Neurophysiological Fluctuation Detects Mental Fatigue Prior to Subjective Awareness

Objective and timely detection of mental fatigue is critical to its understanding and intervention. However, previous studies did not clearly dissociate mental fatigue from other arousal states. Here, we propose an adaptive, cognitively demanding memory recall task to induce mental fatigue, taking into account individual variability while maintaining task engagement. We found a decrease in pupil size and response speed, and enhanced frontal and frontotemporal beta- and gamma-band power of electroencephalography (EEG) after mental fatigue induction, while maintaining a constant level of boredom and motivation. Crucially, we found increased gamma-band power fluctuation, regardless of task difficulty, to robustly detect mental fatigue minutes before self-reported indication. We also found EEG's hyperconnectivity, and its enhanced fluctuation to align with our arousal-cortical model prediction. Overall, this study lays the foundation for effective assessment of mental fatigue and its impacts on cognition.