Action planning and control under uncertainty emerge through a desirability-driven competition between parallel encoding motor plans

Living in an uncertain world, nearly all of our decisions are made with some degree of uncertainty about the consequences of actions selected. Although a significant progress has been made in understanding how the sensorimotor system incorporates uncertainty into the decision-making process, the preponderance of studies focus on tasks in which selection and action are two separate processes. First people select among alternative options and then initiate an action to implement the choice. However, we often make decisions during ongoing actions in which the value and availability of the alternatives can change with time and previous actions. The current study aims to decipher how the brain deals with uncertainty in decisions that evolve while acting. To address this question, we trained individuals to perform rapid reaching movements towards two potential targets, where the true target location was revealed only after the movement initiation. We found that reaction time and initial approach direction are correlated, where initial movements towards intermediate locations have longer reaction times than movements that aim directly to the target locations. Interestingly, the association between reaction time and approach direction was independent of the target probability. By modeling the task within a recently proposed neurodynamical framework, we showed that action planning and control under uncertainty emerge through a desirability-driven competition between motor plans that are encoded in parallel.Author summary: Uncertainty is ubiquitous in our sensorimotor interaction with the external world, and motor decisions regularly have to be made in the face of it. Even after an action is selected, there is residual uncertainty, reflecting the subjective belief about whether the current choice is better compared to the alternative options. Over the past years, several experimental studies have reported that in situations affording more than one possible target of action, individuals delay their decision by moving towards an intermediate location between the potential targets, a strategy consistent with increasing chances of collecting more information before making a decision. This spatial averaging behavior suggests that the brain incorporates uncertainty into the planning and execution of actions. However, there is no strong consensus on how uncertainty is represented and resolved in the sensorimotor system, and affects motor decisions. By training people to perform rapid reaching movements in the presence of two potential targets, we provide evidence that goal location uncertainty is resolved within the action-space and through a desirability-driven competition between motor plans that are encoded in parallel.