Abnormal eye movements found in CNS diseases may be related to the underlying pathophysiology of a particular CNS disease. Although the oculomotor nuclei, their descending primary motor neurons, and the extraocular muscles they innervate ultimately mediate eye movements, higher centers, including premotor areas in the brainstem, midbrain, subcortical areas and cerebral cortex, are also involved in the movements. The oculomotor nuclei and neurons, together with the higher motor centers, functionally comprise five oculomotor systems responsible for different types of eye movements. Of these, the smooth pursuit eye movement system and saccadic eye movement system are of interest, for they seem to be abnormal in CNS disorders where disease in higher subcortical and cortical centers is implicated. Both smooth pursuit and saccadic eye movements have properties that clearly distinguish them and make them easy to measure. Disorders of smooth pursuit eye movements (SPEM) have been found in schizophrenia, as well as other psychiatric disorders characterized by psychotic symptoms. Abnormal SPEM can be found in a variety of CNS illnesses, primarily those that affect the oculomotor nuclei (e.g. brainstem infarction) Abnormal SPEM and abnormalities in saccadic eye movements have also been associated with more diffuse brain diseases such as DAT, Huntington's and Parkinson's Diseases.