Ernst Dow, Thomas Anastasio
Nystagmus is a pattern of eye movement characterized by smooth rota(cid:173) tions of the eye in one direction and rapid rotations in the opposite di(cid:173) rection that reset eye position. Periodic alternating nystagmus (PAN) is a form of uncontrollable nystagmus that has been described as an un(cid:173) stable but amplitude-limited oscillation. PAN has been observed previ(cid:173) ously 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 cir(cid:173) cuits that control eye movement are inherently unstable, but this insta(cid:173) bility is kept in check under normal circumstances by the cerebellum. Circumstances which alter this cerebellar restraint, such as vestibulo(cid:173) cerebellar damage or plasticity due to rotation in darkness, can lead to PAN.