Hyperthermia, but not dehydration, alters the electrical activity of the brain.

Purpose: Whole-body thermal and hydration clamps were used to evaluate their independent and combined impact on the electrical activity of the brain. It was hypothesised that those stresses would independently modify the electroencephalographic (EEG) responses, with those changes being greater when both stresses were superimposed.

Methods: Alpha and beta spectral data (eyes closed) were collected from the frontal, central-parietal and occipital cortices of both hemispheres in resting, healthy and habitually active males (N = 8; mean age 25 years). Three dehydration states were investigated (euhydrated and 3% and 5% mass decrements) in each of two thermal states (normothermia [mean body temperature 36.3 °C] and moderate hyperthermia [38.4 °C]). The combination of those passively induced states yielded six levels of physiological strain, with the EEG data from each level separately examined using repeated-measures ANOVA with planned contrasts.

Results: When averaged across the frontal cortices, alpha power was elevated relative to the occipital cortices during moderate hyperthermia (P = 0.049). Conversely, beta power was generally reduced during hyperthermia (P = 0.013). Neither the alpha nor beta power spectra responded to dehydration, nor did dehydration elevate the heat-induced responses (P > 0.05).

Conclusion: Moderate hyperthermia, but neither mild nor moderate dehydration, appeared to independently alter brain electrical activity. Moreover, the combination of moderate hyperthermia with 5% dehydration did not further increase those changes. That outcome was interpreted to mean that, when those states were superimposed, the resulting neurophysiological changes could almost exclusively be attributed to the thermal impact per se, rather than to their combined influences.