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

Abstract

Voluntary actions are preceded by the Readiness Potential (RP), a slow EEG component generated in supplementary motor area. The RP is usually thought to be specific to internally-driven decisions to act, and 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 gambles. We found no difference in an RP-related motor component when comparing actions driven by strong versus weak evidence, indicating that the RP does not reflect uncertainty. In contrast, the same RP-related component showed higher amplitudes actions performed without external evidence (guesses) than for those performed in response to equivocal, conflicting evidence. This supports the view that the RP reflects the internal generation of action, rather than decision uncertainty.