Upper limb hemiparesis is the most common sequelae in patients, severely impacting their independence and quality of life. Transcranial electrical stimulation (tCES) is a non-invasive and safe treatment, which uses a low direct current or alternating current to change the excitability of the cerebral cortex. It can induces long-term potentiation-like or long-term depression-like effects, thereby modulating the cortical excitability. In recent years, researchers have developed high-definition (HD) devices, which integrate high definition ring electrode configurations and incorporate direct current with theta burst stimulation waveforms. Diverging from traditional transcranial direct current stimulation (tDCS), which applies weak currents (0.5-2 mA) through two large sponge electrodes (25~35 cm^2) externally to the scalp for widespread non-specific cortical stimulation, HD-tES employs an array of small-area electrodes (1 cm^2) to control current distribution over localized cortical regions, thereby enhancing spatial accuracy. However, there is a lack of studies validating the optimal waveform for HD-tES, as well as clinical evidence in subacute stroke populations. The optimal unilateral versus bilateral stimulation modes and their neurological mechanisms for stroke rehabilitation also remain uncertain.

This project comprises two sub-projects:

1. Investigation of Neuromodulatory Effects of Stimulation Waveforms and Optimal Stimulation Waveform for Patients with Stroke : a crossover trial will compare the effects of seven high-definition transcranial electrical stimulation (HD-tES) waveforms on neural excitatory or inhibitory activity in healthy adults. These waveforms include three excitatory wave forms (anodal direct current (aDC), intermittent theta burst stimulation (iTBS), and a combination of aDC and iTBS (aDC+iTBS)) and three inhibitory wave forms (cathodal direct current (cDC), continuous theta burst stimulation (cTBS), and a combination of cDC and cTBS (cDC+cTBS)), along with a sham stimulation condition. Changes in cortical excitability over a 2-hour period before and after stimulation will be tracked.

   Validation of the optimal stimulation waveform for patients with stroke will be conducted through a crossover trial comparing three excitatory HD-tES waveforms for enhancing affected brain activity and three inhibitory HD-tES waveforms for reducing activity in the unaffected hemisphere. Changes in cortical excitability over a 2-hour period before and after stimulation will be monitored.

2. Exploration of the Effects of Unilateral and Bilateral HD-tES in Patients with Subacute Stroke: A triple-blind randomized controlled trial will be conducted to compare four neuromodulatory modes combined with conventional upper limb rehabilitation on brain activity and upper limb function in patients with subacute stroke. These modes include excitatory stimulation on the affected hemisphere (based on results from sub-project one, choosing from aDC, iTBS, or aDC+iTBS), inhibitory stimulation on the unaffected hemisphere (based on results from sub-project one, choosing from cDC, cTBS, or cDC+cTBS), simultaneous bilateral stimulation, and sham stimulation.

The primary outcome will focus on upper limb functional activity and upper limb motor function, with other indicators serving as secondary outcomes.

Inclusion Criteria:

- Sub-Project 1 (Healthy)

1. Adults aged 18 and above.
2. Confirmed right-handedness using the Edinburgh Handedness Inventory.

Sub-Project 1 (Post-stroke patients)

1. Aged 18 and above.
2. Diagnosed with stroke.
3. Post-stroke for more than 6 months.
4. Unilateral hemiparesis.

Sub-Project 2 (Subacute-stroke patients)

1. Aged 18 and above.
2. Diagnosed with stroke.
3. Stroke occurred between 7 days to 6 months ago.
4. Unilateral hemiparesis.
5. Degree of recovery for proximal and distal movements of the affected upper limb is Brunnstrom stage III to V.
6. No severe muscle spasticity in any segments of the affected upper limb (Modified Ashworth Scale ≤ 2).

Exclusion Criteria:

- Sub-Project 1 (Healthy)

1. History of neurological disorders (e.g., stroke, brain tumor, epilepsy), psychiatric disorders (e.g., substance abuse, major depression, schizophrenia, bipolar disorder), or musculoskeletal disorders of the upper limb.
2. Contraindications to transcranial electrical stimulation, including history of epilepsy, atrial fibrillation, presence of metal implants, cardiac pacemakers, convexity skull defects, or increased intracranial pressure.
3. Skin allergies, contact dermatitis, abnormal pain, hypersensitivity to pain, wounds, or ulcers on the head.
4. Participation in other invasive or non-invasive brain stimulation research studies.
5. Pregnancy or lactating women. (If female, must be postmenopausal or surgically sterilized. Fertile women must have a negative pregnancy test result. Fertile female patients engaging in heterosexual intercourse, as well as fertile male patients with fertile female partners, must agree to use effective contraception during the trial period and for 4 months after the last dose of the investigational drug, such as oral contraceptives, dual barrier methods, intrauterine devices, or abstain from sexual intercourse during this period; non-fertile women are those who have undergone bilateral oophorectomy or are postmenopausal.)
6. History of alcohol or substance abuse.
7. Damaged skin at the stimulation site, electrode contact, or device wearing site.
8. Long-term use of central nervous system affecting medications (such as antidepressants, sedatives) or other medications that may affect seizure threshold.
9. Other conditions deemed unsuitable for transcranial electrical or magnetic stimulation by a physician.
10. Affiliation with any research institution/execution unit (e.g., students from NTU, Taipei Medical University).

Sub-Project 1 (Post-stroke patients)

1. Contraindications to transcranial electrical stimulation include a history of epilepsy, atrial fibrillation, presence of metal implants, cardiac pacemakers, convexity skull defects, or increased intracranial pressure.
2. Skin allergies, contact dermatitis, abnormal pain, hypersensitivity to pain, wounds, or ulcers on the head.
3. Severe neurological or psychiatric disorders other than stroke (such as major depression, schizophrenia, substance abuse, organic brain diseases, Parkinson's disease, brain tumors).
4. Use of medications that may lower the seizure threshold.
5. Undergoing other invasive or non-invasive brain stimulation therapies.
6. Pregnancy or lactating women. (If female, must be postmenopausal or surgically sterilized. Fertile women must have a negative pregnancy test result. Fertile female patients engaging in heterosexual intercourse, as well as fertile male patients with fertile female partners, must agree to use effective contraception during the trial period and for 4 months after the last dose of the investigational drug, such as oral contraceptives, dual barrier methods, intrauterine devices, or abstain from sexual intercourse during this period; non-fertile women are those who have undergone bilateral oophorectomy or are postmenopausal.)
7. History of alcohol or substance abuse.
8. Damaged skin at the stimulation site, electrode contact, or device wearing site.
9. Long-term use of central nervous system affecting medications (such as antidepressants, sedatives) or other medications that may affect seizure threshold.
10. Other conditions deemed unsuitable for transcranial electrical or magnetic stimulation by a physician.

Sub-Project 2 (Subacute-stroke patients)

1. Contraindications to transcranial electrical stimulation include a history of epilepsy, atrial fibrillation, presence of metal implants, cardiac pacemakers, convexity skull defects, or increased intracranial pressure.
2. Skin allergies, contact dermatitis, abnormal pain, hypersensitivity to pain, wounds, or ulcers on the head.
3. Severe neurological or psychiatric disorders other than stroke (such as major depression, schizophrenia, substance abuse, organic brain diseases, Parkinson's disease, brain tumors).
4. Use of medications that may lower the seizure threshold.
5. Other muscle and joint problems affecting upper limb function, such as joint contractures, rheumatoid arthritis, myositis ossificans.
6. Severe cognitive impairment or receptive or global aphasia making it difficult to understand instructions.
7. Hemineglect (including visual, auditory, or sensory neglect).
8. Undergoing other invasive or non-invasive brain stimulation therapies.
9. Pregnancy or lactating women. (If female, must be postmenopausal or surgically sterilized. Fertile women must have a negative pregnancy test result. Fertile female patients engaging in heterosexual intercourse, as well as fertile male patients with fertile female partners, must agree to use effective contraception during the trial period and for 4 months after the last dose of the investigational drug, such as oral contraceptives, dual barrier methods, intrauterine devices, or abstain from sexual intercourse during this period; non-fertile women are those who have undergone bilateral oophorectomy or are postmenopausal.)
10. History of alcohol or substance abuse.
11. Damaged skin at the stimulation site, electrode contact, or device wearing site.
12. Long-term use of central nervous system affecting medications (such as antidepressants, sedatives) or other medications that may affect seizure threshold.
13. Other conditions deemed unsuitable for transcranial electrical or magnetic stimulation by a physician.