A Systematic Review of fNIRS-Based Neurovascular Coupling Research: Methodological Landscape and Translational Considerations for Spaceflight.

BACKGROUND: The foundation of cognition is neurovascular coupling (NVC), which is defined as a connection between cerebral blood flow and neuronal activity. NVC can be affected by the situation the person is present in, for example, spaceflight conditions such as sleep deprivation, hypoxia, and microgravity. Therefore, monitoring the phenomena of NVC and how it is being affected under different conditions is very important. There are several tools available for the monitoring of NVC, and among them all, a promising tool is functional near-infrared spectroscopy (fNIRS) that offers benefits like portability and motion-tolerance. However, there is currently no thorough study that links the work being done in the field of fNIRS-based imaging directly to spaceflight requirements. OBJECTIVE: To systematically review fNIRS-based NVC research, summarize methodological approaches and physiological findings, and distinguish gaps limiting translation of fNIRS utilization in the spaceflight application. METHODS: Following PRISMA 2020 guidelines, we searched PubMed, Web of Science, and Scopus for the studies in the last 10 years, i.e., from January 2015 through August 2025, in which NVC was studied in healthy adults utilizing fNIRS. Two reviewers were involved in screening the articles and extracting data. RESULTS: Of 1,267 studies, 66 papers met the inclusion criteria. Cognitive paradigms predominated (40.9%), especially working memory tasks (n=10). Multimodal designs were observed in 54.5% of studies, most typically combining electroencephalography (EEG) and fNIRS (40.9%). Environment stressors were studied in 33.3% of studies: hypoxia/altitude (6.1%), sleep deprivation (7.6%), mental stress/workload (16.7%), exercise/physical stress (3.0%) and thermal stress (4.5%). The prefrontal cortex was the primary region studied (59.1%). No studies investigated the combined effects of stressors, and monitoring lasted only up to 4 hours. Since spaceflight involves multiple stressors over days or months, and considering the limited operational relevance (6.1%) and lack of spaceflight validation, these issues are open questions for translation rather than flaws in the studies themselves. CONCLUSIONS: Although controlled laboratory studies provide a solid methodological foundation, this evidence, primarily focused on mechanistic questions rather than spaceflight, does not currently support the use of fNIRS-based NVC monitoring during space missions. Critical translational issues still need to be resolved, including understanding the impact of combined stress protocols, relying more on short-term lab tests, and designing tasks that are ecologically valid. Solving these challenges is vital before such monitoring can be implemented. Collaboration across fields like neuroscience, aerospace engineering, and space medicine is essential to bridging these gaps.