This Phase I/IIa Proof-of-Concept (PoC) trial is designed to assess the effect of adding a single and repeated low dose (15mg/d) of pipamperone (PIP) for 6 weeks to stable treatment with an effective dose of risperidone (RIS) or paliperidone (PAL) on functional MRI tests and clinical outcome of chronic schizophrenic patients with residual, so-called 'positive' symptoms, as well as on cognition, motivation, subjective well-being of patients, negative symptoms, general psychopathological symptoms and safety/tolerability.