Intranasal photobiomodulation as an energy-efficient, largely parameter-insensitive alternative to transcranial photobiomodulation

<jats:p>Background: Photobiomodulation (PBM) is an emerging non-invasive light-based brain stimulation technique that can alter cortical oscillations and is currently being pursued for improving cognition and treating neurological and psychiatric conditions. Nearly all human EEG evidence comes from transcranial PBM (tPBM) applied to the forehead, where light must traverse the scalp and thick skull, requiring protocols to compensate with high surface irradiance. Intranasal PBM (iPBM) can reach the anterior skull base at a fraction of that irradiance and has also been shown to modulate cerebrospinal fluid dynamics, yet it has been studied almost exclusively as an adjunct to tPBM, leaving its cortical effects in isolation, and its energy-efficiency relative to the transcranial route, unknown. Objective: To define the spatiotemporal EEG response to pulsed iPBM delivered alone, determine whether stimulation parameters or individual biology moderate it, and compare the energy-efficiency of iPBM and tPBM in the same participants. Methods: High-density EEG was collected from forty-six healthy young adults during pulsed iPBM and tPBM spanning a parameter space of varying wavelengths, pulsation frequencies, and irradiances. Percent change in band power from a within-session pre-stimulation baseline was tested with spatiotemporal cluster-based permutation tests. Linear mixed-effects models with backward elimination assessed stimulation and biological moderators (sex, nostril-to-cortex distance). Energy-efficiency, defined as the percent change in band power per J/cm2 of delivered surface energy, was compared between routes within each subject in delivery route-specific cluster regions of interest (ROI) (Wilcoxon signed-rank tests, Benjamini-Hochberg false discovery rate). Results: iPBM alone produced significant spatiotemporal clusters in theta, beta, and gamma power, with anterior increases and posterior decreases; no delta or alpha clusters survived correction. Beta and gamma effects appeared at stimulation onset and persisted even after stimulation ended, whereas theta effects strengthened after stimulation ended. No predictor survived elimination in any band, time window, or cluster ROI: response magnitude was independent of wavelength, pulsation frequency, irradiance, sex, and nostril-to-cortex distance. Notably, although iPBM delivered roughly twenty times less surface energy than tPBM (~0.6-1.1 vs ~12-24 J/cm2), it produced EEG changes of similar magnitude, and its energy-efficiency exceeded that of tPBM in seven of eight eligible comparisons, with median iPBM-to-tPBM efficiency ratios of 14-32 (all FDR q&lt;0.05) Conclusions: Delivered in isolation, pulsed iPBM elicits a robust cortical EEG signature closely resembling that of tPBM, is insensitive to the stimulation parameters and individual factors tested, and achieves this at a small fraction of the delivered surface energy. As a result, delivery route, not surface irradiance alone, should be treated as a primary variable in PBM dose reporting and protocol design.</jats:p>