*4.2. Electrophysiological Results*

ANOVA showed the significant interaction of Cueing condition x Hemisphere (F(2.17, 15.19) = 6.16; ε = 0.72; adjusted *p* value < 0.00001). Effect size for this significant interaction was η2*p* = 0.47. Post-hoc comparisons among means showed that alpha power was significantly stronger over the RH than LH for the LCmot condition only (*p* < 0.000028; for these effects see maps of Figure 5a–d as well as the mean and SE values drawn in Figure 6).

Further post-hoc contrasts between the alpha power values obtained for the different cueing conditions within the two hemispheres also indicated that alpha power was overall greater in the CC than NC condition over both hemispheres (i.e., *p* < 0.00001 for LH, and *p* < 00001 for RH, respectively). Alpha power was also greater in the LC than in the NC condition over both hemispheres. Conversely, alpha power was decreased in the LCmot compared to the CC condition (*p* < 0.00001 for LH, and *p* < 0.0033 for RH, respectively). Post-hoc tests also showed a difference between alpha power values for both the spatially informative conditions LC and LCmot over the LH (*p* < 0.00001), but not the RH. Again, alpha showed an enhanced power in the CC than in the LC condition over the LH (*p* < 0.008), but not the RH, despite an intriguing trend (*p* < 0.062).

The ANOVA also yielded a significant triple interaction of Respiratory condition, Hemisphere and Electrode factors (F (1.74; 12.15) = 3.72; ε = 0.58; adjusted *p* value < 0.05; effect size η2*p* = 0.35) regardless of the cueing condition (For the topographic distribution across electrode sites and hemispheres in the two respiratory conditions see the maps drawn in Figure 7), while for the mean and SE values relative to the afore-mentioned findings see Figure 8). Post-hoc comparisons among means indicated that hypoxia significantly increased alpha power (*p* < 0.0005) determining a prominent alpha synchronization over the right-sided parieto-occipital scalp site (PO8), but not the left occipito-parietal (PO7) site. Furthermore, alpha power was greater over the right (PO8; *p* < 0.004) than the left occipito-parietal (PO7) site.

**Figure 5.** Three-dimensional (3D) maps of the topographical distribution of alpha power over the scalp as a function of the cueing-conditions grand-averaged over the respiratory conditions, and drawn from the top, the back, the right, and the left points of view, respectively. Time = 0 to 1500 ms, frequency = 7.5–12.5 Hz. (**a**): NC condition; (**b**): CC condition; (**c**): LC condition; (**d**): LCmot condition. Note that the scale of alpha power values (in μ<sup>V</sup>2) is unalike in the different attentional-cueing conditions. Note also that while the alerting but spatially unpredictable, exogenous-orienting of attention to target (CC) condition showed the highest alpha power, the fully uncued, exogenous-orienting of attention to target (NC) condition was characterized by the lowest alpha power.

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**Figure 6.** Mean and SE values of alpha power (in μ<sup>V</sup>2) as a function of cueing conditions and hemispheres collapsed across respiratory conditions. Note that for an easier readability, alpha power values were multiplied by 10 before being plotted.

**Figure 7.** 3D topographical scalp maps of alpha power as a function of normoxia (left side) and hypoxia (right side) respiratory conditions grand-averaged across the attentional-cueing modes, and drawn from the top, the back, the right, and the left viewpoints, respectively. Worth of note is the prominent lateralization of alpha synchronization induced by hypoxia over the right-sided occipital and parietal scalp areas as compared to normoxia.

**Figure 8.** Mean and SE values of alpha power plotted as a function of respiratory mode, hemisphere, and electrode site. Note that also in this case alpha power values were multiplied by 10 before being plotted.

Additionally, hypoxia induced EEG alpha synchronization over the mesial-occipital areas of both the LH and RH (*p* < 0.04 and *p* < 0.0072, respectively). Hypoxia also induced stronger alpha synchronization than in ambient-air over the left-sided dorsolateral prefrontal scalp site (*p* < 0.0011) (see Figure 8 again for all these findings). Overall, however, at these anterior scalp regions EEG alpha synchronization was significantly stronger over the RH than the LH, but in air condition only (*p* < 0.034). Moreover, in hypoxia alpha synchronization showed to be much more prominent over the right parieto-occipital than over the temporo-parietal (*p* < 0.001) and the prefrontal (*p* < 0.046) scalp sites, while this topographical difference was much reduced in ambient-air condition and over the left hemisphere (*p* < 0.0011).
