Effects of reward bundling on male rats' preference for larger-later food rewards.

In both humans and nonhumans, prior research demonstrates increased preference for larger-later over smaller-sooner rewards when rewards are bundled together in a series (i.e., when an operant choice produces multiple discrete reward deliveries, as opposed to only a single delivery). These findings can be predicted using a traditional hyperbolic delay-discounting model. The present study was designed to examine the parametric effects of the size of the reward bundle on larger-later reward preference in male rats. During a reward-bundling phase, rats were exposed to bundle-sizes of either 1 (i.e., no bundling), 3, or 9 rewards. Rats in the Bundle-size 9 group showed significantly greater larger-later reward preference across a range of delays (0-17.5 s) than rats in any other group, but no other significant differences in choice were observed between groups. In addition, when choice for unbundled rewards was assessed following the reward-bundling phase, rats in the Bundle-size 9 group showed a significant increase in larger-later reward preference (compared to a pre-test). Obtained data from the reward-bundling phase are compared to model-predicted data, and potential mechanisms of the observed increase in self-control for unbundled rewards are discussed.