Algorithms for the clinical analysis of nystagmus eye movements

Dynamic accommodation measurement using Purkinje reflections and machine learning

Quantifying eye movement is important for diagnosing various neurological and ocular diseases as well as AR/VR displays. We developed a simple setup for real-time dynamic gaze tracking and accommodation measurements based on Purkinje reflections, which are the reflections from front and back surfaces of the cornea and the eye lens. We used an accurate eye model in ZEMAX to simulate the Purkinje reflection positions at different focus distances of the eye, which matched the experimental data. A neural network was trained to simultaneously predict vergence and accommodation using data collected from 9 subjects. We demonstrated that the use of Purkinje reflection coordinates in machine learning resulted in precise estimation. The proposed system accurately predicted the accommodation with an accuracy better than 0.22 D using subject's own data and 0.40 D using other subjects' data with two-point calibration in tests performed with 9 subjects in our setup.

Automated nystagmus detection: Accuracy of slow-phase and quick-phase algorithms to determine the presence of nystagmus

PURPOSE

To verify the accuracy of automated nystagmus detection algorithms.

METHOD

Video-oculography (VOG) plots were analyzed from consecutive patients with dizziness presenting to a neurology clinic. Data were recorded for 30 s in upright position with fixation block. For automated nystagmus detection, slow-phase algorithm parameters included mean and median slow-phase velocity (SPV), and slow-phase duration ratio. Quick-phase algorithm parameters included saccadic difference and saccadic ratio. For verification, two independent blinded assessors reviewed VOG traces and videos and coded presence or absence of nystagmus. Assessor consensus was used as reference standard. Accuracy of slow-phase and quick-phase algorithm parameters were compared, and ROC analysis was performed.

RESULTS

Among 524 analyzed VOG traces, 99 were verified as nystagmus present and 425 were verified as nystagmus absent. Prevalence of nystagmus in the sample population was 18.9%. In ROC analysis, areas under the curve of individual algorithm parameters were 0.791-0.896. With optimal thresholds for determining presence or absence of nystagmus, algorithm sensitivity (70.7-87.9%), specificity (71.8-84.0%), and negative predictive value (91.7-96.4%) were ideal, but positive predictive value (38.8-53.4%) was not ideal. Combining algorithm parameters using logistic regression models mildly improved detection accuracy.

CONCLUSION

Both slow-phase and fast-phase algorithms were accurate for detecting nystagmus. Due to low positive predictive value, the utility of independent automated nystagmus detection systems is limited in clinical settings with low prevalence of nystagmus. Combining parameters using logistic regression models appears to improve detection accuracy, indicating that machine learning may potentially optimize the accuracy of future automated nystagmus detection systems.

Frequency dependence of vestibuloocular reflex thresholds

How the brain processes signals in the presence of noise impacts much of behavioral neuroscience. Thresholds provide one way to assay noise. While perceptual thresholds have been widely investigated, vestibuloocular reflex (VOR) thresholds have seldom been studied and VOR threshold dynamics have never, to our knowledge, been reported. Therefore, we assessed VOR thresholds as a function of frequency. Specifically, we measured horizontal VOR thresholds evoked by yaw rotation in rhesus monkeys, using standard signal detection approaches like those used in earlier human vestibular perceptual threshold studies. We measured VOR thresholds ranging between 0.21 and 0.76°/s; the VOR thresholds increased slightly with frequency across the measured frequency range (0.2-3 Hz). These results do not mimic the frequency response of human perceptual thresholds that have been shown to increase substantially as frequency decreases below 0.5 Hz. These reported VOR threshold findings could indicate a qualitative difference between vestibular responses of humans and nonhuman primates, but a more likely explanation is an additional dynamic neural mechanism that does not influence the VOR but, rather, influences perceptual thresholds via a decision-making process included in direction recognition tasks.

Improved algorithm for classification of ocular nystagmus

An improved algorithm for classification of nystagmus was designed allowing the sorting of response segments even in severely non-linear patients and subjects with abnormally large phase shifts. The algorithm employs a model-based approach that was developed by Rey and Galiana. The improved classification algorithm consists of two essential stages. In the first stage the eye velocity response is classified to obtain initial estimates of the slow phase eye velocity intervals. In the second stage, the slow phase estimates are used to identify a response phase shift and nonlinearity, and compensate for their effects. Multiple tests on simulated data and experimental data obtained from clinical subjects are presented. The results of the tests demonstrate that the algorithm is able to analyze the patient data with a high accuracy even in the presence of noise, eye-blinks and other artifacts.

Vestibular function in patients with cochlear implantation

The aim of this work was to determine the influence of cochlear implantation (CI) on vestibular canal and otolithic function. Between 1995 and 1999, 15 patients (6 females, 9 males; 9-77 years old) underwent a vestibular examination before and after CI. Electronystagmography was performed between 5 and 8 days after CI in 9 patients, and with a time delay of 2-24 months in 10 patients. Pre- and postoperative evaluation included electronystagmography with caloric (44 degrees C, 30 degrees C, ice-water) and pendular rotatory testing. Otolithic function was measured postoperatively using off-vertical axis rotation (OVAR) in six patients. Preoperative data (n = 14) showed areflexia on caloric and rotatory pendular testing in deafness cases due to meningitis (n = 2) and in 2/5 patients with sudden idiopathic bilateral deafness. Two patients suffering from an idiopathic deafness had a unilateral hyporeflexia. Vestibular function was normal in the other eight patients. Immediately following CI, among patients with normal preoperative canal function, three developed vertiginous symptoms with spontaneous nystagmus, which disappeared within days to weeks. Later, postoperative canal evaluation was normal in 5/8 patients (62%) with initially preserved vestibular function: areflexia was measured ipsilaterally to the implanted ear in 1 patient and contralaterally in 2 patients. Hyporeflexia was measured ipsilateral to the implanted ear in two patients. OVAR examination, performed 2-19 months after surgery, showed a preserved otolithic function in all 6 tested patients. Transient vertigo on electrical CI stimulation was described in only one patient during the first postoperative weeks. The following conclusions can be drawn. Patients with deafness due to meningitis had an eradicated vestibular function. In other etiologies, vestibular function was most often preserved. CI did not usually abolish vestibular function, but the canal function was disturbed temporarily in 20% of cases. Otolithic function was preserved in all six CI patients tested in this series.

Responses of sympathetic outflow to skin during caloric stimulation in humans

We previously showed that caloric vestibular stimulation elicits increases in sympathetic outflow to muscle (MSNA) in humans. The present study was conducted to determine the effect of this stimulation on sympathetic outflow to skin (SSNA). The SSNA in the tibial and peroneal nerves and nystagmus was recorded in nine subjects when the external meatus was irrigated with 50 ml of cold (10 degrees C) or warm (44 degrees C) water. During nystagmus, the SSNA in tibial and peroneal nerves decreased to 50 +/- 4% (with baseline value set as 100%) and 61 +/- 4%, respectively. The degree of SSNA suppression in both nerves was proportional to the maximum slow-phase velocity of nystagmus. After nystagmus, the SSNA increased to 166 +/- 7 and 168 +/- 6%, respectively, and the degree of motion sickness symptoms was correlated with this SSNA increase. These results suggest that the SSNA response differs from the MSNA response during caloric vestibular stimulation and that the SSNA response elicited in the initial period of caloric vestibular stimulation is different from that observed during the period of motion sickness symptoms.

Yaw sensory rearrangement alters pitch vestibulo-ocular reflex responses

Ten male subjects underwent two types of adaptation paradigm designed either to enhance or to attenuate the gain of the canal-ocular reflex (COR), before undergoing otolith-ocular reflex (OOR) testing with constant velocity, earth horizontal axis and pitch rotation. The adaptation paradigm paired a 0.2 Hz sinusoidal rotation about an earth vertical axis with a 0.2 Hz optokinetic stimulus that was deliberately mismatched in peak velocity or phase and was designed to produce short-term changes in the COR. Preadaptation and postadaptation OOR tests occurred at a constant velocity of 60 degrees/sec in the dark and produced a modulation component of the slow phase velocity with a frequency of 0.16 Hz due to otolithic stimulation by the sinusoidally changing gravity vector. Of the seven subjects who showed enhancement of the COR gain, six also showed enhancement of the OOR modulation component. Of the seven subjects who showed attenuation of the COR gain, five also showed attenuation of the OOR modulation component. The probability that these two cross-axis adaptation effects would occur by chance is less than 0.02. This suggests that visual-vestibular conditioning of the yaw axis COR also induced changes in the pitch axis OOR. We thus postulate that the central nervous system pathways that process horizontal canal yaw stimuli have elements in common with those processing otolithic stimuli about the pitch axis.

Plasticity of responses to off-vertical axis rotation

This study assessed whether a change in the magnitude of the angular vestibulo-ocular reflex (VOR) influences the magnitude of the linear VOR, thereby suggesting a common gain element for these reflexes. The responses to linear acceleration using yaw off-vertical axis rotation (OVAR) at 30 degrees tilt were recorded before and after an adaptation protocol designed to increase the angular VOR gain. Subjects included eight asymptomatic healthy young individuals. Eye movements, recorded with electro-oculography, were analyzed to yield gain of the horizontal angular VOR and the magnitude of the modulation and bias components of the response to OVAR. Results indicated that there was no consistent influence of angular VOR gain on the eye movement response to OVAR. Since previous studies have shown that responses to earth horizontal axis rotation are influenced consistently by angular VOR gain, our study suggests that high intensity otolithic stimulation is required to observe changes in the linear VOR following modification of the angular VOR.

Intratympanic gentamicin for treatment of intractable Meniere's disease: a preliminary report

Topical administration of aminoglycoside antibiotics in the middle ear can achieve "chemical labyrinthectomy" in patients with intractable Meniere's disease. Herein we report our results of intratympanic gentamicin therapy in 21 patients using two different dosing protocols, twice weekly and twice daily (b.i.d.). Both hearing and vertigo outcome were evaluated. Complete control of episodic vertigo was achieved initially in 20 of 21 patients (95.2%). However, 6 of 20 responders (30%) developed relapsing symptoms within 12 months. Retreatment was successful in 75% of these patients. Overall, hearing was preserved or improved in 62% of cases, worse in 24%, and not yet tested in 14%. When the cumulative dose of gentamicin was < or = 4 injections in the first week, only 1 of 14 (7.1%) lost hearing. Intratympanic gentamicin offers better risk/benefit outcome than other invasive therapies for intractable Meniere's disease.

Classification of human rotation test results using parametric modeling and multivariate statistics

The usefulness of vestibular testing is directly related to the accuracy of the test interpretations. Two factors, subjective analysis of large test data sets and failure to make appropriate age corrections, tend to reduce test accuracy. Correction of these problems can be accomplished by application of physiologically based models of vestibular function and multivariate classification techniques to the test data, thereby creating a more objective test interpretation procedure. Herein we report our results on the use of this strategy for analysis of sinusoidal harmonic acceleration (SHA) test interpretation. For each patient, models reduce the large set of SHA test variables to three key parameters: asymptotic gain, vestibulo-ocular reflex time constant, and bias. In addition, the new technique objectively adjusts these parameters for the patient's age. Finally, each patient's set of parameters are statistically classified as either normal or as unilateral peripheral deficit. Based on learning sets of 57 normals and 30 patients with a full unilateral peripheral deficit, this new technique resulted in a misclassification rate between the categories of normal and full unilateral loss of 3.4%, comparing favorably to the present method's misclassification rate between normal and abnormal of 13.8%. We also analyzed and classified a test group consisting of patients with possible partial unilateral deficits using the same classification function as the normal and full unilateral learning sets. Even though the classifier was not optimized for the partial group, results seemed favorable relative to the human interpreter. These results validate the accuracy and utility of physiological parametric models and multivariate statistical classification in SHA test interpretation.

Vestibular function test anomalies in patients with chronic fatigue syndrome

Chronic fatigue syndrome (CFS) is distinguished by the new onset of debilitating fatigue that lasts at least 6 months, concomitant with other symptoms to be described later. Many CFS patients complain of disequilibrium, yet the exact type of the balance dysfunction and its function and its location (peripheral vs. central) have not been described. Herein we report results of vestibular function testing performed on 11 CFS patients. These results revealed no predominant pattern of abnormalities. Patients typically performed below average in dynamic posturography testing, with a significant number of falls in the tests requiring subjects to depend heavily on the vestibular system. One patient had abnormal caloric testing, while 3 had abnormally low earth vertical axis rotation (EVA) gains at the higher frequencies tested. As a group, the average gain of EVA was significantly lower than normals in the 0.1 - 1.0 Hz range (p < 0.05). In earth horizontal axis rotation, the CFS group had a higher than normal bias value for the optokinetic (OKN) and eyes open in the dark conditions (p < 0.05), but had normal scores during visual vestibular reflex testing. Five of the 11 subjects had an abnormal OKN bias build up over the course of the run, equal to or actually exceeding the 60 degrees/s target velocity by as much as 14 degrees/s. Altogether, these results are more suggestive of central nervous system deficits than of peripheral vestibular disfunction.

Effect of target velocity upon horizontal axis otolith-visual interactions

Visual-vestibular interactions were assessed for eleven human subjects during earth-horizontal axis rotation. The apparatus consisted of a rotating chair and an independently controlled rotating optokinetic surround. Subjects underwent ten different test runs where vestibular and optokinetic stimuli were given independently and in combination. The resultant nystagmus slow component velocity was analyzed. When vestibular stimuli were given, the typical slow component velocity response consisted of an exponential decay to a non-zero baseline value (bias component). Superimposed on this was a cyclic modulation of the slow component velocity whose period was equal to the time required for one complete revolution. Our data indicate that the addition of visual input to otolith input does not affect the slow component velocity modulation component during earth horizontal axis rotation. The average bias component during otolith stimulation alone was much lower than the stimulus velocity. The bias component during optokinetic stimulation produce velocity dependent saturation. Thus, neither input alone was adequate to produce a bias component that matched the higher stimulus velocities. In contrast, the average bias component during otolith-visual interaction runs produced responses that were nearly equal to the relative target velocity. This occurred despite large individual variability of the otolith alone and optokinetic response alone. Thus, the brain compensates to match the target velocity when otolith and visual stimuli are presented together.

A review of computerized electronystagmography technology

Analysis of nystagmus using manual methods is time consuming, reliant on considerable experience and subject to observer bias. Like many electrophysiological responses, nystagmus waveforms are suited to acquisition and analysis by digital computer techniques. Development of computerized eye movement analysis procedures commenced in the late 1960s. Initially programs were mostly researched-based with the data often acquired on FM tape recorders and evaluated on mini-computers. As computer technology evolved and equipment prices decreased, development of portable software and hardware for use in general clinical assessment became feasible. The development of computer programs for the acquisition and analysis of eye movements is described and the limitations of analysis procedures discussed.

Human vertical eye movement responses to earth horizontal pitch

The vertical eye movements in humans produced in response to head-over-heels constant velocity pitch rotation about a horizontal axis resemble those from other species. At 60 degrees/s these are persistent and tend to have non-reversing slow components that are compensatory to the direction of rotation. In most, but not all subjects, the slow component velocity was well characterized by a rapid build-up followed by an exponential decay to a non-zero baseline. Super-imposed was a cyclic or modulation component whose frequency corresponded to the time for one revolution and whose maximum amplitude occurred during a specific head orientation. All response components (exponential decay, baseline and modulation) were larger during pitch backward compared to pitch forward runs. Decay time constants were shorter during the backward runs, thus, unlike left to right yaw axis rotation, pitch responses display significant asymmetries between paired forward and backward runs.

An algorithm separating saccadic from nonsaccadic eye movements automatically by use of the acceleration signal

An algorithm is described to discriminate automatically between saccades and slow eye movements. Sampled data of the eye position have been used to calculate the momentary acceleration of the eye. The higher acceleration values of the saccadic eye movements as opposed to the slow compensatory or pursuit eye movements served to differentiate between the two. The method is demonstrated by search-coil data in squirrel monkeys.

Plasticity of the human otolith-ocular reflex

The eye movement response to earth vertical axis rotation in the dark, a semicircular canal stimulus, can be altered by prior exposure to combined visual-vestibular stimuli. Such plasticity of the vestibulo-ocular reflex has not been described for earth horizontal axis rotation, a dynamic otolith stimulus. Twenty normal human subjects underwent one of two types of adaptation paradigms designed either to attenuate or enhance the gain of the semicircular canal-ocular reflex prior to undergoing otolith-ocular reflex testing with horizontal axis rotation. The adaptation paradigm paired a 0.2 Hz sinusoidal rotation about a vertical axis with a 0.2 Hz optokinetic stripe pattern that was deliberately mismatched in peak velocity. Pre- and post-adaptation horizontal axis rotations were at 60 degrees/s in the dark and produced a modulation in the slow component velocity of nystagmus having a frequency of 0.17 Hz due to putative stimulation of the otolith organs. Results showed that the magnitude of this modulation component response was altered in a manner similar to the alteration in semicircular canal-ocular responses. These results suggest that physiologic alteration of the vestibulo-ocular reflex using deliberately mismatched visual and semicircular canal stimuli induces changes in both canal-ocular and otolith-ocular responses. We postulate, therefore, that central nervous system pathways responsible for controlling the gains of canal-ocular and otolith-ocular reflexes are shared.

Changes in the dynamics of the vertical vestibulo-ocular reflex due to linear acceleration in the frontal plane of the cat

The vertical and horizontal components of the vestibulo-ocular reflex (VOR) were recorded in alert, restrained cats who were placed on their sides and subjected to whole-body rotations in the horizontal plane. The head was either on the axis or 45 cm eccentric from the axis rotation. During off-axis rotation there was a change in the linear force acting on the otolith organs due to the presence of a centripetal acceleration along the animal's vertical axis. Otolith forces (defined to be opposite to the centripetal acceleration) directed ventrally with respect to the animal (negative) decreased both the amplitude and time constant of the first-order approximation to the slow phase eye velocity of the vertical vestibulo-ocular reflex (VVOR). Otolith forces directed dorsally (positive) increased the amplitude and time constant. The effects were greater for the up VOR. The asymmetry in the VVOR time constant also depended on the otolith forces, being less in the presence of negative otolith forces that caused the resultant otolith force to move ventrally, towards the direction along which gravity normally acts when the animal is in the upright position. The effects of otolith forces on the up VVOR were independent of whether the animals were tested in the dark or in the light with a stationary visual surround (i.e., during visual suppression). In contrast, the changes in the time constant of the down VVOR were smaller during visual suppression. Simulations of the eye velocity storage mechanism suggest that the gain of the feedback in the storage integrator was modified by the angle between the resultant otolith force and an animal-fixed reference. This could be the animal's vertical, i.e., the direction along which gravity normally acts. For larger angles the feedback was less and the amplitude and time constant of the VVOR increased. The transformation of the otolith input was the same for both the up and down VOR, even though the final effect on the eye velocity was asymmetric (larger for up VOR) due to a separate, asymmetric gain element in the velocity storage feedback pathway.

The horizontal vestibulo-ocular reflex during linear acceleration in the frontal plane of the cat

Horizontal and vertical eye movements were recorded in alert, restrained cats that were subjected to whole-body rotations with the horizontal semicircular canals in the plane of rotation and the body centered on the axis or 45 cm eccentric from the axis of rotation. Changes in the horizontal vestibulo-ocular reflex (HVOR) due to the resultant of the linear forces (i.e., gravity and linear acceleration) acting on the otolith organs were examined during off-axis rotation when there was a centripetal acceleration along the animal's interaural axis. The HVOR time constant was slightly shortened when the resultant otolith force was not parallel to the animal's vertical axis. This effect was independent of the direction of the otolith force relative to the direction of the slow phase eye velocity. No effect on the HVOR amplitude was observed. In addition to changes in the HVOR dynamics, a significant vertical component of eye velocity was observed during stimulation of the horizontal canals when the resultant otolith force was not parallel with the animal's vertical axis. The effect was greater for larger angles between the resultant otolith force and gravity. An upward or downward component was elicited, depending on the direction of the horizontal component of eye velocity and the direction of the resultant otolith force. The vertical component was always in the direction that would tend to align the eye velocity vector with the resultant otolith force and keep the eye movement in a plane that had been rotated by the angle between the resultant otolith force and gravity.

The vestibulo-ocular reflex in the cat during linear acceleration in the sagittal plane

The horizontal and vertical components of the vestibulo-ocular reflex (VOR) were recorded in alert cats that were rotated with their head placed on or 45 cm eccentric from the axis of rotation. During off-axis rotation there was a centripetal acceleration along the animal's naso-occipital axis that changed the direction and the magnitude of the resultant otolith force in the animal's sagittal plane. When the animal was upright and eccentric from the axis of rotation, the horizontal VOR (HVOR) had a shorter time constant and smaller amplitude compared to the on-axis HVOR. The effect was symmetrical for both directions of the naso-occipital linear acceleration. When the animal was on its side and faced away from the axis of rotation, there was a decrease in the time constant of the down VOR. When the animal faced the opposite direction, the down VOR time constant was increased. No statistically significant effect was found on the amplitude of the VVOR and the time constant of the up VOR.

Parametric classification of segments in ocular nystagmus

A new method for nystagmus classification, using system identification techniques, is presented. We formulate a system whose input is head position and whose output is eye position. We approximate this system with an autoregressive with exogenous input (ARX) model which relates the input and output (transfer function) regardless of the temporal profile for the sensory stimulation. The system is then identified using a least squares criteria and three indicators are produced. From these a flag is produced that marks slow and fast phases as well as blinks and bad data segments. Tests with simulated and real data are presented and indicate that the segment classification is remarkably insensitive to recording noise and that it is more robust than previous techniques. Operator intervention is minimal. We expect the method to be applicable for all types of ocular nystagmus. Here, however, we illustrate our results only in the context of the vestibuloocular reflex (VOR). A discussion explains how this method can also be applied for optokinetic (OKN) or pursuit nystagmus.

Vestibular recruitment in Meniere's disease

The aim of this study was to search for vestibular recruitment in a sample of patients with Meniere's disease. Recruitment was defined as an abnormal growth of response with increasing stimulus intensity. Twenty-nine patients were tested with sinusoidal rotation of three different magnitudes at four different frequencies. We also searched for auditory recruitment in each patient via tests of auditory brain stem responses, acoustic reflexes, and loudness balance and discomfort level. Analysis of vestibular responses indicated, on average, a linear relationship between stimulus magnitude and response magnitude, ie, doubling the stimulus magnitude resulted in twice the response magnitude. Meniere's patients did not yield results significantly different (although they were more variable) from those of the normal subjects. The vestibular responses of patients with auditory recruitment did not differ systematically from those of patients without auditory recruitment. We conclude that vestibular recruitment, if it exists in patients with Meniere's disease, is not demonstrated by sinusoidal rotational testing.

Microcomputer based analysis of nystagmus eye movement

An algorithm to assist in the computation of routine peripheral nystagmus slow phase velocity (SPV) has been implemented on an Apple based data acquisition system. Waveforms stored by the computer may be output to a dot matrix printer to complement conventional strip-chart recorder output. An estimate of SPV is available after each caloric session. Analysis modes range from a summarized output for inclusion in the clinical report to a detailed beat by beat report comprising waveforms, markers and tabulated results. Problematic signals, beyond the scope of the automatic analysis, may be inspected interactively using a software controlled cursor.

Nystagmus responses in a group of normal humans during earth-horizontal axis rotation

Horizontal eye movement responses to earth-horizontal yaw axis rotation were evaluated in 50 normal human subjects who were uniformly distributed in age (20-69 years) and equally divided by gender for each decade. The subjects were rotated with eyes open in the dark, using clockwise and counterclockwise 60 degree/s velocity trapezoids. The nystagmus slow component velocity (SCV) was analysed using four parameters: Amp, Bias, Mod and Tau. Amp and Tau characterize the canal-ocular reflex to constant velocity steps, while Mod and Bias characterize the "AC" and "DC" components of the otolith-ocular reflex. Results indicated that intersubject variability was larger than that seen in earth-vertical axis data. Tau depended significantly (p less than 0.05) upon subject gender, while Mod increased monotonically with age decade. Linear regression showed a positive correlation between pairs of SCV magnitude parameters (Amp, Bias and Mod), suggesting a common scaling effect. In addition, there was a negative correlation between the value of the decay time constant Tau and each of the three magnitude parameters. Thus, despite large intersubject variability, parameters that describe earth-horizontal yaw axis responses are loosely interrelated and some of them vary significantly with gender and age.

A fuzzy set theoretical approach to automatic analysis of nystagmic eye movements

A new method for computer analysis of nystagmic eye movements in vestibulo-ocular (VOR) and optokinetic (OKN) reflexes is developed. A fuzzy set theoretical approach is used to construct the slow cumulative eye position (SCEP) curve by eliminating fast components (saccades) from eye movement signal. These procedures are able to perform automatically some pattern recognition tasks traditionally used--in classical interactive programs--when human operators distinguish between fast- and slow-phases of eye movements. The structure of the algorithm is as follows. 1) A fuzzy clustering of slow- and fast-phases is made. An iterative method is used to refine the membership function of slow-phases, step by step, until a sufficiently discriminating membership function is obtained. 2) Saccades are detected and removed from the eye position signal. 3) SCEP is then built by interpolating between slow phases. 4) A weighted least-squares curve fitting is made. Weighting coefficients are obtained from the last membership function resulting from iterations in step 1). This curve fitting is referenced to the SCEP and the parameters of VOR and OKN are calculated using this last curve. This approach permits an analysis of nystagmic eye movements with high reliability even when the data are of modest quality. The definitive innovative feature of the program is that it allows entirely automatic analysis without participation of a human operator. The main algorithm being independent of the shape of stimulus, the program can be generalized to fit any type of simulation.

The chinchilla's vestibulo-ocular reflex

The horizontal vestibulo-ocular reflex (VOR) was measured and characterized in seven adult chinchillas using 0.01 to 1.0 Hz angular velocity sinusoids. Gains were less than compensatory, and were variable from day to day but phases were highly repeatable both within and between animals. The best fitting transfer function to the average data of all animals had a dominant time constant of 7.5 sec, and an adaptation operator with a time constant of 24.0 sec. There were certain nonlinearities in the horizontal VOR of this animal, and it was difficult to elicit a robust optokinetic response. Results are discussed in relation to similar measurements in other species.

Eyes open versus eyes closed: effect on human rotational responses

The effect of eyelid closure on the response to rotational vestibular stimulation was assessed by evaluating 16 normal human subjects with both earth vertical axis (EVA) and earth horizontal axis (EHA) yaw rotations with either eyes closed (EC) or eyes open in the dark (EOD). Results indicated that for EVA rotation, the subjects' responses were of larger magnitude and less variable with EOD than with EC. However, for EHA rotation, responses were of larger magnitude and equally variable with EC as compared to EOD. Our data also indicated that the quality of the EHA response with EC was altered because eyelid closure influenced the amount of periodic gaze. We conclude that eyelid closure has an effect upon both canalocular and otolithocular reflexes and suggest that both EVA and EHA rotational testing be performed with EOD rather than with EC.

A microcomputer system for clinical signal analysis of vestibular laboratory

A vestibular laboratory system was implemented for clinical testing and medical examination of patients with balance disorders. Programs for analysis of postural control and eye movements were made for a microcomputer. The microcomputer system can be used by a nurse without technical education. The system has now been used for two years at the laboratory.

Earth horizontal axis rotational responses in patients with unilateral peripheral vestibular deficits

We evaluated the vestibulo-ocular reflex (VOR) of five patients with surgically confirmed unilateral peripheral vestibular lesions. Testing used both earth vertical axis (EVA) and earth horizontal axis (EHA) yaw rotation. Results indicated that the patients had short VOR time constants, asymmetric responses to both EVA and EHA rotation, and normal EHA modulation components. These findings suggest that unilateral peripheral vestibular loss causes a shortened VOR time constant even with the addition of dynamic otolithic stimulation and causes an asymmetry in semicircular canal-ocular reflexes and one aspect of otolith-ocular reflexes.

Influence of gravity on cat vertical vestibulo-ocular reflex

The vertical vestibulo-ocular reflex (VOR) was recorded in cats using electro-oculography during sinusoidal angular pitch. Peak stimulus velocity was 50%/s over a frequency range from 0.01 to 4.0 Hz. To test the effect of gravity on the vertical VOR, the animal was pitched while sitting upright or lying on its side. Upright pitch changed the cat's orientation relative to gravity, while on-side pitch did not. The cumulative slow component position of the eye during on-side pitch was less symmetric than during upright pitch. Over the mid-frequency range (0.1 to 1.0 Hz), the average gain of the vertical VOR was 14.5% higher during upright pitch than during on-side pitch. At low frequencies (less than 0.05 Hz) changing head position relative to gravity raised the vertical VOR gain and kept the reflex in phase with stimulus velocity. These results indicate that gravity-sensitive mechanisms make the vertical VOR more compensatory.

Eye movements induced by gravitational force and by angular acceleration: their relationship

The relationship between putative 'canal' and 'otolith' function was studied by using both vertical axis and horizontal axis (barbecue spit) rotations. The vestibulo-ocular reflex (VOR) with eyes closed of 7 normal subjects during vertical axis rotation was characterized (0.02 to 1.67 Hz) using pseudorandom acceleration. Gain and phase points were calculated. The long time constant of the VOR was then estimated from the phase points using a simple linear systems model. The same subjects were also tested with eyes closed using earth horizontal axis rotations. Constant velocity rotation (60 degrees/sec) produced a periodic oscillation in the slope of the slow component of nystagmus known as the 'modulation component'. The amplitude of this component was averaged for clockwise and counterclockwise runs. For the 7 subjects, the amplitude of averaged modulation component correlated with the low-frequency phase values (r = 0.82 to 0.89) and VOR long time constant estimates (r = -0.95) obtained during vertical axis rotations. These data suggest a complementary relationship between responses to linear and to angular accelerations. Subjects having eye movement responses which are less sensitive to low-frequency angular acceleration also tend to have relatively greater responses to changing linear accelerations.

Effects of age, sex and stimulus parameters upon vestibulo-ocular responses to sinusoidal rotation

We evaluated the vestibulo-ocular reflex (VOR) in 50 normal human subjects, uniformly distributed in age 20-59 years, and evenly distributed in sex by decade, using sinusoidal rotation over the frequency range of 0.005 to 1.0 Hz and three peak amplitudes of 25, 50, and 100 deg/sec. Age variations. The gain magnitude of the normal human VOR appears to decrease with increasing age. This trend becomes statistically significant at p less than 0.05 for the 0.005 and 0.01 Hz test points. Sex-related variations. There were no statistically significant differences in gain magnitude between the sexes but females had higher average gains than males for the lowest test frequencies. The phase data showed a trend with the average phase points for females being less than those for males. This difference was statistically significant at p less than 0.05 for the 0.005 Hz test frequency. Dynamic range. Increasing the stimulus amplitude by a factor of four yielded a small but statistically significant decrease in gain magnitude, thus suggesting a mild saturation-type of nonlinearity.

The modulation component of nystagmus during earth-horizontal rotation: relationship with gaze angle

Periodic fluctuations in gaze angle and nystagmus slow phase velocity (SPV) were characterized using 6 normal subjects rotated at constant angular velocity about their rostral-caudal body axis with that axis in the horizontal orientation. The periodicity of the fluctuations corresponded to the angular frequency of rotation. The magnitude of the SPV fluctuations was moderately well correlated with those of the gaze fluctuations. Four vector quantities were found to characterize these data fairly completely, yet compactly.

Computer simulation of fixation suppression of vestibular nystagmus in normal persons

Fixation suppression of vestibular nystagmus was tested in normal humans during passive, nonperiodic angular rotation. The eye movement responses of the subjects were simulated using a simple mathematical model of visual-vestibular interaction. Agreement between observed and simulated responses was reasonably close, indicating that the model is a realistic representation of the physiologic mechanisms involved. The model was then used to assess the relative contributions of the pursuit and optokinetic systems to fixation suppression. It showed that, under these testing conditions, fixation suppression is mediated almost entirely by the pursuit system.

Postural stability and rotational tests: their effectiveness for screening dizzy patients

Results of independently interpreted computerized stationary platform posturography and white noise rotational tests were compared with diagnoses for 110 patients. Using the criterion that an abnormal result from either test classified the subject as abnormal, the sensitivity estimate for the pair of tests was 78% for persons with diagnoses known to result in vestibular dysfunction. The specificity estimate was 90%. Both a vestibulo-ocular and a vestibulo-spinal test were required for effective screening. Consecutive tests performed over five days showed the rotation test to be much less variable than posturography. Rotation testing is therefore preferred for following the performance of patients having disorders thought to cause fluctuating vestibular systems. Interobserver reliability rates were 83% for posturography and 93% for rotation tests.

Intersubject variability in VOR responses to 0.005-1.0 Hz sinusoidal rotations

Two alternate hypotheses concerning intersubject variability in normal human VOR responses were tested. Both experimental gain and phase data and linear systems parameter fits to that data supported the hypothesis that individual experimental data points varied in a systematic rather than a random fashion about ensemble mean values. A simple two parameter linear systems model was found to characterize normal data, but there was little or no correlation between the values of these two parameters, i.e., they were independent variables.

Comparison of vestibulo-ocular and vestibulospinal screening tests

A comparison of a newly developed vestibulospinal screening test and two vestibulo-ocular screening tests (caloric and rotation) was performed on two groups of patients, one with Meniere's disease (MD) the other with benign paroxysmal positional nystagmus (BPPN) and vertigo. Vestibulospinal screening tests were slightly more sensitive (not statistically significant) than caloric tests for detection of a peripheral vestibular disorder. Caloric, rotation, and vestibulospinal screening tests were specific neither for MD nor BPPN groups. The caloric test false-positive rate for the involved ear was unacceptably high for the BPPN group. The rotation test false-positive rate was the highest of the three tests for both groups. The vestibulospinal test combined with either the caloric or the rotational tests increased the sensitivity for detection of a peripheral vestibular disturbance, particularly for the MD patients.