Modulation of cerebral blood flow and cognition by hyperthermia and hypoxia: An electroencephalographic event-related...

Cerebral blood flow (CBF) is essential for sustaining neuronal metabolism and cognitive performance; however, the precise relationship between perfusion and cognition remains unclear. Although ageing and disease are associated with progressive declines in CBF and cognitive impairment, the acute effects of altered CBF under environmental stressors have not been elucidated fully. The influence of environmental stress on cognitive function is likely to depend on the degree of stress (e.g., its intensity and duration). Therefore, it is necessary to carry out a systematic review of a large number of studies, and objective evidence is required to build a comprehensive dataset. This review summarizes research examining the effects of mild to moderate passive heat stress (an increase in core temperature of ∼1.0°C-1.5°C) and acute hypoxia on cognitive processing, as evaluated using electroencephalographic event-related potentials (EEG-ERPs), with the aim of facilitating future cross-experimental comparisons. During mild or greater hyperthermia, CBF decreases owing to blood flow redistribution and hypocapnia-induced by hyperventilation, whereas during hypoxia, CBF can either increase or decrease depending on the conditions (e.g., exposure time or intensity). To standardize comparisons, this review focuses on acute hypoxic exposures, during which CBF tends to decrease. Although it is undeniable that the content summarized here might be somewhat selective, it is hoped that this foundation will contribute to the future development of constructive and objective evaluations. Current evidence indicates that acute fluctuations in CBF are unlikely to predict cognitive outcomes. Rather, both heat and hypoxic stress appear to impair neural activity through mechanisms beyond perfusion alone.