Patients with schizophrenia are deeply affected by the positive symptoms, negative symptoms and cognitive impairment. A generalized cognitive deficit could be frequently observed and traced back to early stage of the disease. Currently medication intervention significantly improves positive symptoms through dopamine receptor modification, leaving alone the negative symptoms and cognitive impairment. Besides dopamine dysregulation, more and more attention had been paid to the association of N-methyl-D-aspartate (NMDA) type glutamate receptor and schizophrenia, focusing on the neurobiological and cognitive biomarkers change. Memantine, an uncompetitive antagonist of NMDA type glutamate receptor approved as cognitive enhancer for Alzheimer's disease, is a potential candidate for preventing cognitive decline of schizophrenia. Previous randomized clinical trials failed to demonstrate its efficacy on chronic schizophrenic patients and it might be related to the chronic and irreversible disease process. There is also study supporting that memantine induces change in mismatch negativity (MMN) in frontal cortex of healthy subjects. This study will compare the MMN change of healthy subjects and the population of early schizophrenia, who has persistent neurobiological, cognitive biomarkers or negative symptoms despite subsided positive symptoms. Both male and female aged 20-45 years old outpatients with a length of illness for less than 5 years since first diagnosed as schizophrenia, currently receiving treatment by atypical antipsychotics in a relatively stable condition will be recruited. Healthy subjects will be recruited comparing their age and sex. We plan to recruit 10 subjects for both patient and healthy subjects with a total of 20 participants. All participants will receive general clinical, cognitive and event-related potential (ERP) evaluation as baseline before taking medication. Twenty mg of memantine will be given 4 hours before ERP retest. The analyses will be performed based on the change of ERP using paired-t sample test. Baseline clinical and cognitive symptoms will be analyzed as possible confounders.