Salehpour F. et al. - Transcranial photobiomodulation and cognitive performance in healthy young adults: a meta-analysis

This meta-analysis included 6 studies conducted in healthy young adults and reported an improvement in cognitive performance following transcranial photobiomodulation, with a standardized mean difference of 0.833 (95% CI 0.458 to 1.209). The authors note high heterogeneity, modest methodological quality, and asymmetry suggestive of publication bias.

What this study measured

This is a systematic review with a meta-analysis—that is, a quantitative synthesis—rather than an isolated study. The research question was whether transcranial photobiomodulation improves cognitive function in healthy adults.

The search covered MEDLINE via PubMed, EMBASE, SCOPUS, Web of Science, and the Cochrane Library through March 2019. ProQuest and Google Scholar were also searched to identify unpublished literature. The selection was limited to procognitive effects in healthy adults.

Key Findings

The figures reported by the authors are as follows.

• 871 studies were identified during the initial search; 9 publications were selected after applying the criteria.
• 7 studies conducted in healthy young adults, aged 17 to 35, and 2 studies in healthy subjects aged 49 and older.
• A meta-analysis based on 6 full-text publications involving young adults.
• Improvement in cognitive outcomes of 0.833 in standardized mean difference (95% CI 0.458 to 1.209, based on 14 comparisons).
• The asymmetry of the funnel plot was confirmed by Egger’s and Begg’s regression tests (values of 0.030 and 0.006).
• After a trim-and-fill analysis, two missing studies were estimated, and the adjusted effect size ranged from 0.761 (95% CI 0.573 to 0.949) to 0.949 (0.779 to 1.120).

How Transcranial Photobiomodulation Works

The mechanism remains hypothetical and is not assessed in this meta-analysis. Red and near-infrared light is thought to be absorbed by cytochrome c oxidase, an enzyme in the mitochondrial respiratory chain, which would increase ATP production in neurons. Modulation of cerebral blood flow and tissue oxygenation is also suggested, as is an effect on oxidative stress. This review provides no biological evidence.

Limits You Should Know

The authors are explicit: there is a high degree of heterogeneity in the data, the overall quality score of the studies is modest, and the number of included studies is small. They write that these shortcomings must be addressed before concluding that transcranial photobiomodulation is a cognitive enhancement intervention in healthy individuals.

The asymmetry of the funnel plot, confirmed by the Egger and Begg tests, suggests a publication bias. After reanalysis, this asymmetry disappeared in the attention subgroup but persisted in the memory subgroup: the finding is therefore less robust for memory than for attention.

The findings pertain to healthy young adults and do not apply to individuals with cognitive impairments. Finally, the literature review ends in March 2019 and does not take into account studies published since then.

What This Means in Practice

This meta-analysis does not support the claim that transcranial photobiomodulation is an established method for improving cognitive performance. The authors conclude that there is a significant effect in the analyzed data, but they refrain from concluding that the method is a validated pro-cognitive intervention. An improvement in memory or attention should therefore not be taken for granted.

Frequently Asked Questions

What does a standardized mean difference of 0.833 mean?

It is a dimensionless measure that expresses the difference between two groups relative to the variability of the measurements. A value close to 0.8 is conventionally described as a large effect. In this meta-analysis, it is accompanied by a 95% confidence interval ranging from 0.458 to 1.209, which reflects considerable uncertainty.

Is the effect just as pronounced on memory as it is on attention?

No. Upon further analysis, the asymmetry in the funnel plot disappeared in the attention subgroup but persisted in the memory subgroup. This persistence suggests that the results pertaining to memory are more susceptible to publication bias. The authors therefore do not present the effect on memory as equivalent.

Do these results apply to older adults?

No. The meta-analysis was based on 6 studies involving young adults. Two of the studies included in the review involved participants aged 49 and older, but they were not included in the calculations. Therefore, no specific conclusions can be drawn for this age group.

Reference

Salehpour F, Majdi A, Pazhuhi M, Ghasemi F, Khademi M, Pashazadeh F, Hamblin MR, Cassano P. Transcranial Photobiomodulation Improves Cognitive Performance in Young Healthy Adults: A Systematic Review and Meta-Analysis. Photobiomodulation, Photomedicine, and Laser Surgery, 2019. PMID 31549906. DOI 10.1089/photob.2019.4673. https://pubmed.ncbi.nlm.nih.gov/31549906/

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.