Discounting of future rewards and punishments in rats

Abstract

Temporal reward discounting describes the decrease of value of a reward as a function of delay. Decision-making between future aversive outcomes is much less studied, and there is no clear decision pattern across studies: while some authors suggest that human and non-human animals prefer sooner over later painful shocks, others found the exact opposite. In a series of three experiments, Long-Evans rats chose between differently timed electric shocks and rewards in a T-maze. In experiment 1, rats chose between early and late painful shocks with identical, long reward delays, in experiment 2, they chose between early reward and early shocks, or late rewards and late shocks, in experiment 3, they chose between early and late rewards, with identical, short delays to the shock. We tested the predictions of two competing hypotheses: the aversive discounting theory assumes that future shocks are discounted, and, hence, less unpleasant than early shocks. The utility from anticipation theory implies that rats derive negative utility from waiting for the shock; late shocks should, hence, be more unpleasant than early shocks. We did not find unanimous evidence for either theory. Instead, our results are more consistent with the post-hoc idea that shocks may have negative spill-over effects on reward values – the closer in time a shock is to a subsequent reward, the stronger the reward is devalued. Interestingly and consistent with our theory, we find that, depending on the temporal shock-reward contiguity, rats can be brought to prefer later over sooner rewards of identical magnitudes.

SIGNIFCANCE STATEMENT

It is well understood how animals discount future rewards, but much less is known how they choose between future aversive outcomes. We designed a novel task to examine decision-making between differently timed electric shocks and rewards. Although rats revealed clear preferences for sooner over later shocks, we found no evidence that they derived negative utility from waiting for the shocks (dread), nor that they discounted the disutility of future shocks. Instead, their choices were consistent with the novel hypothesis that shocks have negative, time-dependent spill-over effects on the utility of subsequent rewards. Consistent with this, we find that, depending on the temporal shock-reward contiguity, rats can be brought to prefer later over sooner rewards, thus promoting self-control.