This study will investigate the effects of 1 mA, 1.5 mA, and 2 mA of bilateral- tDCS on cortical activity and motor learning in healthy individuals

Transcranial direct current stimulation (tDCS) is one of the non-invasive brain stimulation (NIBS) techniques. It delivers weak direct current through the scalp via two electrodes. tDCS has been shown to modulate cortical excitability in polarity-specific effects; anodal increases cortical excitability, while cathodal decreases it. tDCS can be applied in two distinct montages: unilateral (an active electrode is applied over the cerebral cortex, while a reference electrode is applied over the contralateral orbit) and bilateral-tDCS (applying two electrodes simultaneously over both cerebral hemispheres). Many tDCS studies have been used as an add-on therapy in stroke patients which aimed to enhance motor re-learning after stroke. Anodal tDCS has been used to enhance cortical excitability in the lesioned hemisphere while cathodal tDCS is used to decrease it in the non-lesioned hemisphere and thus to rebalance the interhemispheric inhibition (IHI). However, there were recent studies reporting the reverse effect of cathodal-tDCS on cortical excitability at high doses. However, the effects of bilateral-tDCS among different doses on cortical excitability and on motor performance are still controversial.

The present study will be investigated the effects of different intensities of bilateral-tDCS on cortical activity and functional outcomes in healthy individuals. A single session of different intensities of bilateral-tDCS (i.e., 1 mA, 1.5 mA, 2 mA vs. sham) will be combined with serial reaction time task (SRTT) in healthy individuals. Cortical activity (i.e., brain symmetry index (BSI)) will be measured as a primary outcome. The reaction time of serial reaction time task will be served as the secondary outcome. Cortical activity will be evaluated before and after the intervention, while reaction time will be evaluated before, during, and after the intervention.

Inclusion Criteria:

• 1. Age 18-29 years old
• 2. Righ-hand dominant according to the Edinburgh handedness inventory
• 3. No injury to both upper and lower limb for the past 6 months

Exclusion Criteria:

• 1. Presence of any neurological disorders
• 2. Presence of metal implantation, intracranial shunt, cochlear implantation, or cardiac pacemakers
• 3. Presence of opened wound or infectious wound around scalp
• 4. History of epilepsy or any neurological antecedent or unstable condition which can lead to seizure
• 5. Body mass index (BMI) > 30 kg/m2
• 6. Received hormonal treatment and/or drugs that increase sleepiness and affect movement control
• 7. Ischemic heart disease and peripheral vascular ischemia
• 8. Last stage of kidney disease and liver disease