Efficacy Study on Cognitive Functions in Schizophrenic Patients

Primary objectives * To compare neurocognitive effects of amisulpride with those of risperidone in patients with chronic schizophrenia, as assessed by the general cognitive index, a measure of overall cognitive functioning in schizophrenia Secondary objectives * Secondary analyses will be conducted to determine how the two atypical agents\' neurocognitive effects compare with regard to their profile of therapeutic action (based on individual cognitive domain scores in seven cognitive domains, including speed of processing, attention/vigilance, working memory, verbal learning and memory, visual learning and memory, reasoning and problem solving and social cognition); * Investigate whether amisulpride elicits more improvement on negative symptoms compared to risperidone treatment, as measured by the total score on the Scale of the Assessment of Negative Symptoms (SANS) 8 and by the Negative Symptom Subscale of the Positive and Negative Symptom Scale (PANSS); * Assess whether amisulpride improves overall functioning and individual domains of psychotic symptoms compared to risperidone as measured by the Clinical Global Impression (CGI), and the total and positive and general psychopathology subscale scores of PANSS and by the individual domains of SANS, respectively; * Evaluate the safety and tolerability of amisulpride and risperidone based on the study completion rates, and frequency of abnormal laboratory values, prolactin serum concentrations and on the Simpson Angus Scale for Extrapyramidal Symptoms (SAS) 10 and the Abnormal Involuntary Movement Scale (AIMS).