Frequency-Dependent Effects of Transcranial Photobiomodulation on Cortical Excitability

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.