Lee T.L. et al. - Transcranial Photobiomodulation and Cognitive Function: A Systematic Review

A systematic review published in 2022 identified 35 human studies on transcranial photobiomodulation and cognition, of which 29 (82.9%) reported an improvement. The authors note that only half of the included clinical trials were randomized and controlled, which significantly limits the scope of this finding.

What this study measured

This is a systematic review of the human literature, not a meta-analysis: the authors did not calculate an overall quantitative effect; rather, they counted the studies that did and did not report a benefit. The level of evidence is therefore not that of a quantitative synthesis.

The search covered PubMed, Scopus, and Web of Science, from 1987 to May 2022. The functions assessed were learning and memory, attention, executive functions, language, and overall cognition, in both healthy adults and adults with cognitive impairment.

Key Findings

The results are broken down as follows.

• A total of 35 studies were identified.
• 29 studies, or 82.9%, report an improvement in cognitive function following transcranial photobiomodulation.
• All 9 of the studies conducted among participants with subjective memory complaints, mild cognitive impairment, or dementia reported positive results.
• Seven studies—representing 87.5% of those involving patients with traumatic brain injury—report a positive outcome.
• In patients who had suffered a stroke, a series of trials showed a favorable trend in neurological deficits before the Phase III trial was terminated early due to a lack of statistical significance.
• Most common protocol in clinical populations: near-infrared light at 810 nm, irradiance of 20 to 25 mW/cm², fluence of 1 to 10 J/cm².
• Other wavelengths detected: visible red from 630 to 635 nm, near-infrared with maxima from 1060 to 1068 nm.

How Transcranial Photobiomodulation Works

The mechanism proposed in the literature remains hypothetical. Red and near-infrared light is thought to be absorbed by cytochrome c oxidase, an enzyme in the mitochondrial respiratory chain, which would increase the production of ATP available to neurons. Other mechanisms have been suggested: modulation of cerebral blood flow and tissue oxygenation, as well as effects on oxidative stress and local inflammatory processes. This review does not measure any of these parameters; it reports only cognitive outcomes.

Limits You Should Know

The main limitation is pointed out by the authors themselves: only half of the clinical trials reviewed were randomized controlled trials. The others are based on weaker study designs, which are susceptible to the placebo effect and expectation bias.

The negative result should not be glossed over: the series of trials conducted in patients who had suffered a stroke was halted in Phase III due to a lack of statistical significance.

The 82.9% proportion of positive studies also reflects publication bias. Finally, the heterogeneity of the lighting parameters makes it impossible to establish a standard protocol.

What This Means in Practice

This review provides grounds for further research, not for concluding that efficacy has been established. No improvement in memory or attention can be taken for granted on this basis alone. The authors conclude that transcranial photobiomodulation appears to improve cognitive function, while noting that further investigation is necessary. For any cognitive complaint, the appropriate course of action remains a medical evaluation.

Frequently Asked Questions

Does transcranial photobiomodulation really improve memory?

This systematic review of 35 studies reports cognitive improvement in 29 of them, including studies on memory. This count does not constitute evidence of efficacy, as only half of the included clinical trials were randomized and controlled. The review does not provide any quantified overall effect that would allow for the quantification of an expected benefit.

What wavelength is used in these studies?

The most common protocol used in clinical populations involved near-infrared light at 810 nm. Other wavelengths are mentioned in the review, ranging from visible red light between 630 and 635 nm to near-infrared light with peaks from 1060 to 1068 nm. This diversity partly explains why no single standard protocol has emerged.

Why were the trials involving stroke patients halted?

The journal reports that a series of clinical trials had shown a favorable trend regarding neurological deficits in the early phases. The program was then terminated prematurely in Phase III due to a lack of statistical significance. This is a negative result, which contrasts with the favorable data observed in other indications.

Reference

Lee TL, Ding Z, Chan AS. Can transcranial photobiomodulation improve cognitive function? A systematic review of human studies. Ageing Research Reviews, 2022. PMID 36371017. DOI 10.1016/j.arr.2022.101786. https://pubmed.ncbi.nlm.nih.gov/36371017/

This summary is provided for informational purposes only and does not constitute medical advice. Photobiomodulation is not a substitute for prescribed treatment. Consult a healthcare professional.