Electroencephalographic Biomarkers of Relaxation: A Systematic Review and Meta-analysis

Alpha oscillations (8-13 Hz), which are prominent in human EEG, have long been considered a neural marker of relaxation. However, the extent to which different frequency bands and electrode positions of the EEG reflect relaxation remains unclear. This systematic review and meta-analysis assessed the associations between EEG components and concurrently measured the reference indices of relaxation in healthy adults. A comprehensive database search and screening employing preset criteria identified 54 studies that involve 2569 participants published from January 1940 to March 2025 for qualitative synthesis. These studies utilized various reference relaxation measures, such as electrocardiographic (ECG) indices associated with parasympathetic nervous system activity and introspective indices obtained through questionnaires. Risks of bias were assessed based on the risk of bias assessment tool for nonrandomized studies. A meta-analysis of 31 studies employing a random-effects model revealed positive correlations between relaxation indices and the power of alpha oscillations in three specific combinations of EEG channel regions and reference index types: frontal channels with all reference indices, central channels with ECG-related indices, and occipital channels with questionnaire-based indices. No significant correlations were observed between relaxation indices and other EEG frequencies or channels. These findings indicate that alpha oscillations in different scalp regions may represent distinct aspects of the relaxation response based on the type of reference measure used.