## 3 Experiments  
### Basic Concept of the Whole Measurement Experiment  
The whole experiment is practically divided into four measurement phases (\(M\!F_{\text{1,2,3,4}}\)) preceded by the initial phase \(IF_{0}\)--see the principal measurement schedule in Figure 6. The phase \(IF_{0}\) serves as preparation and manipulation of the measurement instruments--testing the wireless connection between the PPG sensor and the data-storing device, setting audio levels on the mixer device for phonation recording, etc. Prior to each experiment, the air in the room was disinfected by a UV germicidal lamp for 15 min to minimize risk of COVID-19 infection--the phonation signal recording must be performed without any protective face shield or respirator mask. In the case of the measuring phases \(M\!F_{\text{1}}\) and \(M\!F_{\text{4}}\), the tested person sits at the desk in the MRI equipment control room, while for the measurement in the phases \(M\!F_{\text{2}}\) and \(M\!F_{\text{3}}\), the person lies on the bed inside the shielding metal cage of the MRI device. Each of the measuring phases starts with PPG signal recording--the operation called \(PPGx_{\text{1}}\) (where \"\(x\)\" represents the number of the current measuring phase) with duration \(T_{\text{DUR}}\) equal to 80 s. Then, the phonation signal is recorded with the pick-up microphone. The signal consists of stationary parts of the vowels a, e, i, o, and u with a mean duration of 8 s interlaced by pauses of 2-3 s. Each vowel phonation was repeated three times, so 5 \(\times\) 3 = 15 records per person were obtained altogether in every individual measuring phase (total of 55 in the whole experiment). The active measurement is finished by the second PPG signal sensing (operation \(PPGx_{\text{2}}\)--also with \(T_{\text{DUR}}\) = 80 s, so the summary duration of all the measuring \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Stressed State** & **Normal Condition** \\ \hline \(PPG_{\text{RANGE}}\) [\%] & Increase & Decrease or constant \\ \(HP_{\text{PIP}}\) [\%] & Increase & Decrease \\ \(HR_{\text{REL}}\) [\%] & Higher positive (+) & Negative (–) or small \\ \(H\!R_{\text{MAR}}\) [\%] & Higher & Smaller \\ \(ORI_{\text{REL}}\) [\%] & Smaller & Higher \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Corresponding changes of PPG signal properties for stressed and normal states. Figure 6: Principal measurement schedule applied in all measurement experiments. phases is about 5-7 min. Between each two consecutive measurement phases, a working time delay (\(WTD_{1-3}\)) with time duration 5-10 min is applied. Therefore, the expected experimental duration is about 50 min in its entirety (without the \(IF_{0}\) phase). During \(WTD_{1}\), the tested person moves from the desk to the MRI device and adapts to the space of the scanning area to stabilize physiological changes in the cardiovascular system after changing body position from sitting to lying. Some people can also have a negative mental feeling inside the MRI tomograph. Both types of changes can evocate the stress that can be detected by the PPG and phonation signals. It holds mainly for \(WTD_{2}\) when the tested person is exposed by negative stimuli consisting of mechanical vibration and acoustic noise generated by the running MRI device during execution of the MR scan sequence. The last \(WTD_{3}\) delay part is planned for movement of the tested person to the desk in the control room and short relaxation after changing position from lying to sitting and returning to the \"normal\" laboratory conditions. Importance weights for input parameters \(SP_{\text{GMM}}\) and \(SP_{\text{PPG}}\) entered to the fusion process were set experimentally as shown in Table 2.  
In this study, two small databases of the phonation and PPG signals from eight healthy voluntary non-smokers were collected and further processed. The examined persons were the authors themselves and their colleagues: four females (F1, F2, F3, and F4) and four males (M1, M2, M3, and M4).