Electrophysiological measures of conflict and reward processing are associated with decisions to engage in physical effort.

Anterior cingulate cortex (ACC), a key brain region involved in cognitive control and decision making, is suggested to mediate effort- and value-based decision making, but the specific role of ACC in this process remains debated. Here we used frontal midline theta (FMT) and the reward positivity (RewP) to examine ACC function in a value-based decision making task requiring physical effort. We investigated whether (1) FMT power is sensitive to the difficulty of the decision or to selecting effortful actions, and (2) RewP is sensitive to the subjective value of reward outcomes as a function of effort investment. On each trial, participants chose to execute a low-effort or a high-effort behavior (that required squeezing a hand-dynamometer) to obtain smaller or larger rewards, respectively, while their brainwaves were recorded. We replicated prior findings that tonic FMT increased over the course of the hour-long task, which suggests increased application of control in the face of growing fatigue. RewP amplitude also increased following execution of high-effort compared to low-effort behavior, consistent with increased valuation of reward outcomes by ACC. Although neither phasic nor tonic FMT were associated with decision difficulty or effort selection per se, an exploratory analysis revealed that the interaction of phasic FMT and expected value of choice predicted effort choice. This interaction suggests that phasic FMT increases specifically under situations of decision difficulty when participants ultimately select a high-effort choice. These results point to a unique role for ACC in motivating and persisting at effortful behavior when decision conflict is high.