Congenital Nystagmus Waveforms Research Articles

A new measure of nystagmus acuity.

To construct a new visual acuity measuring function for congenital nystagmus (CN) patients by studying the relationships between acuity, velocities and positions of the eye. After assessing the relationship between acuity, movement velocities and positions of the eye separately, a new function, which we call the automated nystagmus acuity function (ANAF), was constructed to measure the visual acuity of CN patients. Using a high-speed digital video system working at 500 frames per second, each eye was calibrated during monocular fixation. Twenty-six recorded nystagmus data were selected randomly. Using nystagmus waveforms, the best vision position (foveation period) and visual acuity were analyzed in three groups of subjects, and then all outputs were compared with the well-known expanded nystagmus acuity function (NAFX) and ANAF. Standard descriptive statistics were used to summarize the outputs of the two programs. Foveation periods were brief intervals in the CN waveform when the image was on or near the fovea and eye velocity was relatively slow. Results showed good visual acuity happened during the period when velocity was low and the eye position was near the zero position, which fitted the foveation periods. The data analyzed with NAFX and ANAF had a correlation coefficient of 0.934276, with an average error of -0.00973. The results from ANAF and NAFX analyses showed no significant difference. The NAFX manually identifies foveation eye positions and produces accurate measurements. The ANAF, however, can be calculated simply using the factors eye position and velocity, and it automatically calculates the ANAF without the need to manually identify foveation eye positions.

Beating the Beat: Reading Can Be Faster Than the Frequency of Eye Movements in Persons With Congenital Nystagmus

Rapid serial visual presentation (RSVP) of text has been reported to foster higher reading rates than presentation in a continuous text (CT) format, possibly because scanning eye movements are minimized. We investigated how this might be relevant for persons with congenital nystagmus (CN). We evaluated whether reading rates differ in persons with CN for RSVP versus CT presentation of single sentences under otherwise similar conditions. In a second experiment, we presented unrelated words to observers with CN in RSVP format while measuring their eye movements to determine whether reading can occur during the high-velocity, nonfoveating periods of the CN wave form. Both sentences and random words were selected from the MNRead corpus and displayed at 2x, 4x, or 8x the threshold word size on a 21-inch computer monitor. Subjects with CN have virtually equivalent maximum reading speeds of 449 and 448 words per minute, respectively, for RSVP and CT presentation of sentences. Typically, reading rates were faster than the frequency of CN, which suggests that subjects could read during the nonfoveating periods of the nystagmus waveform. This finding was confirmed using random words that, unlike those in sentences, cannot be inferred from contextual cues. Examination of eye movements recorded during reading indicated that random words are read correctly with 47% to 65% accuracy (depending on word size) during the nonfoveating periods of the CN waveform. A clinical implication of these results is that reading performance in persons with CN should be facilitated by large text sizes that remain legible during a greater fraction of the CN waveform.

Eye movement instabilities and nystagmus can be predicted by a nonlinear dynamics model of the saccadic system

The study of eye movements and oculomotor disorders has, for four decades, greatly benefitted from the application of control theoretic concepts. This paper is an example of a complementary approach based on the theory of nonlinear dynamical systems. Recently, a nonlinear dynamics model of the saccadic system was developed, comprising a symmetric piecewise-smooth system of six first-order autonomous ordinary differential equations. A preliminary numerical investigation of the model revealed that in addition to generating normal saccades, it could also simulate inaccurate saccades, and the oscillatory instability known as congenital nystagmus (CN). By varying the parameters of the model, several types of CN oscillations were produced, including jerk, bidirectional jerk and pendular nystagmus. The aim of this study was to investigate the bifurcations and attractors of the model, in order to obtain a classification of the simulated oculomotor behaviours. The application of standard stability analysis techniques, together with numerical work, revealed that the equations have a rich bifurcation structure. In addition to Hopf, homoclinic and saddlenode bifurcations organised by a Takens-Bogdanov point, the equations can undergo nonsmooth pitchfork bifurcations and nonsmooth gluing bifurcations. Evidence was also found for the existence of Hopf-initiated canards. The simulated jerk CN waveforms were found to correspond to a pair of post-canard symmetry-related limit cycles, which exist in regions of parameter space where the equations are a slow-fast system. The slow and fast phases of the simulated oscillations were attributed to the geometry of the corresponding slow manifold. The simulated bidirectional jerk and pendular waveforms were attributed to a symmetry invariant limit cycle produced by the gluing of the asymmetric cycles. In contrast to control models of the oculomotor system, the bifurcation analysis places clear restrictions on which kinds of behaviour are likely to be associated with each other in parameter space, enabling predictions to be made regarding the possible changes in the oscillation type that may be observed upon changing the model parameters. The analysis suggests that CN is one of a range of oculomotor disorders associated with a pathological saccadic braking signal, and that jerk and pendular nystagmus are the most probable oscillatory instabilities. Additionally, the transition from jerk CN to bidirectional jerk and pendular nystagmus observed experimentally when the gaze angle or attention level is changed is attributed to a gluing bifurcation. This suggests the possibility of manipulating the waveforms of subjects with jerk CN experimentally to produce waveforms with an extended foveation period, thereby improving visual resolution.

Congenital nystagmus: hypotheses for its genesis and complex waveforms within a behavioral ocular motor system model.

Attempts to simulate dysfunction within ocular motor system (OMS) models capable of exhibiting known ocular motor behavior have provided valuable insight into the structure of the OMS required for normal visual function. The pendular waveforms of congenital nystagmus (CN) appear to be quite complex, composed of a sustained sinusoidal oscillation punctuated by braking saccades and foveating saccades followed by periods of extended foveation. Previously, we verified that these quick phases are generated by the same mechanism as voluntary saccades. We propose a computer model of the ocular motor system that simulates the responses of individuals with pendular CN (including its variable waveforms) based on the instability exhibited by the normal pursuit subsystem and its interaction with other components of the normal ocular motor control system. Fixation data from subjects with CN using both infrared and magnetic search coil oculography were used as templates for our simulations. Our OMS model simulates data from individuals with CN during fixation and in response to complex stimuli. The use of position and velocity efference copy to suppress oscillopsia is the key element in allowing for normal ocular motor behavior. The model's responses to target steps, pulse-steps, ramps, and step-ramps support the hypothetical explanation for the conditions that result in sustained pendular oscillation and the rules for the corrective saccadic responses that shape this underlying oscillation into the well-known family of pendular CN waveforms: pendular (P), pseudopendular (PP), pendular with foveating saccades (Pfs), and pseudopendular with foveating saccades (PPfs). Position error determined the saccadic amplitudes of foveating saccades, whereas stereotypical braking saccades were not dependent on visual information. Additionally, we propose a structure and method of operation for the fixation subsystem, and use it to prolong the low-velocity intervals immediately following foveating saccades. The model's robustness supports the hypothesis that the pendular nystagmus seen in CN is due to a loss of damping of the normal pursuit-system velocity oscillation (functionally, it is pursuit-system nystagmus--PSN).

Effects of tenotomy surgery on congenital nystagmus waveforms in adult patients. Part I. Wavelet spectral analysis

Congenital nystagmus (CN) is an aperiodic oscillatory eye movement disorder. Horizontal rectus tenotomy with simple re-attachment has been proposed as a therapy for CN. This therapy might affect vision and/or eye movements. Another paper deals with improvements in visual acuity. This and the companion paper examine changes in eye movements. In this study, we examined the effect of tenotomy on nystagmus waveforms using wavelet spectral analysis. No common effect was found across the patients on the wavelet spectra of the CN beat, suggesting that tenotomy surgery has no effect, or only a quite small effect, on the waveform structure of CN.

Effects of tenotomy surgery on congenital nystagmus waveforms in adult patients. Part II. Dynamical systems analysis

Congenital nystagmus (CN) is an aperiodic oscillatory eye movement disorder of unknown etiology. We examined the effect of horizontal rectus tenotomy with simple re-attachment on the dimensionality of the dynamical mechanism underlying CN. The correlation dimensions (CDs) were calculated from eight patients who had tenotomy surgery. We found no significant differences in the CDs that could be associated with the surgery. The change in dimensionality was less than 5% on average. The results suggest that the tenotomy has no effect, or only a quite small effect, on the underlying mechanism of the CN beats.

Characteristics of braking saccades in congenital nystagmus.

Several of the characteristic waveforms of congenital nystagmus (CN) contain braking saccades. We test the hypothesis that braking (including foveating) saccades, while not always satisfying the standard relationships for saccades, are normal; any differences are due to the presence of high-velocity, slow-phase eye movements. Better measurements of saccadic properties, including position- and velocity-based measures and skewness, can eliminate some of this apparent distortion. We also evoked an analogous effect in normal subjects by use of a ramp-step-ramp stimulus. Finally, we used a model to further demonstrate this distortion in the saccades of normals, deviating from their intended magnitude as a function of the magnitude of the opposing velocity. The saccadic analysis methods developed herein are applicable to all saccades made during ongoing eye movements, whether normal or pathological. The above findings support the hypothesis that the braking saccades integral to many CN waveforms have normal characteristics and are the result of a normal saccadic system's responses to a slow-eye-movement oscillation.

Characterisation of congenital nystagmus waveforms in terms of periodic orbits

Because the oscillatory eye movements of congenital nystagmus vary from cycle to cycle, there is no clear relationship between the waveform produced and the underlying abnormality of the ocular motor system. We consider the durations of successive cycles of nystagmus which could be (1) completely determined by the lengths of the previous cycles, (2) completely independent of the lengths of the previous cycles or (3) a mixture of the two. The behaviour of a deterministic system can be characterised in terms of a collection of (unstable) oscillations, referred to as periodic orbits, which make up the system. By using a recently developed technique for identifying periodic orbits in noisy data, we find evidence for periodic orbits in nystagmus waveforms, eliminating the possibility that each cycle is independent of the previous cycles. The technique also enables us to identify the waveforms which correspond to the deterministic behaviour of the ocular motor system. These waveforms pose a challenge to our understanding of the ocular motor system because none of the current extensions to models of the normal behaviour of the ocular motor system can explain the range of identified waveforms.

Best catch from NANOSNET.

Best catch from NANOSNET.

Modelling of congenital nystagmus waveforms produced by saccadic system abnormalities.

Models of the mechanisms of normal eye movements are typically described in terms of the block diagrams which are used in control theory. An alternative approach to understanding the mechanisms of normal eye movements involves describing the eye movement behaviour in terms of smooth changes in state variables. The latter approach captures the burst cell firing against motor error (difference between target gaze angle and current gaze angle) phase plane behaviour which is found experimentally and facilitates the modelling of variations in burst cell behaviour. A novel explanation of several types of congenital nystagmus waveforms is given in terms of a saccadic termination abnormality.

Stereothresholds in persons with congenital nystagmus and in normal observers during comparable retinal image motion

Despite rapid oscillations of the eyes, visual acuity is close to normal in many observers with congenital nystagmus (CN). This study investigated whether binocular hyperacuity thresholds are also close to normal in observers with CN. To do so, we assessed stereothresholds for horizontally and vertically separated line targets in three normal observers and six observers with idiopathic horizontal CN. Stereothresholds in normal observers are better than in the observers with CN, especially for horizontally-separated targets and very small inter-line separations. Stereothresholds remain better in normal observers than in the observers with CN, even in the presence of conjugate retinal image motion simulating that in jerk nystagmus. However, when the simulated CN wave form also includes disconjugate position variability of the foveation periods, normal observers’ stereothresholds become similar to those of approximately half of the observers with CN. We conclude that stereothresholds in observers with CN are degraded by the more-or-less constant motion of the retinal image, by excessive vergence instability and, in some observers, by a neural sensory deficit.

Congenital nystagmus: In search of simplicity on the other side of complexity

To the Editor: This letter is in response to a recent editorial, “Congenital Nystagmus: In Search of Simplicity on the Other Side of Complexity,“1 written about the paper, “Clinical and Ocular Motor Analysis of Congenital Nystagmus in Infancy.“2 The paper in question (1) is the first prospective study of the waveforms of congenital nystagmus (CN) in infants; (2) contains the largest population so studied; (3) is the first such study to use accurate and sensitive ocular motor recording techniques; and (4) provides the first demonstration that infants do exhibit the same CN waveforms as adults, with breaking saccades, foveating saccades, and foveation periods (ie, they are not all pendular, as others have suggested). We recognize the connection in those CN patients with sensory disorders and also provide an explanation of the mechanism by which afferent disorders can facilitate the disorder of the smooth pursuit system to properly calibrate and thereby result in CN. This was done in the text and in Figure 11. Also, despite the adequate functioning of the smooth pursuit in CN subjects (ie, the velocity gain is normal), the readers of our paper can arrive at the conclusion that CN may be directly caused by the normally damped oscillations of the smooth pursuit system. We are well aware of, and largely responsible for, the overwhelming evidence that smooth pursuit is “intact” in CN (see references 19 and 20 in our paper). We were also careful to differentiate between those conditions that mightfacilitate the development of CN and what current evidence suggests is the direct cause. We would not presume to coerce any clinicians into altering their decisions to continue to use the terms sensory CN and motor CN. We welcome Dr Hamed’s wish to “bridge the gap between basic scientists and pediatric ophthalmologists” (although that was not an issue for us-one of the authors of our paper is a pediatric ophthalmologist). For the record, (1) CN is a single motor sign reflecting the primary oscillations of smooth pursuit and the secondary attempts by the saccadic and fixation subsystems to acquire the target of interest and prolong its capture by the fovea; (2) afferent deficits are neither necessary nor sufficient for the development of CN; (3) CN can only be reliably and repeatedly diagnosed and differentiated from other forms of nystagmus with the aid of ocular motor recordings; and (4) the diagnosis of CN should always prompt the ophthalmologist to assess the afferent visual system, despite the absence of direct causality. We will continue to investigate in the hope of clarifying and to report in the hope of converting the existing confusion on this side to the simplicity of understanding on the other.

Manifest latent and congenital nystagmus waveforms in the same subject A need to reconsider the underlying mechanisms of nystagmus

We report a unique case of a horizontal manifest latent nystagmus (MLN) that is converted to a congenital nystagmus (CN) on covering the subjectþs only seeing eye. The subject is a 65-year-old female who, at five days after birth, had her right eye ennucleated. A prosthetic eye was fitted soon after. Horizontal eye movements were recorded using an IRIS (Skalar Medical) infrared system. The effect of visual feedback on primary and secondary gaze, smooth pursuit, optokinetic responses, and the role of selective attention on fixation stability were examined. On primary gaze with the left eye open and under normal viewing conditions, the patient exhibited a typical MLN waveform. The slow phase was a decreasing velocity and the fast phase beat to the left. On covering the left eye, the nystagmus slow phase immediately increased in velocity and the fast phase beat to the right. Removal of visual feedback (darkness or stabilising the retinal image) also brought about a change from MLN to CN. Both the MLN and CN states were strongly influenced by gaze. The decreasing velocity MLN slow phase was also seen to change to a grossly extended slow phase during periods of visual disengagement. On occasions, these oscillations became a short burst of right-beating CN whenever the patient lost interest in the visual task. This is the first report of an adult subject who can exhibit either of two separate and sustained nystagmus states, an MLN or a CN, whenever the only eye is covered. Two explanations are offered to account for the presence of the two separate waveforms. Central to our interpretation is that the neural integrator has an eccentric null and that it is under a variable gain control. Furthermore, we propose that future models of nystagmus generation will need to include retinal image and attention control loops.

The effect of abnormal fixational eye movements upon visual acuity in congenital nystagmus

Purpose. The purpose of this study was to investigate the role that abnormal eye movements play in the degradation of visual acuity. Methods. Visual acuity was measured monocularly in 10 normal subjects (26.7 ± 4.3 years) and 5 subjects with congenital nystagmus (34.9 ± 8.8 years), using Regan Repeat Letter charts (RRL) and a logMAR based test (LogMAR Crowded Acuity Test (CAT)) while eye movements were continuously recorded using a commercially available infrared limbal eye tracker (Type 54, Optoelectronic Developments, UK). The eye tracker was controlled via a virtual oscilloscope (Viewdac, Keighly Instruments, UK) on an IBM PC clone (Opus Technology 486). Results. The mean visual acuity obtained with RRL was significantly higher than that obtained by CAT in the subjects with congenital nystagmus. A significant correlation was found between the root mean square value of the nystagmus wave-form and the angular extent of CAT. Linear regression analysis revealed a correlation between the duration of the foveation periods and the linear acuity of the subjects with congential nystagmus. The nystagmus waveforms also demonstrated increased amounts of high frequency components (HFC: >3.0Hz) when compared to the normal eyes. Conclusions. The results of this study reveal 1) RRL charts provide a measure of a subjects resolution limit which is less dependent on eye movements; 2) the duration of foveation periods has a significant effect on visual acuity measurements obtained using a linear test chart format; 3) the predominance of high frequency components in the congenital nystagmus waveforms lead to short foveation periods adding to the degree of fixation instability.

Generation of braking saccades in congenital nystagmus

Objectives: We examined eye movement records of two congenital nystagmus (CN) subjects, whose waveforms contained braking saccades, to test the hypothesis that eye velocity, rather than eye position, is the more important criterion for braking saccade generation. Specifically, we wished to determine the criteria and timing used by their ocular motor systems in triggering these unique saccades. Materials and methods: For the records analyzed, eye movements were measured by either scleral search coil or IR limbic reflection and data were sampled at rates of 200–488 Hz with a resolution of 12 bits for analysis by custom software. Both position and velocity were used to determine critical timing points in CN cycles, including saccadic onset, duration, offset, and magnitude. Phase planes at various times (between 40 and 70 ms) prior to saccade onset helped determine (using foveation window criteria for best acuity) the conditions to generate a braking saccade. Results: Braking saccades do brake CN slow phases, with average slowing (unrelated to braking-saccade size) of 62 and 119% for the two waveforms studied. At 70 ms prior to braking saccades, both eye position and velocity usually still satisfied the criteria for good acuity established in the previous foveation period; by 40 ms, velocity no longer did. Thus, high eye velocity was the only criterion that could be used for saccade generation. Braking saccades result in longer foveation times per second for CN waveforms. Conclusions: Eye velocity is the main criterion used to trigger braking saccades and the determination to trigger them occurs closer to 40 than 70 ms prior to their execution. Braking saccades can increase the nystagmus acuity function and allow better acuity.

Effects of acupuncture on foveation characteristics in congenital nystagmus

AIMS/BACKGROUNDTo examine the effects of acupuncture at the sternocleidomastoid muscles on foveation characteristics in congenital nystagmus.METHODSSix patients with congenital nystagmus (CN) received a series of treatments consisting of two needles...

Dynamical systems analysis: a new method of analysing congenital nystagmus waveforms.

Congenital nystagmus is an oculomotor disorder in which fixation is disrupted by rhythmical, bilateral involuntary oscillations. Clinically these eye movements have been described with some degree of success in terms of their peak-to-peak amplitude, frequency, mean velocity and waveform shape. However, it has not proved possible to diagnose any underlying pathology from the nystagmus characteristics. Here, we propose a new approach to understanding the nystagmus using dynamical systems theory. Our approach is based on the use of delay embedding techniques, which allow one to relate a time series of scalar observations to the state space dynamics of the underlying dynamical system. Using this approach we quantify the dynamics of the nystagmus in the region of foveation and present evidence to suggest that it is low-dimensional and deterministic. Our results put new constraints on acceptable models of nystagmus and suggest a way to make a closer link between data analysis and model development. This approach raises the hope that techniques originally developed to stabilise chaotic systems, by using small perturbations, may prove useful in the control of nystagmus.

Ocular movement during reading in patients with congenital nystagmus.

When a healthy person reads sentences arranged horizontally, the eye movement pattern is a series of regular saccades and rest periods. However, eye movements while patients with oculomotor disturbances are reading remain to be defined. In this study, 4 patients with congenital nystagmus (CN) were studied. Coordinated movements of eye and head are also studied by a new apparatus with terrestrial magnetism sensor. While subjects were reading, an eye movement pattern was superimposed by the CN waveform and showed few rest periods. Eye velocity of less than 4 degrees/s was noted in almost every cycle of the eye movements while the patients with CN were reading. When they were requested to read sentences and simultaneously rotate their head naturally, the head movements had the effect of cancelling the eye instability. This finding may be interpreted as a kind of "eye-head coordination".

Evidence Suggesting Individual Ocular Motor Control of Each Eye (Muscle)

Current models of the ocular motor system are usually presented in their most reduced form, are unilateral in architecture, and precise yoking is presumed. Although this simplifies the models, it does not accurately simulate the actual neuroanatomy and limits the models to simple, stereotyped responses. Studies of normal humans and monkeys have demonstrated striking disconjugacies in normal responses. Normal saccades may be disconjugate, or 1 eye may exhibit a dynamic overshoot. Asymmetric vergence can result in disconjugate saccades, unequal magnification spectacles cause differential saccadic gain adjustment, and saccades to unequal disparities also cause unequal saccades in the 2 eyes. In strabismus, deviated eyes typically do not mimic the movements of the fixating eye nor do their latent or congenital nystagmus waveforms duplicate those of the fixating eye. In spasmus nutans, each eye oscillates independently of the other. In achiasmatic dogs, uni-ocular saccades and uni-ocular nystagmus waveforms are seen; the same may be true in human achiasma. These data from both normals and those with abnormalities suggest that current models for ocular motor control are inadequate representations of the actual system. The inability of unilateral, yoked control (or even bilateral, yoked control) system models to duplicate the ocular motor responses of binocular mammals suggests that their ocular motor systems evolved from the bilateral, independent control systems seen in chameleons. One need only postulate a yoking overlay superimposed on two independent control systems to achieve conjugacy (bilateral, yoked, independent control) of the eyes. Abnormalities producing grossly disconjugate eye movements may then be simulated using the independent control of each eye released by a deficiency in the yoking overlay. Independent control of each eye coupled with the essential bilateral brain stem architecture implies that each individual muscle is driven by independent populations of neurons (burst cells, neural integrator cells, etc.). The agonist muscles of each eye are usually coordinated (yoked) but may function independently if the task dictates or if binocularity did not develop. Models based on the above architecture would be robust and could duplicate the many responses (both normal and abnormal) possible from the neurophysiological system.

Hereditary congenital nystagmus and gaze‐holding failure

Congenital nystagmus (CN) may be due to an instability of the neural integrator responsible for gaze holding. This longitudinal study tests that hypothesis, investigates the saccadic instabilities of relatives, and assesses the effects of afferent stimulation on both the CN and the coexisting gaze-holding failure. We recorded four siblings who had CN and gaze-holding failure while fixating in primary position and lateral gaze. In lateral gaze, the CN waveforms were superimposed on the centripetal drift caused by the gaze-holding failure; the drift time constants ranged from 300 to 1,450 msec. CN waveforms lacked extended foveation periods. Saccadic instabilities were present in the father and two clinically unaffected siblings; the mother's eye movements were normal. We conclude that CN in the subjects of this study, and in others with idiopathic CN, is not due to gaze-holding abnormalities, and we speculate that development of the fixation reflexes that produce CN foveation periods requires some minimal foveation interval during which the target image is in the foveal area with low retinal slip velocity and acceleration.