Changes in electro-cortical activity during head out water immersion

Thermoneutral head-out water immersion (HOWI) is well known as analogue for simulating the fluid shifts observed during weightlessness. Various studies have demonstrated that HOWI can induce cardiovascular changes similar to those observed in spaceflight, however no investigations have studied the effects of HOWI on brain function thus far. While magnetic resonance imaging, the state-of-the-art for brain imaging, is bound to specialized laboratory, recent advances in portable electroencephalography (EEG) technology allow studying brain function in various settings. The aim of this study was therefore to determine the effect of fluid shifts during short-term HOWI on electro-cortical activity, with respect to cardiovascular changes. 32 young healthy men were exposed to either 60 min of seated HOWI or seated dry control (CON), preceded by a 10 min baseline seated measurement. 3 periods of 2-min electro-cortical activity and cardiovascular data (impedance cardiography and continuos blood pressure monitoring) were recorded with eyes closed: 1) immediately before intervention, 2) after 30 min, and 3) after 60 min of each condition, respectively. EEG was determined by a 20-channel wireless system (International 10-20-system) at the following electrode sites: Fp1, Fp2, F7, F3, Fz, F4, F8, C3, Cz, C4, T7, T8, P7, P3, Pz, P4, P8, O1, O2 with a common mode (CMS) and driven right leg electrode (DRL) placed on the right mastoid for differential referencing. Artefact-corrected and epoched data were subjected to FFT based spectral power analysis in the following sub-bands alpha-1 (7.5-10.0 Hz), alpha-2 (10.0-12.5 Hz), beta-1 (12.5-16.0 Hz), beta-2 (16.0-20.0 Hz) and beta-3 (20-28 Hz). In addition, exact low-resolution brain electromagnetic tomography (eLORETA) was used to determine the 3-dimensional distribution of electrical activity at each voxel in the neuro-anatomic Montreal Neurological Institute space. Cardiovascular data and static FFT EEG were analysed by a 2 x 3 repeated measures ANOVA; functional localization of EEG data was assessed using a non-parametric randomization procedure. The level of significance was set at alpha = 0.05 for all testing. A significant interaction between group and time was observed for cardiovascular recordings. Specifically, CO increased by about 1 L min-1, whereas MAP and TPR decreased (approx. -15 mmHg and 300 dyn s cm-5, respectively) during HOWI compared to CON (p < 0.05). These changes were complemented by reductions in beta frequency bands in HOWI vs. CON (p < 0.05). In addition, there was also a trend for reduced alpha bands during HOWI. eLORETA analysis revealed that these changes could be attributed to electro-cortical changes within the left temporal and left parietal brain areas (decline for alpha-2 and beta-1 activity, p < 0.05). For the first time we showed that resting state electro-cortical activity is reduced during HOWI compared to age-matched controls. Moreover, these alterations seem to be particularly pronounced for the left temporal and parietal hemisphere, suggesting a link between cardiovascular stress and changes in electro-cortical activation of these regions. The simultaneous decrease in MAP and TPR suggest that the present findings are likely mediated by variations in baroreceptor discharge as a result of the external hydrostatic pressure associated with HOWI.