Abstract
BACKGROUND: Excessive heat exposure can lead to hyperthermia in humans, which impairs physical performance and disrupts cognitive function. While heat is a known physiological stressor, it is unclear how severe heat stress affects brain physiology and function. METHODS: Eleven healthy participants were subjected to heat stress from prolonged exercise or warm water immersion until their rectal temperatures (T(re)) attained 39.5°C, inducing exertional or passive hyperthermia, respectively. In a separate trial, blended ice was ingested before and during exercise as a cooling strategy. Data were compared to a control condition with seated rest (normothermic). Brain temperature (T(br)), cerebral perfusion, and task-based brain activity were assessed using magnetic resonance imaging techniques. RESULTS: T(br) in motor cortex was found to be tightly regulated at rest (37.3°C ± 0.4°C (mean ± SD)) despite fluctuations in T(re). With the development of hyperthermia, T(br) increases and dovetails with the rising T(re). Bilateral motor cortical activity was suppressed during high-intensity plantarflexion tasks, implying a reduced central motor drive in hyperthermic participants (T(re) = 38.5°C ± 0.1°C). Global gray matter perfusion and regional perfusion in sensorimotor cortex were reduced with passive hyperthermia. Executive function was poorer under a passive hyperthermic state, and this could relate to compromised visual processing as indicated by the reduced activation of left lateral-occipital cortex. Conversely, ingestion of blended ice before and during exercise alleviated the rise in both T(re) and T(br) and mitigated heat-related neural perturbations. CONCLUSION: Severe heat exposure elevates T(br), disrupts motor cortical activity and executive function, and this can lead to impairment of physical and cognitive performance.