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

This study investigates the frequency-dependent neuromodulatory effects of transcranial photobiomodulation (tPBM) over the primary motor cortex (M1) on cortical excitability and fine motor performance in healthy young adults. Using a randomized, double-blind, sham-controlled, 5-arm crossover design, 20 healthy participants will undergo five experimental conditions separated by a mandatory 7-day washout period: Continuous Wave (CW), 10 Hz pulsed tPBM, 40 Hz pulsed tPBM, 100 Hz pulsed tPBM, and a Sham control. To isolate pulse frequency while holding peak intensity constant, all active interventions will utilize an identical peak irradiance, with pulsed modes operating at a 50% duty cycle (delivering 50% of the cumulative energy dose relative to CW). Primary outcomes include corticospinal excitability measured via single- and paired-pulse Transcranial Magnetic Stimulation (TMS), alongside fine motor speed and dexterity assessed via a computerized finger-tapping task.