Frequency-Dependent Effects of Transcranial Photobiomodulation With Same Peak Irradiance on Cortical Excitability and Fine Motor Performance

Transcranial photobiomodulation (tPBM) has emerged as a promising non-invasive tool to modulate neural activity through the absorption of near-infrared (NIR) photons by mitochondrial cytochrome c oxidase (CCO). However, whether the biological effects on the human cortex are driven solely by cumulative energy delivery or are significantly modulated by pulse frequency remains a critical question in neurophysiology. This study implements a rigorous within-subject crossover design to systematically isolate the effects of pulse frequency from total dosimetric parameters. Participants and Screening: A sample of 20 healthy volunteers (aged 18-35) will be recruited. Potential candidates will undergo a strict screening protocol to ensure safety and baseline homogeneity. Exclusion criteria include any contraindications to magnetic fields assessed by the Transcranial Magnetic Stimulation Adult Safety Screen (TASS; Rossi et al., 2021), psychiatric conditions according to DSM-5, a history of neurological disorders, the use of psychotropic medications within the last 12 months, or prior participation in an interventional neuromodulation study within the preceding 6 months. Experimental Design \& Intervention: Enrolled participants will complete five experimental sessions in a randomized, counterbalanced order to eliminate carryover or sequence effects, separated by a mandatory washout period to ensure the return of cortical excitability to baseline. In each session, a clinical-grade near-infrared system will be applied over the primary motor cortex (M1) hot spot. The five experimental arms consist of: Continuous Wave (CW) tPBM - NIR light delivered continuously at baseline peak irradiance, 1x total cumulative energy dose; 10 Hz Pulsed tPBM - NIR light pulsed at 10 Hz with identical peak irradiance as CW and a 50% duty cycle, 0.5x total dose relative to CW; 40 Hz Pulsed tPBM - NIR light pulsed at 40 Hz with identical peak irradiance as CW and a 50% duty cycle, 0.5x total dose relative to CW; 100 Hz Pulsed tPBM - NIR light pulsed at 100 Hz with identical peak irradiance as CW and a 50% duty cycle, 0.5x total dose relative to CW; sham Control - a dedicated low-intensity red light photobiomodulation device emitting visible red light at sub-therapeutic levels to maintain participant blinding. Double-blinding will be enforced for the participant and investigator. Alphanumeric codes will mask the active protocols on the user interface, and participants will wear opaque safety goggles. Outcome Measures: Multiple neurophysiological, behavioral, and safety endpoints will be collected immediately pre-intervention (baseline) and post-intervention. Neurophysiology (TMS): Motor Evoked Potential (MEP) amplitude (primary excitability index); Intracortical Facilitation (ICF); Short-Interval Intracortical Inhibition (SICI); and Cortical Silent Period (CSP). Behavioral Performance (FTT): Evaluated via a dedicated Android application tracking total number of taps, variability of the inter-tap interval (vITI), spatial resultant sum (Σ\|\|Δr\|\|), and the 95% confidence ellipse area (X,Y).Safety and Hemodynamics: Systemic tolerability will be recorded via the Systematic Assessment for Treatment Emergent Events - Systematic Inquiry (SAFTEE-SI). Hemodynamic variations will be closely monitored through independent analyses of Systolic Blood Pressure (SBP), Diastolic Blood Pressure (DBP), and Heart Rate (HR). Data will be processed using Linear Mixed-Effects Models to account for the repeated-measures structure of the crossover design.

• Total Number of Taps - FTT [Срок оценки: Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)]
• Variability of the Inter-Tap Interval (vITI) - FTT [Срок оценки: Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)]
• Spatial Resultant Sum (Σ||Δr||) - FTT [Срок оценки: Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)]
• 95% Confidence Ellipse Area (X,Y) - FTT [Срок оценки: Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)]
• Adverse Events and Tolerability (SAFTEE-SI) [Срок оценки: Baseline, before all interventions and one week after the last intervention]