Inclusion Criteria:  diagnosis of schizophrenia or schizoaffective disorder; clinically stabilized on antipsychotic: a stable dose of antipsychotic medication for >4 weeks; age 18-50 years, and presence of negative symptoms (based on PANSS): a negative subscore ≥16 points and [one of items N1-N7 scoring ≥4 or two items N1-N7 scoring ≥3]  Exclusion Criteria:  any significant neurological illness; mental retardation history of head injury with loss of consciousness for more than 1 hour history of epileptic seizures or epileptic activity on the baseline EEG (evaluated by an expert in clinical EEG and epilepsy), alcohol or drug abuse within the past 3 months, and depressive episode or antidepressant treatment in the past 4 weeks.