Disturbance of neurologic function in disorders of the central nervous system is expressed as an altered activation pattern in functional networks employed by specific tasks and can be studied by functional imaging modalities, e.g., positron emission tomography. Language, a complex brain function, is based on the interplay of a distributed network in which partial functions are executed in various centers, the primary language areas. These areas are hierarchically organized and activated according to the complexity of the specific language task. The specialization of different centers and the lateralization of integrative functions into the dominant (usually left) hemisphere are achieved by collateral and transcallosal inhibition of secondary language areas which normally are not employed for performance of a specific language task.