Nystagmus is a pattern of eye movement characterized by smooth rotations of the eye in one direction and rapid rotations in the opposite direction that reset eye position. Periodic alternating nystagmus (PAN) is a form of uncontrollable nystagmus that has been described as an unstable but amplitude-limited oscillation. PAN has been observed previously only in subjects with vestibulo-cerebellar damage. We describe results in which PAN can be produced in normal subjects by prolonged rotation in darkness. We propose a new model in which the neural circuits that control eye movement are inherently unstable, but this instability is kept in check under normal circumstances by the cerebellum. Circumstances which alter this cerebellar restraint, such as vestibulocerebellar damage or plasticity due to rotation in darkness, can lead to PAN.