The study will investigate whether cognitive behavioral psychotherapy (CBT) combined with prefrontal transcranial direct current stimulation (tDCS) is more efficacious with regard to symptom reduction in depressed patients than CBT combined with sham-tDCS or CBT alone.

Brain stimulation techniques are widely seen as promising treatment alternatives for patients not responding to or tolerating psychotropic medication. In particular, transcranial direct current stimulation (tDCS) is of special interest due to its potential to be used by a large number of patients because of its comparably ease of usage and good tolerability. Thus, a large number of studies investigating clinical effects of tDCS have been performed with statistically significant effects but that are of moderate clinical relevance. Clinical studies have mainly focused on the prefrontal cortex (PFC) as the main stimulation target based on findings of numerous studies indicating the lateral PFC to be a key dysfunctional node within brain networks involved in the pathophysiology of depression.

Studies in clinical and healthy participants indicate that tDCS is capable of positively augmenting prefrontal functions that are relevant for a successful cognitive behavioral therapy. More specifically, it has been shown that tDCS is capable of improving reappraisal strategies as well as the use of cognitive control techniques .

To date, previous studies have mainly addressed global antidepressant effects of tDCS and not effects on more circumscribed phenotypes mediated by top-down PFC processes such as impaired or biased emotional learning processes. All these trials have applied the stimulation to patients while being in a resting position. Nonetheless, recent neuropsychological studies indicate that tDCS effects appear to be "activity dependent", meaning that the stimulation effects are greater when the brain region being stimulated is simultaneously engaged in a cognitive task. Therefore, in the present study we will apply tDCS to patients with unipolar major depressive disorder during cognitive behavioral therapy (CBT), a well-established and highly effective psychotherapeutic treatment for depression.

Inclusion Criteria:

- unipolar major depressive disorder

Exclusion Criteria:

• neurological diseases or relevant psychiatric diseases other than major depressive disorder
• current medication other than SSRI or Mirtazapine
• manic episodes (lifetime)
• psychotic symptoms (lifetime)
• treatment with psychotherapy within the past 2 years
• treatment with electroconvulsive therapy (lifetime)