Combining fMRI and tDCS in Neuropathic Pain

Inclusion Criteria: * 1\. Age 20 or older and 90 or younger. 2. Ability to give informed consent 3. Neuropathic pain and/or nonpainful sensory symptoms resulted from I or II I. Peripheral neuropathy confirmed by clinical symptoms nerve conduction study, autonomic function test, quantitative sensory test, or skin biopsy. II. Stroke involving somatosensory pathway confirmed by neuroimage 4. Agree not to take caffeine, alcohol, tea and drugs with significant nervous system effects for 48 hours before each study session. Exclusion Criteria: * 1\. Presence of sever systemic diseases, including severe ischemic heart disease or congestive heart failure (left ventricular ejection fraction \< 30%), severe lung diseases with dyspnea, severe generalized edema, and systemic infection 2. Presence of major neurological disorders, including brain tumor, head trauma and infection or inflammation of nervous system. 3\. Presence of major depression or any major mental disorders 4. Presence of a cardiac pacemaker, brain stimulator, aneurysm clips or metal implants in the head, unless these devices are certified to be MRI-compatible 5. Pregnancy 6. Claustrophobia or any other contraindications to MRI 7. Inability to give informed consent.