Abstract
Abstract: :
Purpose: Fusion Maldevelopment Nystagmus Syndrome (FMNS)—formerly known as Latent/Manifest Latent Nystagmus—is thought to be due to a tonic imbalance in the optokinetic system. We wished to investigate the effects of full–field (±20° horizontally by ±10° vertically) sinusiodal and square wave gratings on the waveforms of subjects with FMNS. Methods: Horizontal and vertical ocular motility recordings were made using high–speed (500 Hz) videography during viewing of targets with and without fixation points and/or gratings. Trials were performed for each eye viewing monocularly, and with both eyes viewing. The spatial frequency of the gratings ranged over eight octaves, from 0.125 to 16.0 cycles/degree (the upper limit of the video display). Data for each condition were analyzed for instantaneous frequency of the nystagmus, peak velocity of the fast phases, and average velocity of the slow phases. Results: When viewing the gratings, subjects exhibited a decrease in their nystagmus (approximately 50%), as determined by both fast–phase peak velocities and average slow–phase velocities (with a concommitant decrease in the average instantaneous frequency) when compared to the no–grating condition. The amount of damping was dependent of which eye was viewing, the "visual attention" level of the subject, and on the spatial frequency of the grating being viewed. Conclusions: Textured backgrounds such as moderate–frequency, full–field gratings appear to have similar effects on FMN waveforms as those reported in the case of the ocular following response in normal subjects. This suggests their possible use as a simple and inexpensive method to improve FMNS subjects’ visual acuity in day–to–day activities. We also demonstrated that FMNS, like Infantile Nystagmus Syndrome (INS)—formerly known as Congenital Nystagmus—has a stronger attentional dependence than previously reported.
Keywords: nystagmus • ocular motor control • visual acuity