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 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.

Figure 1

Schematic diagram showing equivalence between ID and PD protocols at the mathematical indifference point (odds against reinforcing = 4). Arrows represent nose-poking for large reward by rats. In the PD protocol, there are an average of four "unlucky" events (omissions) before a "lucky" event (5-pellet food delivery). In the ID protocol, the delay interval should be at least four times the time-out (delay = 15 × 4 = 60 seconds) to reproduce a 5-pellet delivery with approximately the same temporal characteristics as in the PD protocol. To turn this ideal situation into a more realistic account, four times the spontaneous waiting observed between two consecutive trials (further 10 × 4 = 40 seconds) shall be added to delay duration (see Table 1). To optimize performance for long-term benefit, choice should be LL/L before the mathematical indifference point, and SS beyond it.

Figure 2

Choice behavior in rats (n = 8) tested with either the intolerance-to-delay (ID) or the probability discounting (PD) protocols, shown during subsequent daily sessions. Data represent the mean (± SEM) choice (%) of the larger reinforcer, delivered either after a delay (LL in the ID protocol) or with a certain degree of probability (LLL in the PD protocol). In correspondence to each odds level, delay duration to be run was calculated a priori (see also Table 1). Equivalence between odds/probability and delay was re-estimated a posteriori (see e.g. dotted line). The vertical line shows the mathematical indifference point in the PD task, separating the field before (left to the line) from beyond (right to the line) it. The horizontal line shows the level of rats' indifferent choice, separating preference for LL/L (above the line) from preference for SS (below the line). * p < 0.05 when comparing across protocols within a given daily session.

Figure 3

Choice behavior in rats (n = 8) tested with either ID or PD protocols. Data are the same as shown in Figure 2, but the ID-task curve has been scaled according to the a posteriori re-estimation of equivalence between odds/probability and delay (see also Table 1). The asterisks denote daily sessions at which points of the PD tasks were significantly different from the corresponding points of the ID task. It is clearly evident that the delay dimension produced aversion for LL, whereas the probabilistic dimension produced a preference for LLL.