The purpose of this study is to determine whether noninvasive brain stimulation associated with motor learning offers an additional benefit than motor learning alone in patients with stroke.

The investigators hypothesis is that active transcranial direct current stimulation (tDCS) combined with constraint induced movement therapy (CIMT) will induce a greater motor function improvement as compared with sham tDCS combined with CIMT.

In this 3 experiments study we will evaluate the effect of tDCS and constraint induced movement therapy (CIMT) in stroke patients (exp 1) as well as in healthy subjects (control population - exp 2). Finally we will investigate the effect of tACS - transcranial alternating current stimulation - as an alternative to tDCS, on motor function in healthy subjects (exp 3). The two experiments on tDCS are parallels (exp 1 & 2), while the experiment on tACS uses a crossover design (exp 3).

For stroke patients (Exp1), our primary outcome measure is a clinical scale (Jebsen-Taylor Hand Function Test), while for healthy subjects (Exp 2) we investigate the effect of tDCS and CIMIT on cortico-spinal excitability using TMS - transcranial magnetic stimulation. Finally, for the third experiment using tACS, our primary outcome is cortical oscillation, as measured by EEG - electroencephalogram.

Inclusion Criteria:

All subjects must be between the ages of 18-90 and must not be pregnant.

Additional Entry criteria for Stroke subject enrollment:

1. First-time clinical ischemic or hemorrhagic cerebrovascular accident - evidenced by a radiological (or physician's) report;
2. Weakness, defined as score of less than 55 (out of 66) on arm motor Fugl-Meyer (FM) scale
3. Stroke onset >6 months prior to study enrollment.

Exclusion Criteria:

1. Significant pre-stroke disability;

2. Major depression, as defined by Hamilton Depression (HAM-D) scale grater then or equal 17 if needed (history of depression before the stroke);

3. Any substantial decrease in alertness, language reception, or attention that might interfere with understanding instructions for motor testing;

4. Excessive pain in any joint of the paretic extremity (not applicable to severe stroke subjects);

5. Contraindications to single pulse Transcranial Magnetic Stimulation (TMS) (TMS will be used to measure cortical excitability) such as metal head implants

   • history of seizures
   • unexplained loss of consciousness
   • metal in the head
   • frequent or severe headaches or neck pain
   • implanted brain medical devices.

6. Contraindications to tDCS

   • metal in the head
   • implanted brain medical devices

7. Advanced liver, kidney, cardiac, or pulmonary disease;

8. A terminal medical diagnosis consistent with survival < 1 year;

9. Coexistent major neurological or psychiatric disease as to decrease number of confounders;

10. A history of significant alcohol or drug abuse in the prior 6 months;

11. Use of carbamazepine and amitriptyline;

12. Subjects may not be actively enrolled in a separate intervention study targeting stroke recovery and

13. Subjects may not have already received constraint induced motor therapy and/or tDCS treatment for stroke;

14. History of epilepsy before stroke (or episodes of seizures within the last six months).

15. Subjects with global aphasia and deficits of comprehension

16. Pregnancy