Preparing to act follows Bayesian inference rules

Predictive brain theories suggest that human brain sets-up predictive models to anticipate incoming sensory evidence. Recent studies demonstrated these models to be integrated already in sensory areas, shaping even perceptual outcomes. Here, we hypothesized that this integration process informs the entire functional hierarchy, thus scaling all the way down to the motor system. Operationally, we propose that cue-oriented cortico-spinal excitability (CSE) modulation serves the pre-activation of motor representations aligned to prior-congruent decisions. To this end, 62 participants completed a probabilistic discrimination task while we delivered bilateral single-pulse TMS over the two primary motor cortices (M1s) and recorded motor-evoked potentials (MEPs) to assess motor excitability associated to prior-congruent vs. incongruent actions separately encoded by the two hands. Our findings revealed that prior expectations shaped CSE well before action execution, predominantly by inhibiting the M1 cortex coding for the prior-incongruent action. Importantly, this physiological modulation underpinned the prior-induced bias in participants’ choices, highlighting the link between motor preparatory modulation and actual decision-making. Furthermore, we observed significant interindividual variability in prior-driven CSE modulations, revealing two distinct predictive strategies: the believers’ style, who heavily rely on prior, and the empiricists one, who downplay its role, maintaining CSE level mostly unbiased. Crucially, autistic and schizotypal traits drove these differences in prior-driven motor strategy, with believers characterized by schizotypal traits, whereas empiricists displaying autistic-like features. These results demonstrate how predictive models are integrated into action representations and highlight the role of prior-driven CSE modulations as a potential marker able to intercept interindividual differences in predictive styles.