Mental fatigue causes significant activation of the prefrontal cortex: A systematic review and meta‐analysis of fNIRS studies

Mental fatigue, a psychobiological prevalent and underestimated condition, is defined by increased lethargy and impaired concentration. This condition is not restricted by age and is exacerbated by various predisposing factors. Prolonged mental fatigue in occupational environments raises the probability of accidents or fatalities. Its fundamental mechanism is largely obscure and inherently subjective, thus there is no universally accepted parameter for its detection. Recently, there has been an increase in research that focuses on the use of functional near-infrared spectroscopy (fNIRS) to observe changes in brain hemoglobin during mental fatigue. Thus, this study assessed the reliability of oxygenhemoglobin and deoxyhemoglobin as fatigue biomarkers and conducted a systematic review and meta-analysis of studies which used fNIRS to monitor mental fatigue. The findings revealed significant activation of the prefrontal lobe under mental fatigue, and its activation level is intricately associated with the monitoring of diverse states during mental fatigue. Importantly, the type of induced mental fatigue and whether pre-trial training was provided to subjects were independent of the prefrontal lobe activation level. Overall, fNIRS proves to be an effective tool in tracking brain activity during mental fatigue, with a highly active prefrontal cortex acting as a dependable indicator for early identification of mental fatigue.