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.
Voluntary Participation: Participants who are clear of their cognitive faculties, capable of understanding all experimental procedures, and who provide written informed consent prior to enrollment.
Neurologically and Psychiatrically Healthy Status: Individuals with no current or prior history of neurological, neurodevelopmental, or psychiatric conditions;
Age Range: Young adult volunteers aged between 18 and 35 years.
Use of any continuous psychotropic medication within the past 12 months.
Any current psychiatric diagnosis based on the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5).
Diagnosis of any neurological disorder capable of altering cortical excitability (e.g., seizures/epilepsy, cerebrovascular accidents/stroke, brain tumors).
Any clinical, psychological, or social condition that, in the investigator's opinion, places the participant at an increased risk, compromises participant safety, or precludes full compliance and successful completion of the study protocol.
Crossover
Transcranial Photobiomodulation
Combination productTranscranial photobiomodulation (tPBM) is a non-invasive, non-thermal neuromodulatory modality that utilizes low-power coherent (laser) or non-coherent (light-emitting diodes, LEDs) light sources within the red (lambda = 600-700 nm) and near-infrared (NIR; lambda = 700-1100 nm) spectral windows to modulate cortical function. Structurally tailored to penetrate superficial anatomical barriers-including the scalp, skull, and meninges-tPBM delivers photons directly to the cerebral cortex.
Motor Evoked Potential (MEP) Amplitude - TMS
Single-pulse TMS will be applied over the primary motor cortex (M1) hotspot to elicit Motor Evoked Potentials (MEPs) recorded via electromyography (EMG) from the target muscle (e.g., first dorsal interosseous, FDI). Peak-to-peak MEP amplitude (mV) will be measured at a stimulation intensity adjusted to evoke a baseline response of approximately 1 mV. This outcome reflects overall baseline corticospinal excitability and its modulation following the tPBM protocol.
Intracortical Facilitation (ICF) - TMS
Evaluated using a paired-pulse TMS protocol consisting of a subthreshold conditioning stimulus (80% of resting motor threshold, RMT) followed by a suprathreshold test stimulus (120% RMT) at a long interstimulus interval (ISI) of 10 ms. The outcome is expressed as the ratio of the conditioned MEP amplitude to the unconditioned test MEP amplitude. ICF is primarily mediated by cortical glutamatergic circuits and NMDA receptor activity.
Short-Interval Intracortical Inhibition (SICI) - TMS
Assessed via a paired-pulse TMS paradigm using a subthreshold conditioning stimulus (80% RMT) followed by a suprathreshold test stimulus (120% RMT) at a short interstimulus interval (ISI) of 3 ms. The resulting SICI value is quantified as the percentage of inhibition of the conditioned MEP relative to the unconditioned test MEP. This parameter indexes local intracortical inhibitory interneuron activity mediated by GABA\_A receptors.
Cortical Silent Period (CSP) - TMS
Induced by applying a single suprathreshold TMS pulse (120% RMT) over the M1 hotspot while the participant maintains a stable, isometric voluntary contraction of the target muscle (e.g., 20% of maximum voluntary contraction). The CSP duration (ms) is measured from the onset of the MEP to the return of rectified background EMG activity. CSP duration provides a precise marker of interhemispheric and intracortical inhibition mediated by GABA\_B receptors.
Total Number of Taps - FTT
The cumulative count of valid screen contacts executed by the participant's index finger within a fixed, standardized testing interval (e.g., 30 seconds). This metric serves as a behavioral index of maximal motor execution speed and tapping frequency.
Variability of the Inter-Tap Interval (vITI) - FTT
Calculated as the standard deviation (or coefficient of variation) of the temporal intervals between consecutive screen contacts (in milliseconds). This outcome quantifies the temporal rhythmic precision and stability of the central motor program.
Spatial Resultant Sum (Σ||Δr||) - FTT
The cumulative Euclidean distance calculated across all sequential tap coordinates on the 2D Android screen interface. This parameter reflects spatial dispersion and motor drift, tracking the continuous precision of the targeted finger-pointing trajectory.
95% Confidence Ellipse Area (X,Y) - FTT
Computed as the total geometric surface area (in squared millimeters, mm2) of the bivariate error ellipse that encompasses 95% of the coordinates of all performed taps on the horizontal (X) and vertical (Y) axes. This spatial metric quantifies overall motor accuracy and targeting consistency.
Systolic Blood Pressure (SBP)
Systemic systolic blood pressure will be quantified in millimeters of mercury (mmHg) using a calibrated, digital oscillometric upper-arm sphygmomanometer. Measurements will be recorded under standardized resting conditions at pre-intervention (baseline) and immediately post-intervention across all five crossover arms to monitor potential acute alterations in peak arterial pressure induced by transcranial photobiomodulation.
Diastolic Blood Pressure (DBP)
Systemic diastolic blood pressure will be quantified in millimeters of mercury (mmHg) using the same automated oscillometric system. Measurements will be recorded concurrently with the systolic parameters at baseline and immediately post-intervention across all five crossover sessions to evaluate potential shifts in minimum arterial pressure during cardiac relaxation.
Heart Rate (HR)
Heart rate will be quantified in beats per minute (bpm) concurrently with the blood pressure evaluation using the same automated digital system. This parameter serves as an acute physiological indicator of autonomic nervous system balance (sympathovagal tone) before and after the application of continuous or sinusoidal pulsed photobiomodulation protocols.
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Hospital de Clinicas de Porto Alegre
Lead sponsor
Federal University of Rio Grande do Sul
Collaborator
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior.
Collaborator
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