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

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Abstract

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.

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Abbreviations

EEG:

Electroencephalographic

EOG:

Electro-oculographic

E1, E3, E5 and E6:

Electrode positions above and below the left eye, and at the outer canthi of the left and right eyes (respectively)

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Acknowledgements

AMJvdH held an Australian Post-Graduate Award from the University of Wollongong (Australia) throughout this investigation. NAST was supported during the writing of this manuscript by the Brain Pool Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and Information, Communication and Technology (Grant number: 2019H1D3A2A01061171). No other funding was provided by agencies in the public, commercial or not-for-profit sectors.

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Authors and Affiliations

  1. Centre for Human and Applied Physiology, School of Medicine, University of Wollongong, Wollongong, NSW, 2522, Australia

    Anne M. J. van den Heuvel, Benjamin J. Haberley, David J. R. Hoyle & Nigel A. S. Taylor

  2. Australian Centre for Electromagnetic Bioeffects Research, School of Psychology, Illawarra Health & Medical Research Institute, University of Wollongong, Wollongong, NSW, 2522, Australia

    Nigel A. S. Taylor & Rodney J. Croft

Authors
  1. Anne M. J. van den Heuvel
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  2. Benjamin J. Haberley
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  3. David J. R. Hoyle
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  4. Nigel A. S. Taylor
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  5. Rodney J. Croft
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Contributions

AMJvdH, NAST and RJC designed and planned this research, and were involved in data analysis and all phases of manuscript preparation. AMJvdH ran the experiments, and was also responsible for data collection. BJH and DJRH were essential participants in subject preparation, data collection, laboratory operations and manuscript preparation. NAST supervised the thermal physiological aspects of this project, whilst RJC was responsible for the neurophysiological methods. All authors read and approved the final version of the manuscript and its submission for publication, and agree to be accountable for this work, ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship have been listed as authors.

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Correspondence to Nigel A. S. Taylor.

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Communicated by George Havenith.

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van den Heuvel, A.M.J., Haberley, B.J., Hoyle, D.J.R. et al. Hyperthermia, but not dehydration, alters the electrical activity of the brain. Eur J Appl Physiol 120, 2797–2811 (2020). https://doi.org/10.1007/s00421-020-04492-5

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