Subjective feeling of psychological fatigue is related to decreased reactivity in ventrolateral prefrontal cortex

The purpose of this study is to examine the relationship between subjective fatigue and brain function. Twenty-three healthy young volunteers participated in this study. Relationships were investigated between subjective fatigue assessed using visual-analogue scale (VAS) score and sleep duration, and cerebral cortex reactivity during a verbal fluency task by 52-channel near-infrared spectroscopy (NIRS). The VAS score negatively correlated with oxygenated hemoglobin concentration ([oxy-Hb]) increases in the bilateral channels over the regions from the ventrolateral part of the frontal lobe to the upper part of the temporal lobe during the verbal fluency task. Sleep duration in the previous night positively correlated with [oxy-Hb] increases in the bilateral channels over the dorsolateral prefrontal lobe also during the verbal fluency task. No significant correlations between the VAS score and sleep duration in the previous night with [oxy-Hb] increases were found during a control task, the left-finger-tapping task. The subjective feeling of psychological fatigue is related to decreased reactivities in the lateral frontal and superior temporal cortices and is unrelated to sleep duration in the previous night, which is reflected in the reactivity in the dorsolateral prefrontal cortices. These results suggest that transient hypofunction and persistent dysfunction in the lateral prefrontal and temporal lobes are among the brain substrates of fatigue. These also demonstrate the advantage of NIRS for investigating brain function during subjective phenomena such as fatigue because it enables examination in a natural setting.