Objective: This study aimed to examine (1) the independent effects of hypoxia on cognitive function and (2) the effects of exercise on cognition while under hypoxia. Methods: Design: Systematic review with meta-analysis. Data sources: PubMed, Scopus, Web of Science, PsychInfo, and SPORTDiscus were searched. Eligibility criteria for selecting studies: randomized controlled trials and nonrandomized controlled studies that investigated the effects of chronic or acute exercise on cognition under hypoxia were considered (Aim 2), as were studies investigating the effects of hypoxia on cognition (Aim 1). Results: In total, 18 studies met our inclusionary criteria for the systematic review, and 12 studies were meta-analyzed. Exposure to hypoxia impaired attentional ability (standardized mean difference (SMD =
۰٫۴), executive function (SMD =
۰٫۱۸), and memory function (SMD =
۰٫۲۶) but not information processing (SMD = 0.27). Aggregated results indicated that performing exercise under a hypoxia setting had a significant effect on cognitive improvement (SMD = 0.3, 95%CI: 0.14
۰٫۴۵, I 2 = 54%, p < 0.001). Various characteristics (e.g., age, cognitive task type, exercise type, exercise intensity, training type, and hypoxia level) moderated the effects of hypoxia and exercise on cognitive function. Conclusions: Exercise during exposure to hypoxia improves cognitive function. This association appears to be moderated by individual and exercise/hypoxia-related characteristics.

Introduction

Cognitive functions are brain-based skills that allow humans to carry out tasks at various levels of difficulty and that are critical in day-to-day life.1,2 Notably, cognitive performance is possibly affected by environmental cues such as ambient temperature and altitude.3,4 For instance, increasing altitude and the ensuing severity of hypoxia may attenuate oxygen delivery to the brain tissue. Such exponentially reduced oxygen fraction during inspiration may result in impairment of brain function and cognitive abilities, including executive function, attention, episodic memory, and information processing.5
۷ This occurrence may be detrimental for particular populations, including those in the armed forces, athletes, mountaineers, mountain rescuers, and other high altitude residents who are repeatedly exposed to hypoxic conditions.8
۱۰ Findings from several primary studies11,12 and a previously published review13 have indicated hypoxia-induced cognitive deficits. Regardless of cognitive task types (i.e., central executive vs. nonexecutive tasks) and hypoxic conditions (i.e., hypobaric vs. normobaric hypoxia), low partial pressure of oxygen (PaO2) levels are strongly associated with greater reductions in cognitive function. In contrast, accumulating evidence suggests that hypoxia has no negative effects on cognitive function.14
۱۷ For instance, an experimental study by Lefferts et al.9 showed that response accuracy on cognitive tasks was similar in normoxia compared to hypoxia in a mixed sample of young men and women. Sun et al.18 reported that moderate hypoxia did not alter either reaction time or accuracy in sedentary young adults. Given the conflicting findings in previous studies, along with new publications on this topic, an updated systematic review is needed to evaluate and synthesize the current evidence on the effects of hypoxia on cognition.