## 3 Results  
At first, we analyzed the EEG power spectra at all four stages of the experiment. For stages R1, S1, R2, and S2, Figure 2a-d show the median values of the EEG \(\delta_{1}\)-activity, \(\delta_{2}\)-activity, \(\alpha\)-activity [7; 52; 53; 54; 55; 56; 57; 58], and \(\beta/\alpha\)-activity [59], respectively, averaged over the entire ensemble of subjects. To compare stages S1 and R1, and stages S2 and R2, we plot Figure 2e-h, which shows the normalized differences in spectral indices presented in Figure 2a-d.  
Figure 2: (**a–d**) Maps of the median values of the power of EEG signals averaged over all subjects at R1, S1, R2, and S2 stages of the experiment: (**a**) \(\delta_{1}\)-activity; (**b**) \(\delta_{2}\)-activity; (**c**) \(\alpha\)-activity; (**d**) \(\beta/\alpha\)-activity. Dots indicate the EEG leads. (**e–h**) Normalized differences in the median values of spectral indices at stages S1 and R1, and stages S2 and R2: (**e**) \(\Delta\delta_{1}\)-activity; (**f**) \(\Delta\delta_{2}\)-activity; (**g**) \(\Delta\alpha\)-activity; (**h**) \(\Delta\beta/\alpha\)-activity. Circles indicate the EEG leads that demonstrated statistically significant differences between the stages of rest and stress, while crosses indicate the EEG leads in which these differences were not statistically significant. As can be seen in Figure 2a-c, \(\delta_{1}\)-activity and \(\delta_{2}\)-activity have higher values than \(\alpha\)-activity at all stages of the experiment. The spectral indices of \(\delta_{1}\)- activity and \(\delta_{2}\)-activity show qualitatively similar maps (Figure 2a,b). In particular, at the stages of rest, \(\delta_{1}\)-activity and \(\delta_{2}\)-activity are maximal in the frontal EEG leads. In all leads, \(\delta_{1}\)-activity is statistically significantly less by 55 (34; 71)% (median value with the first and third quartiles) at stress stage S1 than at rest stage R1, and by 54 (42; 71)% at stress stage S2 than at rest stage R2 (Figure 2e). \(\delta_{2}\)-activity is statistically significantly less by 73 (51; 80)% at stress stage S1 than at rest stage R1, and by 58 (43; 76)% at stress stage S2 than at rest stage R2 (Figure 2f). Figure 2d demonstrates the predominance of \(\alpha\)-activity compared to \(\beta\)-activity at all stages of the experiment. At stress stages S1 and S2, a decrease in \(\alpha\)-activity (Figure 2c) and an increase in \(\beta\)/\(\alpha\)-activity (Figure 2d) is observed. At the stages of rest, \(\alpha\)-activity is maximal in parietal EEG leads (Figure 2c). \(\alpha\)-activity statistically significantly decreased in all leads by 22 (10; 39)% at stage S1 and by 28 (12; 45)% at stage S2 compared with stages R1 and R2, respectively (Figure 2g). \(\beta\)/\(\alpha\)-activity statistically significantly increased by 26 (\(-\)6; 83)% in occipital leads at stage S1 and by 32 (6; 73)% in occipital and parietal leads at stage S2 compared with stages R1 and R2, respectively (Figure 2h). It should be noted that in the \(\delta_{1}\)- and \(\delta_{2}\)-ranges, the differences in spectral indices between the stages of stress and rest are significant and more pronounced in all EEG leads, while in the \(\alpha\)- and \(\beta\)-ranges, these differences are less pronounced and statistically significant only in several leads. Thus, the spectral analysis of infra-slow oscillations of brain potentials demonstrates the sensitivity of their dynamics to changes in the psychophysiological state caused by stress-inducing cognitive tasks. Second, we analyzed the couplings between the signals of EEG leads in low-frequency ranges. Figure 3 presents the results of calculating the phase coherence coefficient (1) between EEG oscillations in the \(\delta_{1}\)-, \(\delta_{2}\)-, \(\delta\)-, and \(\theta\)-ranges. Figure 3a-d shows the medians of distributions in the ensemble of mean values of the coherence (1) of a given EEG lead and other leads.