Real-time EEG response to pulsed transcranial photobiomodulation in healthy young adults: Effects of stimulation parameters and skin tone

Abstract

Transcranial photobiomodulation (tPBM) relies on the photochemical stimulation of mitochondrial processes and has already demonstrated some ability to improve human cognition. However, it remains unclear how tPBM modulates neural oscillatory activity in real-time and how stimulation parameters and individual characteristics affect these responses. Capturing these dynamics is essential to understanding how tPBM's photochemical effects translate into effective neuromodulation, especially in diverse populations." Our study provides the first evidence of a cumulative increase in the power of neuro-electric oscillations in 46 healthy young adults with diverse skin tones who underwent 4 minutes of pulsed tPBM using varying laser parameters: two wavelengths (808/1064 nm), two pulsation frequencies (10/40 Hz), and three irradiances (100/150/200 mW/cm2). To capture "online" and "offline" effects, we analyzed minute-by-minute baseline-normalized frequency band power and used mixed-effects modeling to uncover significant predictors of EEG responses among tPBM parameters, skin tone, and sex. tPBM elicited frequency-band specific modulations in EEG power beginning near the stimulation site and propagating posteriorly over several minutes. Increases were cumulative with time, consistent with past fNIRS findings. They were also strongest and most sustained in beta/gamma bands, consistent with past findings of cognitive improvement. Significant parameter effects included stronger high-frequency responses at 808 nm and at 150 mW/cm2, wavelength-dependent frequency effects, larger responses in lighter skin, and parameter-specific sex differences. Both laser parameters and biological factors interact to shape tPBM's spatial and temporal neuromodulation profile and hence, parameter selection is crucial for achieving specific outcomes. Our findings uncover the manner in which tPBM induces neuronal changes, and contribute to a framework for creating more informed and personalized tPBM protocols.