This study of two types of optokinetic nystagmus (OKN) provides evidence that previously reported pursuit impairments in schizophrenics may be due to a cortical dysfunction. Differences in responses to partial-field and full-field OKN stimuli strongly support the hypothesis that there are two functionally distinct but anatomically overlapping mechanisms that can produce nystagmus responses. Partial-field OKN is composed of alternating phases of pursuit and saccadic movements. The slow phases of partial-field OKN, like pursuit eye movements, are of significantly poorer quality in schizophrenics compared with normal controls. Full-field OKN, however, is intact in both groups. Partial-field OKN is an improved test for pursuit abnormalities that reflect disturbances of nonvoluntary attention.