulsive choice have been developed that elegantly represent the underlying processes. This review spotlights experimental research in impulsive choice covering human and nonhuman animals across the domains of learning, motivation, and cognition. Contemporary models of delay discounting designed to explain the underlying mechanisms of impulsive choice are discussed. These models focus on potential candidate mechanisms, which include perception, delay and/or reinforcer sensitivity, reinforcement maximization, motivation, and cognitive systems. Although the models collectively explain multiple mechanistic phenomena, there are several cognitive processes, such as attention and working memory, that are overlooked. Future research and model development should focus on bridging the gap between quantitative models and empirical phenomena.

Effects of the estrous cycle on impulsive choice and interval timing in female rats

Research in humans and animals shows differences in impulsive choice, which is a failure to wait for larger, delayed rewards, when comparing males and females. It is possible that fluctuations in sex hormones (estradiol and progesterone) across the reproductive cycle contribute to sex differences in impulsive choice. The current study delivered an impulsive choice task with peak interval trials to female rats while estrous cycles, the rodent reproductive cycle, were tracked over the course of the task. Female rats were more sensitive to changes in delay in the proestrus phase of the estrous cycle and made more larger-later choices when in estrus, particularly when the delay to the smaller reward was short. Estradiol increases dramatically during proestrus while progesterone peaks during estrus, suggesting that estradiol and progesterone may affect impulsive choice through mechanisms such as delay discounting, delay aversion, and/or timing processes. Analyses of timing of the choice task delays showed inconsistent effects of the estrous cycle across delays, suggesting that reward-timing interactions may have complicated how hormone fluctuations affected interval timing. Further research is needed to determine the mechanism underlying increased larger-later choices during the estrus phase, increased delay sensitivity during the proestrus phase, and variability in interval timing across delays and estrous cycle stages.

A behavioral timing intervention upregulates striatal serotonergic markers and reduces impulsive action in adult male mice

Many studies support the hypothesis that time-based interventions reduce impulsive behavior in rodents. However, few studies have directly assessed 1) how such interventions affect impulsive action rather than impulsive choice, 2) if intervention effects differ by sex, and 3) how time-based interventions affect neurochemistry in regions mediating decision-making and reward. Thus, we assessed how a fixed-interval (FI) intervention initiated during late adolescence and extending into adulthood affected dopaminergic and serotonergic analytes in the frontal cortex and striatum and subsequent impulsive action in adult male and female mice. Beginning on postnatal day (PND) 45, mice were either trained on a progressive series of FI schedules (FI 20, 40, & 60 s) or remained in the home cage. Following the intervention, increases in striatal serotonergic analytes were found in FI-exposed males and females (n = 8/sex/group) with few changes found in the frontal cortex. Impulsive action was assessed in the remaining mice (n = 10/sex/group) using a fixed-ratio waiting-for-reward (FR-wait) task in which completion of an FR-25 component initiated a "free" pellet component in which pellets were delivered at increasing intervals according to a fixed delay increment that varied across sessions. Responses reset the additive delay and initiated a new FR-25 component. FI-exposed males, but not females, showed fewer delay resets and no-wait resets relative to control mice. Importantly, FI-exposure did not affect discrimination reversal performance in either sex. These data suggest that time-based interventions may reduce impulsive action in addition to impulsive choice perhaps with increased male sensitivity. Additionally, time-based interventions appear to operate through striatal serotonergic augmentation.

Testing delay of gratification in rats using a within-session increasing-delay task

In delay discounting, preference reversals refer to shifts in preference from a larger-later reward to a smaller-sooner reward. Steep hyperbolic discounting predicts a preference reversal when a smaller-sooner and larger-later reward both become temporally proximal; prior research is consistent with this prediction. Hyperbolic discounting does not predict a preference reversal, however, after an individual chooses a larger-later reward over a smaller-immediate reward; prior research is inconsistent with this prediction. We sought to replicate and extend these findings using a delay of gratification task in rats. The task included a defection response which allowed rats to reverse their preference after choosing a larger-later sucrose reinforcer to instead obtain a smaller-immediate sucrose reinforcer. In Experiment 1, we found that rats would defect on their choice of the larger-later reinforcer, systematically replicating prior research. We also found that experience on the delay of gratification task led to decreases in defection responses. In Experiment 2, we found that prior experience on an intertemporal choice task, with no opportunity to defect, also led to few defection responses on the delay of gratification task. We discuss our findings in the context of whether inhibitory control or temporal learning could be involved in delay of gratification.