The purpose of this study is to investigate different stimulation intensities and frequencies of transcranial direct and alternating current stimulation over the dorsolateral prefrontal cortex for cognitive improvement in schizophrenia.

In this randomized, double-blind study, 50 patients diagnosed with schizophrenia are randomly assigned into two arms. The first group receives transcranial direct current stimulation (tDCS) at three intensities (2 mA, 3 mA, and sham) and the second group receives transcranial alternating current stimulation (tACS) at three different frequencies (8 Hz, 40 Hz, and sham) over the left dorsolateral prefrontal cortex for 20 min. There are at least 72 hours between stimulation sessions. The patients conduct three cognitive tasks from the CANTAB neuropsychological battery for schizophrenia during stimulation including a working memory task, a cognitive flexibility task, and an emotion recognition task. Mood stability is also evaluated before and after each stimulation session with the Positive and Negative Affect Scale (PANAS).

Inclusion Criteria:

• Diagnosis of schizophrenia based on DSM V,
• Being 18-50 years old (male and female)
• If female, negative urine pregnancy test
• feasibility for tDCS interventions according to safety guidelines
• stable medication regime especially the classical neuroleptics and all CNS-activating medications, if taken, 4-6 weeks before the experiment
• fluency in the native language
• right-handed
• Required written informed consent signed by patients' guardian

Exclusion Criteria:

• pregnancy
• alcohol or substance dependence
• history of seizure
• history of neurological disorder
• history of head injury
• Presence of ferromagnetic objects in the body that are contraindicated for brain stimulation of the head (pacemakers or other implanted electrical devices, brain stimulators, some types of dental implants, aneurysm clips, metallic prostheses, permanent eyeliner, implanted delivery pump, or shrapnel fragments) or fear of enclosed spaces).

Data will be available upon reasonable request from the PI after publication of results

Allocation Concealment is used to perform a random sequence on study participants. Also, non-transparent envelopes sealed with random sequences will be used, in a way that each random sequence is recorded on a card, and the cards are placed in the envelope in order. In order to maintain a random sequence, the envelopes are numbered in the same way on the outer surface. Finally, the lids of the envelopes are glued and placed inside a box. At the beginning of the registration, according to the order of entry of the eligible participants, one of the envelopes will be opened in order and the assigned group of the participant will be revealed.