Nystagmus, nihs TAG muhs, is an involuntary, rhythmical movement of the eyes. The eyes may move from side to side, up and down, in a circle, or in a combination of these movements. The motions may be rapid or slow, and jerky or smooth. Nystagmus occurs normally, as when a person watches scenery from a moving train. This condition may also be produced by diseases of the eye, the ear, or the brain. Some people are born with nystagmus.