Experiment 1: Modulation of Physiological Tremor in Healthy Volunteers Thirty healthy volunteers will undergo TUS targeting the dentate nucleus in a randomized, double-blinded crossover design. Tremor amplitude, induced by a 15 g weight, will be measured using an accelerometer, and EEG will assess neural oscillations and cerebello-thalamo-cortical connectivity.

Stimulation will include short-term (1 minute on/off for 12 minutes) and long-term (30 minutes) protocols, as well as closed-loop TUS for phase-specific effects. This experiment aims to optimize stimulation parameters and explore the dentate nucleus's role in tremor generation.

Experiment 2: Tremor Modulation in Essential Tremor Patients Thirty ET patients will receive TUS targeting the dentate nucleus with optimized parameters from Experiment 1 in a randomized crossover design. The best protocol from previous experiment will be tested here. Tremor amplitude and EEG will be recorded to assess short- and long-term effects of TUS on pathological tremor.

Experiment 1: Modulation of Physiological Tremor in Healthy Volunteers This is a randomized, double-blinded, crossover study involving 30 healthy volunteers. Healthy volunteers will be recruited from campus with a flyer and using social media platforms. Each participant will undergo two conditions: on-target transcranial focused ultrasound stimulation (TUS) to the dentate nucleus and off-target (sham) stimulation. Initial screening will be performed via phone, after which an informed consent will be signed. The full experiment will consist of four separate visit days, of which only the first one will take place at the UZ Leuven department of Radiology. Visit days 2-4 will entirely be performed at the KU Leuven Brainshub facility in gebouw De Nayer.

• Visit 1: Imaging On the first visit, participants will undergo MRI scans to obtain structural (T1-weighted) and Ultrashort Echo Time (UTE) MRI images. These scans will help reconstruct individual head models to estimate skull characteristics and optimize the TUS target engagement. Using the MRI data, simulations will be performed to calculate the optimal position, shape, size, and pressure of the ultrasound beam. This ensures precise targeting of the dentate nucleus on an individual level during subsequent stimulation sessions.
• Visit 2: Immediate effects on Tremor Modulation and Phase Entrainment During this visit, participants will perform a task to induce physiological tremor (e.g., holding a weight on their finger). Tremor amplitude will be measured using an accelerometer. TUS will be applied in two conditions: on-target stimulation to the dentate nucleus and off-target sham stimulation. The stimulation will follow a short-term protocol (1 minute on, 1 minute off, repeated for 12 minutes). EEG will be recorded to assess changes in neural oscillations and phase entrainment in the cerebello-thalamo-cortical network. This visit aims to evaluate the immediate effects of TUS on tremor modulation and neural activity.
• Visit 3: Prolonged effects on Tremor Modulation This visit will evaluate the long-term effects of TUS on tremor modulation. The stimulation protocol will be extended to 30 minutes of sonication to observe whether prolonged stimulation results in stronger or sustained tremor reduction. As in Visit 2, accelerometer data will be collected to measure tremor amplitude, and EEG will be recorded to monitor changes in brain activity over the extended stimulation period. The goal is to assess the durability of the effects and refine the optimal stimulation duration.
• Visit 4: Closed-loop Stimulation for Tremor Suppression On the fourth visit, a closed-loop stimulation protocol will be tested. This involves synchronizing the TUS delivery to either the peak or trough of the tremor oscillation to determine whether phase-specific stimulation enhances tremor modulation. Real-time tremor data will be collected using an accelerometer, and EEG will be used to track neural responses. This visit aims to explore whether closed-loop TUS can provide more precise control over tremor amplitude compared to standard on/off stimulation.

Experiment 2: Tremor Modulation in Essential Tremor Patients Building on the results from Experiment 1, 30 ET patients will participate in a similar crossover design. Patients will be recruited by a neurologist collaborating on this project. The optimized TUS parameters from Experiment 1 will be used to target the dentate nucleus. The most optimal protocol for tremor reduction will be selected during this experiment. Thus, experiment 2 will exist of one visit day for imaging and another visit day to test the most optimal TUS protocol. Similarly, tremor amplitude will be measured using an accelerometer, and EEG will monitor neural activity during stimulation.

Inclusion Criteria:

• Experiment 1: Healthy Volunteers

  1. Voluntary written informed consent of the participant or their legally authorized representative has been obtained prior to any screening procedures
  2. Participants aged 18-55 years
  3. Male of female
  4. Good health with no history of serious mental illness or implanted metal
  5. Willingness to adhere to the TUS and MRI study schedule
  6. Willingness to avoid caffeine and alcohol intake for at least 2 h prior to the investigation

• Experiment 2: ET Patients

  1. Voluntary written informed consent of the participant or their legally authorized representative has been obtained prior to any screening procedures
  2. Participants aged 18-75 years
  3. Male of female
  4. Diagnosis of ET as confirmed from clinical history and examination by a movement disorder neurologist
  5. Willingness to adhere to the TUS and MRI study schedule
  6. Willingness to avoid caffeine and alcohol intake for at least 2 h prior to the investigation

Exclusion Criteria:

• Experiment 1: Healthy volunteers

  1. Currently taking any psychotropic medication
  2. Any head trauma resulting in loss of consciousness
  3. Diagnosed with a serious mental illness
  4. Alcohol or substance abuse or dependence (other than tobacco) in the past week
  5. Currently in treatment for a psychiatric condition
  6. Pregnancy (a test will be scheduled by the research team)
  7. Personal or family history of seizures or epilepsy
  8. Claustrophobia or inability to stay still in the MR scanner environment
  9. Any metal in the body that is not MRI-compatible
  10. Serious history of migraines
  11. Hair in dreadlocks, braids, or weave (not possible to position ultrasound transducer)
  12. orthopedic forearm or hand problems
  13. Inability to adhere to the experimental schedule.

• Experiment 2: ET Patients

  1. Currently taking any psychotropic medication
  2. Implanted with a non-MRI compatible medical device
  3. History of thalamotomy
  4. Skin lesions at stimulation site
  5. Peripheral neuropathy
  6. neurologic exam not consistent with ET
  7. Any head trauma resulting in loss of consciousness
  8. Diagnosed with a serious mental illness
  9. Alcohol or substance abuse or dependence (other than tobacco) in the past week
  10. alcohol or caffeine consumption within 12 hours of study enrollment.
  11. Currently in treatment for a psychiatric condition
  12. Pregnancy (scheduled test)
  13. Personal or family history of seizures or epilepsy
  14. Claustrophobia or inability to stay still in the MR scanner environment
  15. Any metal in the body that is not MRI-compatible
  16. Serious history of migraines
  17. Hair in dreadlocks, braids, or weave
  18. orthopedic forearm or hand problems
  19. Inability to adhere to the experimental schedule.

Both the participant and the investigator applying the TUS will be blinded to the stimulation condition (on-target or active control).