Effects of transcranial photobiomodulation in cerebral circulation and brain neural oscillations: a systematic review

Transcranial photobiomodulation (tPBM) uses the light of lasers or light-emitting diodes to modulate and improve brain function. In this study, the use of tPBM for improving cerebral circulation and modulating brain neural oscillations has been systematically reviewed to determine the useful parameters of tPBM and the grade of its practical recommendation. The PROSPERO code "CRD42023393544". "Google scholar", "Pub Med", "Scopus" and "science direct" online databases were searched based on Preferred Reporting Items for Systematic Reviews and meta-analyses (PRISMA) guidelines. Search keywords were "low level laser therapy AND cerebral circulation" OR "photobiomodulation therapy AND cerebral circulation" OR "low level laser therapy AND brain neural oscillation" OR "photobiomodulation therapy AND brain neural oscillations". Reviews, preprints, non-transcranial PBM, patents, unpublished articles, non-English articles, case reports and non-related articles were excluded. Forty-five studies published from January 2009 to May 2024, including 18 animal studies and 27 human studies, were included. 808 (810) nanometer (nm) and 1064 nm wavelengths, power density of 250 mW/cm² and energy density in the range of 60-120 J/cm² were the most used tPBM parameters. The human studies included 19 randomized clinical trials (RCTs), and the Cochrane Risk of Bias Tool for Randomized Trials (RoB2) was used to assess quality of the RCTs to determine the level of evidence and grade of practical recommendations. According to the grade practice recommendations, tPBM is strongly recommended for modulating brain neural oscillations and improving cognitive processes. In addition, tPBM is recommended for increasing cerebral blood flow, metabolism and oxygenation.