Effects of reward and effort history on decision-making and movement vigor during foraging

Abstract

During foraging, animals explore a site and harvest reward, and then abandon that site and travel to the next opportunity. One aspect of this behavior involves decision-making, while the other involves movement control. These two aspects of behavior may be linked via an underlying desire to maximize a single normative utility: the sum of all rewards acquired, minus all efforts expended, divided by time. According to this theory, the history of rewards, and not just its immediate availability, should dictate how long one should stay and harvest reward, and how vigorously one should travel to the next opportunity. We tested this theory in a series of experiments in which humans used their hand to harvest tokens at a reward patch, and then used their arm to reach toward another patch. After a history of high rewards, the subjects not only shortened their harvest duration, but they also moved more vigorously toward the next reward opportunity. In contrast, after a history of high effort, they lengthened their harvest duration but reduced their movement vigor, reaching more slowly to the next reward site. Thus, a history of high reward or low effort biased decisions by promoting early abandonment of the reward site, and biased movements by promoting vigor.