Mental fatigue decreases complexity: Evidence from multiscale entropy analysis of instantaneous frequency variation in alpha rhythm

Abstract

Mental fatigue (MF) jeopardizes performance and safety through a variety of cognitive impairments and according to the complexity loss theory, should represent "complexity loss" in electroencephalogram (EEG). However, the studies are few and inconsistent concerning the relationship between MF and loss of complexity, probably because of the susceptibility of brain waves to noise. In this study, MF was induced in thirteen male college students by a simulated flight task. Before and at the end of the task, spontaneous EEG and auditory steady-state response (ASSR) were recorded and instantaneous frequency variation (IFV) in alpha rhythm was extracted and analyzed by multiscale entropy (MSE) analysis. The results show that there were significant differences in IFV in alpha rhythm either from spontaneous EEG or from ASSR for all subjects. Therefore, the proposed method can be effective in revealing the complexity loss caused by MF in spontaneous EEG and ASSR, which may serve as a promising analyzing method to mark mild mental impairments.

Keywords: auditory steady-state response; complexity; mental fatigue; multiscale entropy; nonlinear analysis.

Figure 1

Experimental protocol. (A) Experimental flow diagram of EEG acquisition of spontaneous EEG (SPON) and auditory steady-state response (ASSR) in Sober and MF states. (B) Alpha rhythm extraction from SPON and ASSR with and without instantaneous frequency variation method (IFV), producing preprocessed four types of time series, SPON-α, SPON-α-IFV, ASSR-α, and ASSR-α-IFV, before they were analyzed by multiscale entropy methods (MSE) to compare Sober and MF states. (C) Schematic diagram of the coarse graining process, taking scale 2 and scale 3 as examples. (D) An example of MSE comparison.

Figure 2

MSE analysis of SPON-α (A), ASSR-α (B), SPON-α-IFV (C), and ASSR-α-IFV (D) on frontal lobe (channel F3) from four exemplary subjects. The blue and red lines represent the Sober and MF states, respectively. See Section 2.5 for the definitions of terms.

Figure 3

MSE analysis of SPON-α (A), ASSR-α (B), SPON-α-IFV (C), and ASSR-α-IFV (D) on parietal lobe (channel P4) from four exemplary subjects. The blue and red lines represent the Sober and MF states, respectively. See Section 2.5 for the definitions of terms.

Figure 4

MSE analysis of SPON-α (A), ASSR-α (B), SPON-α-IFV (C), and ASSR-α-IFV (D) on temporal lobe (channel T4) from four exemplary subjects. The blue and red lines represent the Sober and MF states, respectively. See Section 2.5 for the definitions of terms.

Figure 5

MSE analysis of SPON-α (A), ASSR-α (B), SPON-α-IFV (C), and ASSR-α-IFV (D) on occipital lobe (channel O1) from four exemplary subjects. The blue and red lines represent the Sober and MF states, respectively. See Section 2.5 for the definitions of terms.

Figure 6

Comparison of MSE from frontal lobe (channel F3) on: (A) alpha rhythm from spontaneous EEG (SPON-α), (B) alpha rhythm from ASSR (ASSR-α), (C) IFV in alpha rhythm from spontaneous EEG (SPON-α-IFV), and (D) IFV in alpha rhythm from ASSR (ASSR-α-IFV). The sign * indicates significance at p < 0.05. Shades indicate the standard errors.

Figure 7

Comparison of MSE from parietal lobe (channel P4) on: (A) alpha rhythm from spontaneous EEG (SPON-α), (B) alpha rhythm from ASSR (ASSR-α), (C) IFV in alpha rhythm from spontaneous EEG (SPON-α-IFV), and (D) IFV in alpha rhythm from ASSR (ASSR-α-IFV). The sign * indicates significance at p < 0.05. Shades indicate the standard errors.

Figure 8

Comparison of MSE from frontal lobe (channel T3) on: (A) alpha rhythm from spontaneous EEG (SPON-α), (B) alpha rhythm from ASSR (ASSR-α), (C) IFV in alpha rhythm from spontaneous EEG (SPON-α-IFV), and (D) IFV in alpha rhythm from ASSR (ASSR-α-IFV). The sign * indicates significance at p < 0.05. Shades indicate the standard errors.

Figure 9

Comparison of MSE from occipital lobe (channel O1) on: (A) alpha rhythm from spontaneous EEG (SPON-α), (B) alpha rhythm from ASSR (ASSR-α), (C) IFV in alpha rhythm from spontaneous EEG (SPON-α-IFV), and (D) IFV in alpha rhythm from ASSR (ASSR-α-IFV). The sign * indicates significance at p < 0.05. Shades indicate the standard errors.

Figure 10

Complexity loss rates of SPON-α, ASSR-α, SPON-α-IFV, and ASSR-α-IFV from all subjects and channels. See Section 2.5 for the definitions of terms. The sign * indicates significance at p < 0.05.