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

Abstract

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 durg-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 amonut of food obtained. In the PD protocol, animals developed a stable preference for LLL reward, maintained it even when SS and LLL options wre 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 no 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 interepret choice behavior in ID and PD protocols differently. © 2006 Adriani and Laviola; licensee BioMed Central Ltd.