Mechanism of tDCS-induced Learning Enhancement - the Role of Serotonin

Inclusion Criteria: * right handedness * unobtrusive neuropsychological screening * ability to provide written informed consent * no pathological findings in head MRI * age: 18 to 35 years (young adults) or 50-80 years (older adults) * Highly effective contraception (Pearl Index \< 1) or reliable abstinence from any heterosexual relationships in women of childbearing potential Exclusion Criteria: * severe internal or psychiatric disease (especially depression or suicidal thoughts) * epilepsy * cognitive impairment (\< SD under age adjusted norm in neuropsychological testing) * concurrent taking of serotonin precursors (tryptophan, 5-HTP) or MAO inhibitors * concurrent taking of tramadol or triptans * concurrent taking of pimozide or linezolid * concurrent taking of other drugs prolonging the QT-interval * long-QT-syndrome * hypokalemia or hypomagnesemia * known intolerance of the study medication * claustrophobia or metallic implants, tattoos (MRI exclusion criteria) * pregnancy or lactation * participation in another drug-interventional clinical trial within the last month or during the entire study * probands that are placed in an institution due to official or judicial order * non-agreement to save and transmit pseudonymised study data within the clinical trial