Motor plans under uncertainty reflect a trade-off between maximizing reward and success

Abstract

When faced with multiple potential movement options, individuals either reach directly to one of the options, or initiate a reach intermediate between the options. It remains unclear why people generate these two types of behaviors. Using the go-before-you-know task (commonly used to study behavior under choice uncertainty) in humans, we examined two key questions. First, do these two types of responses actually reflect distinct movement strategies? If so, the relative desirability (i.e., weighing the success likelihood versus the attainable reward) of the two target options would not need to be computed identically for direct and intermediate reaches. We showed that indeed, when reward and success likelihood differed between the two options, reach direction was preferentially biased toward different directions for direct versus intermediate reaches. Importantly, this suggests that the computation of subjective values depends on the choice of movement strategy. Second, what drives individual differences in how people respond under uncertainty? We found that risk/reward-seeking individuals tended to generate more intermediate reaches and were more responsive to changes in reward, suggesting these movements may reflect a strategy to maximize reward versus success. Together, these findings suggest that when faced with choice uncertainty, individuals adopt movement strategies consistent with their risk/reward attitude, preferentially biasing behavior toward exogenous rewards or endogenous success and consequently modulating the relative desirability of the available options.

Significance Statement

When choosing between multiple options, individuals weigh the relative values of the options (i.e., how rewarding it is and how likely one is to successfully obtain that reward). Moreover, when the outcome is uncertain, individuals typically adopt one of two movement strategies: commit to an option immediately and reach directly for it, or hedge one’s bets by reaching intermediate between the options until more information can be obtained. Here we show that the relative values of the options are not fixed, but change depending on the selected movement strategy (direct or intermediate reaches). These strategies in turn are related to an individual’s tendency to be risk/reward-seeking. Thus option values depend on how individuals choose to respond to uncertainty.