## Materials and Methods
### Measurements and Data Processing
#### 2.2.4 Physiological Signals  
EEG  
To investigate the activity of the central nervous system, signals were measured by an EEG measuring device (EEG-1200, Nihonkohden, Japan) at a sampling rate of 500 Hz. The EEG cap was positioned on the head of the participants, and the EEG signals of 16 channels based on international 10-20 systems (Fp1, Fp2, F3, F4, C3, C4, P3, P4, O1, O2, F7, F8, T3, T4, T5, and T6) were recorded. The raw signals were filtered by a band-pass filter with cutoff frequencies of 1-40 Hz. Artifacts such as electromyography, electrooculography and signal due to body movement and heartbeat were eliminated by filtering using a band-pass filter with cutoff frequencies of 1-40 Hz, and by utilizing a MATLAB program (EEGLAB toolbox) based on independent component analysis. The 10-s segments were then processed by the fast Fourier transform analysis with the Hanning window [29]. Finally, the power spectral density (PSD) and the content of each frequency band (delta wave (1-4 Hz), theta wave (4-8 Hz), alpha wave (8-13 Hz), and beta wave (13-30 Hz)) of 16 channels was calculated. In the  
\begin{table}
\begin{tabular}{c c} \hline
**State (Value)** & **Indicators in Images** \\ \hline Alert (0) & Fast eye blinks, often reasonably regular; apparent focus on driving with occasional fast sideways glances; normal facial tone. \\  & Increase in duration of eye blinks and possible increase in the rate of eye \\  & blinks; increase in duration and frequency of sideways glances; \\  & appearance of “glazed-eye” look, occasional yawning. \\  & Occasional disruption of eye focus; significant increase in eye blink \\  & duration; disappearance of eye blink patterns observed during the alert state; reduction in the degree of eye opening; occasional disappearance of facial tone. \\  & Discernible episodes of almost complete eye closure; eyes are never fully \\  & open; significant disruption of eye focus. \\  & Significant increase in frequency of eye closure episodes; longer \\  & duration of episodes. \\ \hline \end{tabular}
\end{table}
Table 1: Drowsiness level based on facial expression [3].  
previous study [20], we calculated the mean value of the content in each part of the brain (frontal lobe: Pp1, Pp2, F3, and F4, Parietal lobe: P3 and P4, Occipital lobe: O1, O2, and Temporal lobe: T3 and T4), and these integrated parameters for each part of the brain were included in the dataset (4 bands \(\times\) 4 parts = 16 parameters); however, in this study, the parameters for each band of the 16 channels (4 bands \(\times\) 16 channels = 64 parameters) were used instead.  
ECG  
To investigate the activity of the autonomic nervous system, ECG signals were measured by an ECG measuring device (WEB-7000, Nihonkohden, Japan) at a sampling rate of 1000 Hz. Electrodes were attached to the bodies of participants with precordial leads. The peaks of R-waves were detected, and the R-R interval (RRI--the time interval of two successive peaks of R-wave) was calculated from the raw signals by utilizing a MATLAB program (Signal processing toolbox). The 10-s segments of RRI data were utilized to calculate mean RRI values and coefficients of variation of RRI (CVRR). Additionally, the RRI data were processed by the fast Fourier transform analysis with the Hanning window. The PSD of each frequency band was also calculated: low frequency (LF)--0.04-0.15 Hz and high frequency (HF)--0.15-0.45 Hz.