Can transcranial photobiomodulation improve cognitive function? A systematic review of human studies

Transcranial photobiomodulation (tPBM) has been studied for over a decade as a possible cognitive intervention. To evaluate the effect of tPBM for enhancing human cognitive function in healthy adults and remediating impaired cognitive function in adults with cognitive disorders. A systematic literature search from three electronic databases (PubMed, Scopus, Web of Science) was conducted from 1987 to May 2022. The cognitive function being evaluated included learning and memory, attention, executive function, language, and global cognitive function. Of the 35 studies identified, 29 (82.9 %) studies reported positive improvement in cognitive functions after tPBM. All nine studies on participants with subjective memory complaints, mild cognitive impairment, and dementia, showed positive outcomes. Seven (87.5 %) studies on traumatic brain injury (TBI) patients also showed positive results. A series of clinical trials on stroke patients showed positive trends on improved neurological deficit at first, but was prematurely terminated later at phase III due to the lack of statistical significance. One of the most frequently used protocols for clinical populations employed devices delivering near-infrared light (810 nm), the irradiance of 20–25 mW/cm2, and fluence of 1–10 J/cm2. Despite this commonly used protocol, there were other studies explored the potential effectiveness of other wavelengths of light ranging from visible red light (630-635 nm) to invisible near-infrared light (1060-1068 nm). tPBM seems to improve cognitive function. However, only half of the reviewed clinical trials were randomized control trials, further investigation is warranted.