Elevated brain temperature under severe heat exposure impairs cortical motor activity and executive function

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.

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.

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.

Severe heat exposure elevates T_br, disrupts motor cortical activity and executive function, and this can lead to impairment of physical and cognitive performance.