Investigating tDCS as a Treatment for Unipolar and Bipolar Depression

Inclusion Criteria: * 18 years of age or above. * Meets criteria for a DSM-IV Major Depressive Episode with duration of at least 4 weeks. * Total score ≥ 20 on the Montgomery-Asberg Depression Rating Scale at study entry. Exclusion Criteria: * Current episode duration greater than 3 years. * Failed more than 3 adequate antidepressant trials in current episode. * DSM-IV psychotic disorder. * Drug or alcohol abuse or dependence (preceding 3 months). * Inadequate response to ECT in the current episode of depression. * Rapid clinical response required, e.g., high suicide risk. * Significant acute suicide risk, defined as follows: suicide attempt within the previous 6 months that required medical treatment; or ≥ 2 suicide attempts in the past 12 months; or has a clear-cut plan for suicide and states that they cannot guarantee that they will call their regular psychiatrist or the investigator if the impulse to implement the plan becomes substantial during the study; or in the investigator's opinion, is likely to attempt suicide within the next 6 months. * Clinically defined neurological disorder or insult. * Metal in the cranium, skull defects, or skin lesions on scalp (cuts, abrasions, rash) at proposed electrode sites. * Pregnancy. * Concurrent long acting benzodiazepines, ritalin or dexamphetamine medication.