Unsigned value prediction-error modulates the motor system in absence of choice.

Human actions are driven by the pursuit of goals, especially when achieving these goals entails a reward. Accordingly, recent work showed that anticipating a reward in a motor task influences the motor system, boosting motor excitability and increasing overall readiness. Attaining a reward typically requires some mental or physical effort. Recent neuroimaging evidence suggested that both reward expectation and effort requirements are encoded by a partially overlapping brain network. Moreover, reward and effort information are combined in an integrative value signal. However, whether and how mental effort is integrated with reward at the motor level during task preparation remains unclear. To address these issues, we implemented a mental effort task where reward expectation and effort requirements were manipulated. During task preparation, TMS was delivered on the motor cortex and motor-evoked potentials (MEPs) were recorded on the right hand muscles to probe motor excitability. The results showed an interaction of effort and reward in modulating the motor system, reflecting an unsigned value prediction-error signal. Crucially, this was observed in the motor system in absence of a value-based decision or value-driven action selection. This suggests a high-level cognitive factor such as unsigned value prediction-error can modulate the motor system. Interestingly, effort-related motor excitability was also modulated by individual differences in tendency to engage in (and enjoy) mental effort, as measured by the Need for Cognition questionnaire, underlining a role of subjective effort experience in value-driven preparation for action.