Spontaneous nystagmus that periodically reverses direction has been reported in a variety of circumstances: as a variant of congenital nystagmus; in association with acquired neurological lesions, especially in the region of the craniocervical junction; and as a transient manifestation of anticonvulsant intoxication.'-6 Periods of oscillation ranging from a few seconds to minutes have been rep~ rted.~.~ Sometimes this nystagmus is present only in darkness.'It has been reported in blind patients with no other evidence of neurological disease? Within the subjects of these reports exists a group of patients who show an indefatigable nystagmus, in light or darkness, which regularly reverses direction with a period often of 3-4 minutes, and in whom acquired disease of the nervous system seems most likely to be the cause.'It is this form of periodic alternating nystagmus (PAN) that we address here, using clinical studies and control-systems analysis to develop a hypothetical explanation, or model, of the phenomenon. We propose that PAN arises from (1) an instability in the brain-stem neural networks that generate slow phases of vestibular and optokinetic nystagmus,(2) the action of an adaptive network that normally acts to null prolonged, inappropriate nystagmus, and (3) an inability to use retinal-error-velocity information.