The Readiness Potential reflects the internal source of action, rather than decision uncertainty

Abstract Voluntary actions are preceded by a Readiness Potential (RP), a slow EEG component generated in medial frontal cortical areas. The RP is classically thought to be specific to internally-driven decisions to act, and to reflect post-decision motor preparation. Recent work suggests instead that it may reflect noise or conflict during the decision itself, with internally-driven decisions tending to be more random, more conflictual and thus more uncertain than externally-driven actions. To contrast accounts based on endogenicity with accounts based on uncertainty, we recorded EEG in a task where participants decided to act or withhold action to accept or reject visually-presented gambles, and used multivariate methods to extract an RP-like component.. We found no difference in amplitude of this component between actions driven by strong versus weak evidence, suggesting that the RP may not reflect uncertainty. In contrast, the same RP-like component showed higher amplitudes prior to actions performed without any external evidence (guesses) than for actions performed in response to equivocal, conflicting evidence. This supports the view that the RP reflects the internal source of action, rather than decision uncertainty. Significance Statement The EEG Readiness Potential (RP) seen prior to self-initiated actions is often taken as a neural marker of volition (Shibasaki & Hallett, 2006). It has been argued that the RP may instead reflect conflict and uncertainty in the decisions leading up to these actions, raising questions about decades of previous findings. We directly tested these explanations by asking participants to decide whether to act or not in a gambling task, while manipulating both uncertainty and the source of the information (internal or external) guiding the decision. Only the source of information affected the presence of the RP. Therefore, the RP remains a marker of internally-generated voluntary action, not a marker of uncertainty.