Brace yourself: neural and computational insights into the experience of mental effort.

The subjective experience of mental effort is critical for adaptive cognitive control, yet its neural and computational underpinnings remain elusive. Here, we combine hierarchical drift-diffusion modeling and EEG to investigate how trial-by-trial fluctuations in both preparatory and task-related neural activity shape subjective effort ratings. Participants performed an arithmetic task of variable difficulty, choosing task difficulty in advance, which allowed us to isolate neural signatures of preparation (contingent negative variation) and task engagement (P3 amplitude). Computational modeling revealed that participants adjusted decision boundaries based on anticipated difficulty, reflecting heightened caution. Critically, subjective effort ratings tracked this increased caution, likely reflecting the cost of additional accumulation. EEG analyses showed that while subjective effort was sensitive to the P3 amplitude, indicating exerted effort during task performance, it was insensitive to preparatory CNV activity. Our findings offer novel insights into the computations underlying subjective effort, proposing a selective role for exerted, but not preparatory activity.