nystagmus n. A rapid involuntary oscillation of the eyeballs. Upbeating nystagmus can result from (1) drug intoxication, (2) lesions of the cerebellar vermis, and (3) lesions of the medulla at the level of the inferior olive, because of the interruption of the brain stem connections with the cerebellum. (Partial Credit: Hypertext Webster Gateway at UCSD). CLICK HERE to see a movie illustrating nystagmus (Note: You need to have an MPEG movie player for your browser. If you don't, you can download CINEWEB.)