Three Dimensional Vestibular Ocular Reflex Testing Using a Six Degrees of Freedom Motion Platform

Summary

A method is described to measure three-dimensional vestibulo ocular reflexes (3D VOR) in humans using a six degrees of freedom (6DF) motion simulator. The gain and misalignment of the 3D angular VOR provide a direct measure of the quality of vestibular function. Representative data on healthy subjects are provided

Abstract

The vestibular organ is a sensor that measures angular and linear accelerations with six degrees of freedom (6DF). Complete or partial defects in the vestibular organ results in mild to severe equilibrium problems, such as vertigo, dizziness, oscillopsia, gait unsteadiness nausea and/or vomiting. A good and frequently used measure to quantify gaze stabilization is the gain, which is defined as the magnitude of compensatory eye movements with respect to imposed head movements. To test vestibular function more fully one has to realize that 3D VOR ideally generates compensatory ocular rotations not only with a magnitude (gain) equal and opposite to the head rotation but also about an axis that is co-linear with the head rotation axis (alignment). Abnormal vestibular function thus results in changes in gain and changes in alignment of the 3D VOR response.

Here we describe a method to measure 3D VOR using whole body rotation on a 6DF motion platform. Although the method also allows testing translation VOR responses ¹, we limit ourselves to a discussion of the method to measure 3D angular VOR. In addition, we restrict ourselves here to description of data collected in healthy subjects in response to angular sinusoidal and impulse stimulation.

Subjects are sitting upright and receive whole-body small amplitude sinusoidal and constant acceleration impulses. Sinusoidal stimuli (f = 1 Hz, A = 4°) were delivered about the vertical axis and about axes in the horizontal plane varying between roll and pitch at increments of 22.5° in azimuth. Impulses were delivered in yaw, roll and pitch and in the vertical canal planes. Eye movements were measured using the scleral search coil technique ². Search coil signals were sampled at a frequency of 1 kHz.

The input-output ratio (gain) and misalignment (co-linearity) of the 3D VOR were calculated from the eye coil signals ³.

Gain and co-linearity of 3D VOR depended on the orientation of the stimulus axis. Systematic deviations were found in particular during horizontal axis stimulation. In the light the eye rotation axis was properly aligned with the stimulus axis at orientations 0° and 90° azimuth, but gradually deviated more and more towards 45° azimuth.

The systematic deviations in misalignment for intermediate axes can be explained by a low gain for torsion (X-axis or roll-axis rotation) and a high gain for vertical eye movements (Y-axis or pitch-axis rotation (see Figure 2). Because intermediate axis stimulation leads a compensatory response based on vector summation of the individual eye rotation components, the net response axis will deviate because the gain for X- and Y-axis are different.

In darkness the gain of all eye rotation components had lower values. The result was that the misalignment in darkness and for impulses had different peaks and troughs than in the light: its minimum value was reached for pitch axis stimulation and its maximum for roll axis stimulation.

Case Presentation

Nine subjects participated in the experiment. All subjects gave their informed consent. The experimental procedure was approved by the Medical Ethics Committee of Erasmus University Medical Center and adhered to the Declaration of Helsinki for research involving human subjects.

Six subjects served as controls. Three subjects had a unilateral vestibular impairment due to a vestibular schwannoma. The age of control subjects (six males and three females) ranged from 22 to 55 years. None of the controls had visual or vestibular complaints due to neurological, cardio vascular and ophthalmic disorders.

The age of the patients with schwannoma varied between 44 and 64 years (two males and one female). All schwannoma subjects were under medical surveillance and/or had received treatment by a multidisciplinary team consisting of an othorhinolaryngologist and a neurosurgeon of the Erasmus University Medical Center. Tested patients all had a right side vestibular schwannoma and underwent a wait and watch policy (Table 1; subjects N1-N3) after being diagnosed with vestibular schwannoma. Their tumors had been stabile for over 8-10 years on magnetic resonance imaging.

Protocol

1. 6DF Motion Platform

Vestibular stimuli were delivered with a motion platform (see Figure 1) capable of generating angular and translational stimuli at a total of six degrees of freedom (FCS-MOOG, Nieuw-Vennep, The Netherlands). The platform is moved by six electro-mechanical actuators connected to a personal computer with dedicated control software. It generates accurate movements with six degrees of freedom. Sensors placed in the actuators continuously monitored the platform motion profile. The device has <0.5 mm precision for linear and <0.05° for angular movements. Vibrations during stimulation were 0.02°. Reson....

Results

Sinusoidal stimulation light

Figure 4 (top panel) shows for the control group the mean gain of the horizontal, vertical and torsion angular velocity components for all tested sinusoidal stimulations in the horizontal plane in the light. Torsion was maximal at 0° azimuth, whereas vertical had its maximum at 90°. Figure 5 shows the 3D eye velocity gain in the light. Gain varied between 0.99 ± 0.12 (pitch) and 0.54 ± 0.16 (roll). The measured data closely correspond.......

Discussion

This paper describes a method to accurately measure 3D angular VOR in response to whole body rotations in humans. The advantage of the method is that it gives quantitative information about gain and misalignment of 3D angular VOR in all three dimensions. The method is useful for fundamental research and has also potential clinical value e.g. for testing patients with vertical canal problems or patients with ill-understood central vestibular problems. Another advantage of the device is the ability to test transla.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Electric Motion Base MB-E-6DOF/24/1800KG * (Formerly E-CUE 624-1800)	FCS-MOOG, Nieuw-Vennep, The Netherlands
Magnetic field with detector, Model EMP3020	Skalar Medical, Delft, The Netherlands
CED power 1401, running Spike2 v6	Cambridge Electronic Design, Cambridge
Electromagnetic search coils	Chronos Vision, Berlin, Germany