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

The overall goal of this procedure is to determine three dimensional vestibular function in patients with vestibular disorders. This is accomplished by first seating the subject on a motion platform and fasten the seatbelt. Insert scleral search coils onto the subject's eyes.

In order to measure the vestibular ocular reflex in three dimensions, use a vacuum pillow and a bite board to restrain the subject. Next, the platform is activated. It delivers sinusoidal and step stimuli in a random order to test a vestibular system in all three dimensions.

The final step is the offline analysis of the eye coil data to extract the magnitude and alignment of the vestibular ocular reflex. Ultimately, the gain and alignment of the vestibular ocular reflex is used to distinguish normal from abnormal vestibular function. This technique enables us to test the vestibular system in all three dimensions.

This is a main advantage over existing methods such as single access rotating chairs that are used in ENT clinics. This method provides insight in 3D vestibular function in healthy subjects. In addition, the method is used to study vestibular diseases such as Sonoma tumors, vestibular neuritis, and many error disease.

Demonstrating the procedure will be Joyce DITs Kasper Boer, both PhD students and Johan Pell staff member within my research group. To begin this procedure, seat the subject on a chair mounted at the center of a motion platform and restrain them with the four point seatbelt anchored to the base of the platform. During the experiment, record the eye movements of both eyes using 3D scleral search coils with a standard 25 kilohertz two field coil system based on the amplitude detection method of Robinson.

To accomplish this first anesthetize the subject's eyes with a few drops of oxy butane in each eye. Then insert the scleral search coils, which are embedded in silicone into each eye. Once the search coils have been inserted, position the subject's head such that the imaginary line connecting the miis externa with the lower orbital canus or reeds line is within six degrees from the earth.Horizontal.

Next, immobilize the subject's head through the use of a vacuum pillow inflated around the subject's neck. Then have the subject bite down on an individually molded dental impression bite board. The bite board is attached to the cubic frame through a rigid bar and contains two 3D sensors from measuring spurious head movements via angular and linear acceleration.

Then activate the motion platform and raise it to its operating position. Calibrate the horizontal and vertical signals of both scleral search coils individually by instructing the subject to fixate on a series of targets for five seconds each. Then begin a sequence of pre-programmed motions.

The motion platform is capable of generating angular and translational stimuli at a total of six degrees of freedom through the use of six computer controlled electromechanical actuators shown here. In order to define motion, use a standard right-handed coordinate system. The coordinate system is centered at a point midway between the subject's ears and is defined from the subject's point of view.

First, define leftward rotation as positive motion in the Z direction. This is known as Y.Next, define downward motion as positive motion and the Y direction. This is known as pitch.

Finally, define a right word rotation as positive motion in the X direction. This is known as roll. To begin synchronize the platform and eye movement data using a laser beam mounted at the backside of the platform.

The home position is recognized when the laser is projected onto a small photo cell located at the back wall, which is monitored during the procedure, deliver sinusoidal stimuli in both light and dark settings. In the light, have the subject fix their eyes on a continuously lit red LED that is located 177 centimeters in front of them at all times in the darkness. The light is turned on for two seconds, then off before each movement begins.

Next, deliver whole body rotations about the three cardinal AEs via the motion platform, the roc coddle or vertical axis, the interoral axis, and the nasal occipital axis. In addition to stimulation about the cardinal axes, deliver whole body rotations in steps of 22.5 degrees between roll and pitch. Then perform impulse stimulation in a dimly lit environment using the LED as a visual target.

To accomplish this, deliver short duration impulses in each of the three cardinal axes and intermediate horizontal axes at 45 degrees. Repeat each impulse six times and deliver them in a random order. Additionally, vary the motion onset randomly with between 2.5 and 3.5 seconds.

Separating each new motion during the stimulations Acquire eye movement data at a frequency of 1000 hertz. Using a CED data acquisition system. Sample eye position data is shown here for each individual component.

Then convert the raw data of the eye coil signals into angular velocity for each component. The angular velocity data is used to calculate the gain, which is defined as the magnitude of compensatory eye movements with respect to imposed stimulus. Misalignment is instantaneous angle presented in degrees that is calculated in three dimensions between the inverse of the eye velocity axis and the head velocity axis.

An example of misalignment as a function of stimulus. Axis orientation is shown here as a dashed line shown here is a graph for the average results of the gain. For horizontal axis sinusoidal simulation of the control group, the torsional maximum appeared at zero degrees azimuth, whereas the vertical maximum was at both minus 90 degrees and plus 90 degrees azimuth.

The horizontal component shows only baseline measurements. When the vertical and torsion components are combined, you get the predicted value for the three DI velocity gain shown here as a dotted line. The actual values are shown as data points.

The misalignment between the stimulus and response axis averaged over six subjects as shown here. The dotted line represents the predicted values closely corresponding to the actual values. The misalignment was smallest during the pitch and gradually increased towards the role creating a maximum misalignment of 17.33 degrees at 22.5 degrees azimuth.

A significant difference was noticed when comparing the eye velocity gain components in the light versus in the dark. Both the vertical and torsion components were significantly lower in the dark, resulting in an overall lower 3D eye velocity gain. While the misalignment between stimulus and response followed the predicted values during sinusoidal simulation in the light, they do not match predicted values in the dark.

This is mainly due to the influence of the non-zero horizontal component. Impulse stimulation causes only brief disruptions of visual information, but has a qualitatively similar response in gain and misalignment to the sinusoidal stimulation in darkness. The sensitivity of this method is demonstrated when comparing patients with brain abnormalities such as unilateral sonomas to the control patients.

Shown on the left are the gain and misalignment charts of a patient with a 14 millimeter brain tumor. Obvious differences can be seen when comparing these charts to those of control patients. After watching this video, you should have a good understanding how a vestibular test procedure on a sixth degree of motion platform is performed.

It is important to understand how subjects are mounted on this platform, how search curls are inserted, and how to interpret the data.