Utility of Functional Near-infrared Spectroscopy (fNIRS) in Major Depressive Disorder: A Systematic Review

Purpose of the Review: Functional near-infrared spectroscopy (fNIRS) is a non-invasive, portable, and cost-effective neuroimaging tool for monitoring brain activity in real-world settings. Its advantages make it promising for clinical use in assessing major depressive disorder (MDD). This review evaluates current literature on the utility of fNIRS in MDD. Collection and Analysis of Data: Following PRISMA guidelines, English-language studies (2011–2025) from MEDLINE, ScienceDirect, and EMBASE were reviewed. Inclusion criteria: Studies using fNIRS to (a) differentiate MDD from controls, (b) correlate cerebral activity with symptoms, (c) monitor treatment response, or (d) distinguish MDD from other psychiatric disorders. Results: Of 6,312 records, 166 studies met the inclusion criteria. Most found reduced oxygenated hemoglobin (oxy-Hb) responses in prefrontal regions during cognitive and emotional tasks in MDD, especially on verbal fluency. Greater depression severity correlated with lower oxy-Hb, which increased as symptoms improved. fNIRS may also help distinguish MDD from other psychiatric disorders. Conclusions: fNIRS reliably detects reduced prefrontal activation in MDD, reflecting illness severity and treatment response. However, methodological differences limit broader clinical use. Standardized protocols and larger studies are needed for routine clinical adoption.