Passive Hyperthermia Increases Neural Recruitment, Thereby Reducing Processing Efficiency During Executive Task Performance

INTRODUCTION: Heat-induced hyperthermia impairs cognitive performance and contributes to central fatigue during exercise and environmental heat exposure. Electroencephalographic event-related potential (EEG-ERP) studies consistently show reduced P300 amplitudes under heat stress, whereas functional magnetic resonance imaging (fMRI) studies often report expanded blood oxygenation level-dependent (BOLD) activation. PURPOSE: This study examined how passive hyperthermia influences task-dependent neural activation and cognitive processing using both fMRI and EEG-ERP within comparable paradigms. METHODS: Eighteen healthy adults (ten males and eight females) performed Go/No-go and Flanker tasks under normothermic and hyperthermic conditions during fMRI. Passive heating increased external ear canal temperature (Tear) by approximately 1.1°C above baseline. Sixteen participants completed an additional EEG-ERP session with the Flanker task. Behavioral responses, regional BOLD activation, and P300 amplitude and latency were assessed. Cervical arterial blood flow was measured by using a color-coded ultrasound system. RESULTS: The mean heating duration was 53.9 ± 9.2 min across all subjects. Hyperthermia increased heart rate and Tear while reducing internal carotid and vertebral artery blood flow. Reaction times decreased in both tasks, with no change in error rates. fMRI showed expanded activation in bilateral frontal, parietal, temporal, and subcortical regions. Task subtraction analyses (Flanker minus Go/No-go) revealed greater frontal and parietal activation during the complex task. In contrast, P300 amplitude was reduced during hyperthermia. CONCLUSION: Passive hyperthermia expands the spatial extent task-related BOLD activation while reducing P300 amplitude, suggesting reduced processing efficiency. However, the BOLD findings should be interpreted with caution because cerebral perfusion was also reduced. These findings suggest increased neural strain under heat stress, contributing to central fatigue and potentially impairing decision-making in sport and occupational setting.