Disassembling infra-low-frequency neurofeedback: A neurophysiological investigation of its feedback components.

Infra-Low-Frequency Neurofeedback (ILF-NFB), an EEG-based intervention, integrates specific electrode placements with "classic" frequency band (FB; 1-35 Hz) and infra-low-frequency (ILF; <0.1 Hz) components. Despite increasing clinical use, systematic studies of their individual and combined effects on neurophysiology remain limited. To address this gap, we conducted three randomized, sham-controlled, double-blind crossover studies (each with 40 healthy volunteers) examining FB-Only (Study 1), ILF-Only (Study 2), and combined FB&ILF (Study 3) protocols. The primary outcome was a functional connectivity multivariate pattern analysis (fc-MVPA) of resting-state fMRI data, using false discovery rate (FDR)-corrected thresholds (p < 0.05) at voxel and cluster levels. Exploratory analyses included post-hoc seed-to-voxel analyses (Bonferroni-corrected, p-FWE<0.0083), psychophysiological measures (heart rate variability [HRV], respiration), and neurofeedback state questionnaires. For FB-Only and ILF-Only, fc-MVPA interaction effects did not survive FDR correction. For study with combined FB&ILF, fc-MVPA revealed significant interaction effects ([Post-Pre Verum] vs. [Post-Pre Sham]), identifying clusters in parietal, occipital, and cingulate regions. Post-hoc seed-to-voxel analyses using the fc-MVPA clusters as seeds suggested that FB&ILF increased connectivity between posterior midline/parieto-occipital regions and the right dorsolateral prefrontal cortex; no significant changes were observed in sham conditions. HRV showed a significant interaction effect only in the FB&ILF study. Questionnaires revealed greater perceived controllability and signal causation in Verum vs. Sham for FB-Only and FB&ILF, but not ILF-Only. Verum ILF-Only improved concentration and well-being, though participants also reported increased nervousness and sleepiness post-session. Overall, our findings indicate that the combined FB&ILF application induces robust connectivity and autonomic changes, providing initial evidence to support clinical ILF-NFB effects and warranting further mechanistic studies.