VOLUNTARY CONTROL OF CONGENITAL NYSTAGMUS

1. Congenital nystagmus is a poorly-understood, involuntary eye movement disorder. Here we report three patients with an unusual form of congenital nystagmus; in a well-lit room our patients could both suppress and release their nystagmus at will. 2. Using eye movement recordings, we quantified our patients' fixation and tracking behavior. In darkness, all patients had a sustained jerk nystagmus with velocity-increasing slow phases. During pursuit of a small target, two patients generated slow phases in the wrong direction. 3. In one patient we were able to show that in darkness he could decrease the slow-phase velocity of his nystagmus by a factor of 4 by imagining a fixation target. He could do no better with a 0.1-degrees fixation light in a dark room, but he could suppress his nystagmus completely with a 1-degrees fixation light. His pursuit capability also improved as target size increased. 4. We discuss the possible fixation and visual-following systems used by these patients to suppress their nystagmus. We suggest that the fixation system of our patients has both abnormal and normal feedback loops with the latter being under voluntary control. Furthermore, visual mechanisms are also necessary for complete suppression of nystagmus with parafoveal as well as foveal stimulation being important for stable fixation. We also compare our patients with others described previously and suggest a classification of congenital nystagmus based on the functional state of the fixation and the visual-following systems.