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Randomized Controlled Trial
. 2021 Apr;121(4):1061-1071.
doi: 10.1007/s00421-020-04591-3. Epub 2021 Jan 11.

Impact of elevated core temperature on cognition in hot environments within a military context

Edward Tom Ashworth  1 , James David Cotter  2 , Andrew Edward Kilding  3
Affiliations
Randomized Controlled Trial

Impact of elevated core temperature on cognition in hot environments within a military context

Edward Tom Ashworth et al. Eur J Appl Physiol. 2021 Apr.

Abstract

Purpose: Cognition can be impaired during exercise in the heat, potentially contributing to military casualties. To our knowledge, the independent role of elevated core temperature during exercise has not been determined. The aim of the current study was to evaluate effects of elevated core temperature on cognition during physically encumbering, heated exercise, and to determine whether the perceptual cooling effects of menthol preserves cognition.

Methods: Eight participants complete three trials in randomised order: one normothermic (CON) and two with elevated (38.5°C) core temperature, induced by prior immersion in neutral versus hot water The CON trial and one hot trial (HOT) used a water mouth-rinse following each cognitive task of the trial, (HOT) while the other used a menthol mouth-rinse (MENT). Participants walked in humid heat (33°C, 75% relative humidity) in military clothing, completing a cognitive battery of reaction time, perceptual processing, working memory, executive function, cognitive flexibility, vigilance, and declarative memory.

Results: No differences in cognitive performance were observed between any conditions. Near-infrared spectroscopy showed greater oxygenated haemoglobin tissue content in HOT and MENT compared to CON (ΔO2Hb-deO2Hb: 2.3 ± 4.5 µM, p < .024), and lower deoxygenated haemoglobin in MENT than in CON or HOT (p = .017), suggesting higher brain metabolism during the more stressful conditions.

Conclusion: Moderately elevated core (38.5°C) and skin temperature does not appear to impair cognitive performance during exercise despite mildly elevated cerebral metabolism. The effects of menthol remain undetermined due to the lack of heat-mediated cognitive impairment.

Keywords: Cognitive; Exercise; Menthol; Physiology.

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Conflict of interest statement

N/A.

Figures

Fig. 1
Fig. 1
Schematic representation of the experimental design. In each experimental condition participants were initially immersed in water (WI), either heated to 40°C or 36°C. Following this they walked in the heat while completing a cognitive testing battery. Following each task participants were prescribed a mouth-rinse of either water (W) or menthol (M) during a ~ 1 min interlude
Fig. 2
Fig. 2
Physiological measures during cognitive testing while walking in military dress in the heat (33°C, 75% RH) while either normothermic (CON), hyperthermic (HOT) or hyperthermic with the aid of a menthol mouth-rinse (MENT). Measures of rectal temperature (a), skin temperature (b), and heart rate (c) were all obtained throughout the trial. Data are reported as mean ± SD. In a thick lines represent means, while thin lines display individual responses. In b and c MENT and HOT results are offset along the time axis for clarity. *Indicates p < .05 between CON and HOT. #Indicates p < .05 between CON and MENT
Fig. 3
Fig. 3
Perceptual responses to cognitive tasks during a 30-min military-dressed heated treadmill walk in humid heat (33°C, 75% RH) while either normothermic (CON), hyperthermic (HOT) or hyperthermic with the aid of a menthol mouth-rinse (MENT). Perceptions consist of the NASA Task-Load Index (TLX) for cognitive tasks, as well as common heat-related perceptions. Feeling is normalised to a positive scale. αIndicates p < .05 between CON and HOT, βIndicates p < .05 between CON and MENT, no differences were observed between MENT and HOT
Fig. 4
Fig. 4
Estimated oxygen difference in cerebral tissue, calculated using baseline-adjusted cerebral issue oxygenated and deoxygenated haemoglobin measured from the forehead using near-infrared spectroscopy. a displays an individual subjects data over time, while b displays the group means ± SD. Readings were obtained while completing a battery of cognitive tasks during a 30-min heated walk while either normothermic (CON), hyperthermic (HOT) or hyperthermic with the aid of a menthol mouth-rinse (MENT). While there is an overall effect for CON to be lower than MENT and HOT (both p < .025), no differences were found for analyses at each time point using the Holm p-value adjustment
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