Delay aversion but preference for large and rare rewards in two choice tasks: implications for the measurement of self-control parameters

BACKGROUND: Impulsivity is defined as intolerance/aversion to waiting for reward. In intolerance-to-delay (ID) protocols, animals must choose between small/soon (SS) versus large/late (LL) rewards. In the probabilistic discount (PD) protocols, animals are faced with choice between small/sure (SS) versus large/luck-linked (LLL) rewards. It has been suggested that PD protocols also measure impulsivity, however, a clear dissociation has been reported between delay and probability discounting. RESULTS: Wistar adolescent rats (30- to 46-day-old) were tested using either protocol in drug-free state. In the ID protocol, animals showed a marked shift from LL to SS reward when delay increased, and this despite adverse consequences on the total amount of food obtained. In the PD protocol, animals developed a stable preference for LLL reward, and maintained it even when SS and LLL options were predicted and demonstrated to become indifferent. We demonstrate a clear dissociation between these two protocols. In the ID task, the aversion to delay was anti-economical and reflected impulsivity. In the PD task, preference for large reward was maintained despite its uncertain delivery, suggesting a strong attraction for unitary rewards of great magnitude. CONCLUSION: Uncertain delivery generated no aversion, when compared to delays producing an equivalent level of large-reward rarefaction. The PD task is suggested not to reflect impulsive behavior, and to generate patterns of choice that rather resemble the features of gambling. In summary, present data do indicate the need to interpret choice behavior in ID and PD protocols differently

Serotonin Differentially Regulates Short- and Long-Term Prediction of Rewards in the Ventral and Dorsal Striatum

BACKGROUND: The ability to select an action by considering both delays and amount of reward outcome is critical for maximizing long-term benefits. Although previous animal experiments on impulsivity have suggested a role of serotonin in behaviors requiring prediction of delayed rewards, the underlying neural mechanism is unclear. METHODOLOGY/PRINCIPAL FINDINGS: To elucidate the role of serotonin in the evaluation of delayed rewards, we performed a functional brain imaging experiment in which subjects chose small-immediate or large-delayed liquid rewards under dietary regulation of tryptophan, a precursor of serotonin. A model-based analysis revealed that the activity of the ventral part of the striatum was correlated with reward prediction at shorter time scales, and this correlated activity was stronger at low serotonin levels. By contrast, the activity of the dorsal part of the striatum was correlated with reward prediction at longer time scales, and this correlated activity was stronger at high serotonin levels. CONCLUSIONS/SIGNIFICANCE: Our results suggest that serotonin controls the time scale of reward prediction by differentially regulating activities within the striatum

The Role of Serotonin in the Regulation of Patience and Impulsivity

Classic theories suggest that central serotonergic neurons are involved in the behavioral inhibition that is associated with the prediction of negative rewards or punishment. Failed behavioral inhibition can cause impulsive behaviors. However, the behavioral inhibition that results from predicting punishment is not sufficient to explain some forms of impulsive behavior. In this article, we propose that the forebrain serotonergic system is involved in “waiting to avoid punishment” for future punishments and “waiting to obtain reward” for future rewards. Recently, we have found that serotonergic neurons increase their tonic firing rate when rats await food and water rewards and conditioned reinforcer tones. The rate of tonic firing during the delay period was significantly higher when rats were waiting for rewards than for tones, and rats were unable to wait as long for tones as for rewards. These results suggest that increased serotonergic neuronal firing facilitates waiting behavior when there is the prospect of a forthcoming reward and that serotonergic activation contributes to the patience that allows rats to wait longer. We propose a working hypothesis to explain how the serotonergic system regulates patience while waiting for future rewards