Frequency-Dependent Effects of Transcranial Photobiomodulation on Cortical Excitability

Frequency-Dependent Effects of Transcranial Photobiomodulation on Cortical Excitability and Fine Motor Performance: A Randomized, Double-Blind, Sham-Controlled, Crossover Trial

This study aims to investigate the frequency-dependent effects of near-infrared (NIR) transcranial photobiomodulation (tPBM) on human cortical excitability and fine motor performance. A cohort of 20 healthy adult volunteers (aged 18-35 years) will participate in a randomized, double-blind, sham-controlled, single-center study utilizing a within-subject, five-arm crossover design. Each participant will undergo five distinct experimental conditions separated by a strict washout period: active Continuous Wave (CW) tPBM, active pulsed tPBM at three neural oscillation frequencies (10 Hz, 40 Hz, and 100 Hz), and an inactive Sham comparator. All interventions will be delivered using a clinical-grade near-infrared device.

To isolate the specific effects of pulse frequency from overall energy delivery, the total energy dose (cumulative Joules) and total exposure time will be held perfectly constant across all active arms. The pulsed conditions will keeps the average irradiance and cumulative fluence identical to the continuous wave mode. Cortical excitability will be assessed via single- and paired-pulse Transcranial Magnetic Stimulation (TMS) protocols over the primary motor cortex (M1). Fine motor performance will be quantified using a smartphone-based Finger Tapping Test (FTT). Safety and systemic parameters will be tracked using the Systematic Assessment for Treatment Emergent Events - Systematic Inquiry (SAFTEE-SI) and vital signs. The central hypothesis is that distinct modulation frequencies will differentially influence corticospinal excitability and motor execution, independent of the total energy delivered.

Study Overview

• Status: Not yet recruiting
• Conditions: Cortical Excitability; Healthy Volunteers (HV); Transcranial Photobiomodulation
• Intervention / Treatment: Combination product: Transcranial Photobiomodulation

Detailed Description

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) active tPBM at baseline irradiance. 10 Hz near-infrared active tPBM. 40 Hz near-infrared active tPBM. 100 Hz near-infrared active tPBM. Sham Comparator (inactive control). Physical Calibration and Blinding: To ensure flawless dosimetric matching, total energy dose (Joules) and application duration will be kept constant across all active arms. The active pulsed interventions (10 Hz, 40 Hz, and 100 Hz) will utilize a 50% duty cycle with a peak irradiance exactly twice that of the Continuous Wave (CW) mode, thereby ensuring that the average irradiance (mW/cm2) and cumulative fluence (J/cm2) remain strictly identical to the CW. For the Sham condition, the device will emit a minimal, sub-therapeutic visual output of visible red light. This serves as a visual guide mimic to maintain complete participant blinding without transferring active near-infrared photons to the cortex (0 J active NIR dose). 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.

• Study Type: Interventional
• Enrollment (Estimated): 20
• Phase: Phase 1

Contacts and Locations

• Study Contact: Name: Marco A Caldieraro, MD PhD; Phone Number: +555199109-5177; Email: mcaldieraro@hcpa.edu.br
• Study Contact Backup: Name: Victor CS Araújo, MD Msc; Phone Number: +5551997360799; Email: vcaraujo@hcpa.edu.br

Study Locations

• Brazil
  • Rio Grande do Sul
    • Porto Alegre, Rio Grande do Sul, Brazil
      • Hospital de Clinicas de Porto Alegre
      • Contact: Victor CS Araújo, MD Msc; Phone Number: +5551997360799; Email: vcaraujo@hcpa.edu.br
      • Contact: Marco A Caldieraro, MD PhD; Phone Number: +5551991095177; Email: mcaldieraro@hcpa.edu.br

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult
• Accepts Healthy Volunteers: Yes

Description

Inclusion Criteria:

• 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.

Exclusion Criteria:

• 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.
• Presence of any intracranial devices or implants, including cochlear implants and aneurysm clips.
• Severe or uncompensated systemic medical illness that could interfere with study participation or confound physiological outcomes.
• Participation in any other interventional neuromodulation study within the preceding 6 months.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Device Feasibility
• Allocation: Randomized
• Interventional Model: Crossover Assignment
• Masking: Double

Number of Arms

5

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Continuous Wave (CW) tPBM; Active near-infrared (NIR) transcranial photobiomodulation delivered in Continuous Wave (CW) mode using the NIR device, providing a constant baseline irradiance to achieve the target total energy dose (J).	Combination product: Transcranial Photobiomodulation; Transcranial 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.
Experimental: 10 Hz Pulsed tPBM; Active NIR light delivered in pulsed mode at a frequency of 10 Hz. It matched across frequencies to ensure that the average irradiance (mW/cm2) and total energy dose (J) remain identical to the CW baseline within the same exposure time.	Combination product: Transcranial Photobiomodulation; Transcranial 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.
Experimental: 40 Hz Pulsed tPBM; Active NIR light delivered in pulsed mode at a frequency of 40 Hz. It matched across frequencies to ensure that the average irradiance (mW/cm2) and total energy dose (J) remain identical to the CW baseline within the same exposure time.	Combination product: Transcranial Photobiomodulation; Transcranial 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.
Experimental: 100 Hz Pulsed tPBM; Active NIR light delivered in pulsed mode at a frequency of 100 Hz. It matched across frequencies to ensure that the average irradiance (mW/cm2) and total energy dose (J) remain identical to the CW baseline within the same exposure time.	Combination product: Transcranial Photobiomodulation; Transcranial 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.
Sham Comparator: Sham Comparator (Sham tPBM); Inactive transcranial photobiomodulation (tPBM) delivered near-infrared device. To ensure successful participant blinding, the device will be programmed to deliver a minimal, non-therapeutic power output of visible red light (660 nm), which serves as a visual guide mimic but delivers negligible energy to the cortical tissue (0 J of active near-infrared light). Total exposure time, acoustic signals, and equipment interface will be identical to the active arms.	Combination product: Transcranial Photobiomodulation; Transcranial 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.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
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.	Immediately before the intervention (pre-intervention) and 20 minutes after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and 20 minutes after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and 20 minutes after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and 20 minutes after the intervention (post-intervention)

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
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.	Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)
Adverse Events and Tolerability (SAFTEE-SI)	The safety and tolerability profile of the combined neuromodulation protocol will be systematically evaluated using the Systematic Assessment for Treatment Emergent Events - Systematic Inquiry (SAFTEE-SI). This structured instrument will track the incidence, severity, and potential causal relationship of any somatic, neurological, or behavioral symptoms (e.g., headache, scalp discomfort, fatigue, dizziness, or localized thermal sensations) emerging during or after stimulation.	Baseline, before all interventions and one week after the intervention

Other Outcome Measures

Outcome Measure	Measure Description	Time Frame
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.	Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)
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.	Immediately before the intervention (pre-intervention) and immediately after the intervention (post-intervention)

Collaborators and Investigators

• Sponsor: Hospital de Clinicas de Porto Alegre
• Collaborators: Federal University of Rio Grande do Sul; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior.
• Investigators: Principal Investigator: Marco A Caldieraro, Doctor, Federal University of Health Science of Porto Alegre

Study record dates

Study Major Dates

• Study Start (Estimated): July 1, 2026
• Primary Completion (Estimated): July 1, 2027
• Study Completion (Estimated): October 1, 2027

Study Registration Dates

• First Submitted: July 8, 2026
• First Submitted That Met QC Criteria: July 15, 2026
• First Posted (Actual): July 16, 2026

Study Record Updates

• Last Update Posted (Actual): July 16, 2026
• Last Update Submitted That Met QC Criteria: July 15, 2026
• Last Verified: July 1, 2026

More Information

Terms related to this study

• Keywords: Healthy volunteers; Cortical excitability; Transcranial photobiomodulation; Finger Tapping Test
• Other Study ID Numbers: 2025-0082

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No