Whole-Body Hyperthermia Decreases Short-Term Memory and Visual Recognition Memory Functioning but Increases Executive Functioning in Healthy Ageing

Background and Aim: Mammalian homeothermy maintains a consistent core body temperature (rectal; Tre), and it differs depending on different species. Of note, human body temperature is approximately 37 °C, and plays an important role in keeping physiological homeostasis. However, age-related loss of muscle mass decreases the firing rate and results in a lower resting core, muscle and Tre. Physically, temperature-based conduction of velocity of nerves decreases and age-related neural excitability attenuates in older men. Age-related cognitive functioning response to heat stress might be delayed in aged adults. We hypothesized that acute whole-body hyperthermia (Tre increased about 2.5°C) may lead to a greater depression in cognitive functioning in older men than in young men.

Materials and Methods: Eleven young (19–21 years) and nine older (61–80 years) healthy subjects participated in this study. They were immersed in waistline hot water (HW, approximately 43° C) until the young men Tre up to 39.5 °C, and older men Tre reached 39°C, respectively. To access cognitive performance (short-term memory, visual recognition memory and executive function), the Automated Neuropsychological Assessment Metric (ANAM4, Oklahoma, USA) was used.

Results: Although no significant hyperthermia-induced test durations and memorable figures in the forced-choice recognition memory test were found, a decrease in the number of memorable figures was observed to be greater among older men than among young men. Among older men, whole-body hyperthermia was accompanied by a significant decrease in mean reaction time in unpredictable task switching of the odd/even numbers, in parallel with a greater increase in repeating numbers. Among young men, mean digit length significantly dropped in the forward digit-span task, whereas there was no significant difference in the older men.

Conclusion: Age-related delayed and weakened thermoregulation led to a decline in the short-term memory and visual recognition memory functioning. Paradoxically, executive functioning was increased in whole-body acute hyperthermia.