Foveation dynamics in congenital nystagmus I: Fixation

Abstract

Congenital nystagmus (CN) has been described as a 'fixation' nystagmus implying an inability to fixate a target. However, each cycle of CN contains a target-foveation period during which the eye velocity is at, or near, zero. Prolongation of foveation time, reduction of retinal image velocity and cycle-to-cycle foveation repeatability all contribute to increased visual acuity. We developed several methods to accurately measure the dynamics of foveation in CN; their use is illustrated on an individual with typical idiopathic CN and no afferent defects. During eight 5-second intervals of fixation on a stationary target, the horizontal standard deviation (SD) of the mean foveation position (FPOS) was 12.82 minarc and the SD of foveation velocity was 118.36 minarc/sec. The SD of the means of total eye position and of the non-foveating peak of the CN were 43.17 and 25.32 minarc respectively. The mean foveation-time interval (eye velocity less than or equal to 4 degrees/sec) was 57.27 msec. The SD FPOS for the best 1-second interval (4 successive CN cycles), in a typical 5-second record, was 0.71 minarc. Histograms revealed peaks of eye position at 0 +/- 10 minarc and of eye velocity at 0 +/- 240 minarc/sec. The small vertical component of the CN (16 minarc peak-to-peak) had a SD of 6.56 minarc. A nystagmus foveation function related to visual acuity was derived that was more sensitive than CN intensity. The increased visual acuity resulting from the use of convergence or base-out prisms was due to increased foveation time. Although it might appear that CN is a defect of fixation, this individual with CN had strong fixation reflexes in the sense that he was able to accurately (within 1 minarc) achieve (interbeat) and maintain (intrabeat) target foveation for appreciable periods of time. Our data support the hypothesis that individuals with idiopathic CN do not have a primary disturbance of fixation.

Foveation dynamics in congenital nystagmus II: Smooth pursuit

“…To compare target foveation during pursuit with that during fixation, retinal error phase-plane plots were made and the pre-defined foveation window (0-+ 30minarc and 0_+ 240minarc/sec) superimposed on them. This is the same foveation window defined in our study of CN fixation; it is the foundation for both high visual acuity and oscillopsia suppression [9]. The phase plane of Fig.…”

“…The mean of retinal foveation position (REgfp) and its SD were measured at each target velocity using interactive graphics and SD's were compared to the average value obtained during fixation at each gaze angle (15.02 minarc) [9]. Foveation-period means and SD's were calculated in two ways: (1) To facilitate direct comparison to the 5-second records of fixation previously reported [9], the RERfp'S and SD's for all pursuit intervals in each 5-second record (combining pursuit in both directions) were averaged for each target velocity; (2) To preserve possible directional differences, RERfp'S and SD's in each pursuit direction were calculated separately for each target velocity.…”

“…Foveation-period means and SD's were calculated in two ways: (1) To facilitate direct comparison to the 5-second records of fixation previously reported [9], the RERfp'S and SD's for all pursuit intervals in each 5-second record (combining pursuit in both directions) were averaged for each target velocity; (2) To preserve possible directional differences, RERfp'S and SD's in each pursuit direction were calculated separately for each target velocity. Initial analysis was done on a PDP 11/73 computer and more extensive later analysis on an IBM PS/2 using the ASYST software for scientific computing [10] and SigmaPlot for plotting and curve-fitting data.…”

A Distal Model of Congenital Nystagmus as Nonlinear Adaptive Oscillations

“…An important aspect of this model is that it is not possible to proscribe a 'foveation period', at least when defined as a period when the eye position and velocity are within some limits (a rectangle in state space; [19]). The cost depends on the trajectory through state space and will vary between individuals depending on their spatio-temporal contrast sensitivity functions.…”

Ultrastructural Study of Extraocular Muscle in Congenital Nystagmus

“…This is probably due to the visual functional impairment caused by ocular structural abnormalities during the early development. Recent studies by Dell'Osso et al [5][6][7] showed that CN patients had intense fixation reflexes and stable pursuing eye movements with normal vestibuloocular reflexes and proposed that CN was due to unstable efferent impulses from the extraocular muscle. Many other hypotheses about the etiology and pathological mechanism of CN have not been widely accepted.…”