This work reports on the creation of an electroencephalogram (EEG) index and the discovery of a power ratio related to alpha band frequencies (8-13 Hz) that were experimentally found to have a high correlation with human mental fatigue. The EEG was measured from occipital lobe of the human brain by microfabricated dry candle-like microneedle electrodes (CMEs) that can avoid hairs and penetrate through the high-impedance stratum corneum. The newly identified EEG index exhibited higher correlations to mental fatigue states, which were rated subjectively via the visual analogue scale (VAS), than did heart rate variance, which is often used as the objective measure of fatigue levels. Interestingly, it was found that the index trend changed 10 min before the VAS did, which implies that the index could assess the onset of mental fatigue even before the subjects recognised it themselves. The identification of this EEG index, along with the ease of obtaining EEG measurements via CME, which is due to the fact that the CME does not require any skin preparation or conductive gels and can acquire EEGs from hair-covered parts of the scalp, can be expected to promote practical fatigue assessment applications that use EEGs.
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