Noninvasive brain stimulation like tDCS can enhance executive functions like working memory in healthy subjects as well as in patients. To identify the optimal parameters for this intervention in patients with schizophrenia, the investigators first test the effects of different polarities (anodal, cathodal), stimulation intensities (1mA, 2mA) and laterality (left, right) on working-memory performance (nback task) in a sham-controlled cross-over design (n=128). To elucidate mechanisms of action, oscillatory brain activity will be registered with electroencephalography (EEG).