Healthy volunteers will be recruited. All subjects will be tested a battery of neuropsychological tests, then undergo fluorodeoxyglucose-positron emission tomography (PET) and magnetic resonance imaging (MRI). During the PET and MRI, they will be stimulated with transcranial direct current stimulation (tDCS) for 15 minutes. A third of subjects will receive real tDCS on the left prefrontal cortex, a third on the right prefrontal cortex, and the other third will receive sham tDCS.

Inclusion Criteria:

• N/A

Exclusion Criteria:

• History of any neurological or psychiatric diseases;
• Abnormal MRI;
• metal implants or a cardiac pacemaker;
• Pregnant or breastfeeding women (female subjects of child bearing potential will be screened for pregnancy before MRI imaging).
• severe hypertension.
• cardiovascular disease.
• Family history of epilepsy.