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times when there is only a single eye to record from;
however, this does not preclude you from using a binocular recording and analyzing only a single eye. However,
it is important to realize that when using a binocular
goggle, nearly all analysis software will report VOR
data that have been combined or averaged from both
eyes. That being said, it is important to appreciate that
a single eye VOR recording may be more appropriate
if there are significant disconjugate eye movements or
a unilateral ocular pathology that prohibits binocular
recording, particularly if the analysis software does
not permit the ability to choose between a binocular
and monocular analysis. Keep in mind that this rule
applies only to VOR assessment and not during analy-
sis of ocular motor results where the identification of
disconjugate eye movements is always preferred and
can be critical to the differential diagnosis.
Use of Infrared Light. Video-oculography cameras
utilize infrared (IR) light to identify and capture eye
movements by identifying pupil contrast from the
lighter surrounding iris (Figure 13–19). The software’s
capability of identifying and “locking on” to the pupil
allows it to track the movement of each eye over time.
However, various problems can limit the software’s
eye-tracking algorithm. Specifically, problems with
ocular tracking are most commonly the result of eye
makeup. Mascara, in particular, is extremely sensi-
A
B
figure 13–19. Video-oculography (VOG) showing actual eye image (A) versus threshold image (B).
Threshold detection is adjusted to maximize detection of pupil contrast from surrounding iris.
13. ROTATIONAL VESTIBULAR ASSESSMENT 305
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tive to IR light and produces a reflective surface that
the IR light highlights (similar to the pupil contrast).
When this happens, the eye tracking software has difficulty making key distinctions between the pupil and
the eye makeup (Figure 13–20). Such ocular “splat-
ter” can cause significant problems with eye-tracking
algorithms “locking on” to the intended target (pupil),
which subsequently creates an extremely poor signalto-noise ratio and a very poor (noisy) recording. Consequently, the analysis of such noisy recordings is
figure 13–20. Video-oculography (VOG) images showing oversaturation of the pupil
threshold detection due to mascara. This example demonstrates a slight-to-mild impact,
which can often be corrected by adjusting the threshold detection parameters. However,
at times, such corrections are more difficult to resolve and can easily obscure pupil detection, which is depicted by the off-setting of the crosshairs (a) from the actual center of the
eyes (c). Such off-setting can significantly impact the reliability of your data acquisition.
Excessive threshold saturation most often occurs as a result of an inappropriately calibrated threshold detection setting or continuing to test with irreconcilable noise (such as
excessive mascara). As a result, consistent VOG capture is either lost (b) or collected as
noise, significantly impacting the signal-to-noise ratio.
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fraught with poor signal detection and either missing
or uninterpretable data. This is a similar and common
problem often encountered during videonystagmography (VNG) testing. Other deterrents to accurate (i.e.,
“clean”) eye tracking include eyeliner, ptosis, drooping
eyelids, narrow eye slits, significant coloboma, cataract
lens replacement surgery, and macular degeneration.
Miscellaneous Components of the Video Goggles.
There are certain components of the goggles that will
require regular attention and routine maintenance.
First, prior to making any ocular recordings (whether
ocular motor assessment or VOR testing), it is important to properly focus the IR camera image for each
eye. This will ensure accurate measurement of even the
most subtle eye movements (e.g., torsional). Because
each patient’s facial dimensions will vary, the focal
length between the eye and the camera will similarly vary from one patient to another. Therefore, it is
important to refocus each camera prior to testing each
patient. Furthermore, because IR light has a different
focal length than visible light, if you focus the cameras
in a well-lit room and then proceed to test the patient in
a lightproof enclosure, do not be surprised if the camera seems out of focus. This being said, it is often better
to focus each camera with minimal lighting, such as
with the booth’s red light or in a dimly lit room (note
that this is different from the adaptation of the CRP,
which requires 20 minutes).
Second, it is important to maintain care of the goggle’s face mask and headband, as maintaining a lightproof seal against the face is critical to testing, particularly
if you are not using a lightproof enclosure. Cleaning with
an alcohol-based solution should be avoided, as alcohol
can dry out the rubber mask and head strap (if made
from rubber) and cause premature cracking.
Finally, inspection of the IR reflective dichroic mirrors (lens) that are fitted directly in front of the eyes
should be performed prior to each patient test (see
Figure 13–20). It is important to look for any debris,
smudges or even cracks that may prevent the eyetracking algorithm from accurately following eye
movement. Use a soft lens cloth to wipe clean, and
avoid any harsh cleaner (e.g., window cleaner), as this
can damage the IR reflective coating.
TEST PREPARATION AND PATIENT SETUP
Now that we are familiar with the principal hardware
components of the chair, it is important to discuss
key elements that are critical to preparing the patient for
rotational testing. These include pre-test procedures such
as dizziness questionnaires, patient instructions, calibration, and a pre-rotational ocular motor assessment.
Pre-Appointment Fundamentals
When scheduling patients for a rotational assessment,
there are two things that are helpful to address prior
to patients arriving for their appointment. The first is
to give them any pre-test instructions. Pre-test instructions would include avoidance of any mascara that
could obscure ocular tracking or use of alcohol 48 to 72
hours prior to their appointment. Pre-test instructions
would also include avoidance of any medications that
could potentially alter the VOR response (usually suppression of the response) or produce any aberrant findings that would suggest a central pathology. A list of
drug classifications and the respective potential abnormality associated with each can be found in Table 13–1.
Termination of any medication should be done in conjunction with the patient’s primary medical provider,
as cessation of certain medications may not be possible
or medically advisable.
A second pre-appointment fundamental is to have
the patient fill out a dizziness questionnaire. There are
many questionnaires to choose from such as the Dizziness Handicap Inventory (Jacobson & Newman, 1990).
Questionnaire(s) can be sent to patients prior to their
appointment, which can facilitate the use of their time
during their actual vestibular assessment. Included
with the questionnaire could be a brief letter or brochure that briefly discusses the tests the patient will be
receiving, but being careful to do so in a nonthreatening way. It is sometimes helpful to include images of
the rotational chair, as it can be somewhat threatening
for some patients when viewing it for the first time.
Patient Setup
When the patient arrives, it is important to familiarize
the patient with the chair. Making them feel comfortable and safe are the priorities. This will help to reduce
any anxiety, which could deleteriously impact your
ocular data. When placing the patient in the rotational
chair, the safety restraints and harnesses should be fit
tightly against the patient without sacrificing comfort.
A firm contact between the goggles and the face is also
critical, as any slippage of the goggle during testing can
create spurious (often reduced) eye movement data.
In addition, the head restraint posts and head/chin
straps should be tightened firmly against the head to
table 13–1. Vestibular Assessment Abnormalities Associated with Various Drug Classifications
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Drug Classification Abnormality
Alcohol (ethanol)
Aminoglycoside antibiotics
Streptomycin
Gentamicin
Anticonvulsants
Dilantin (phenytoin)
Tegretol (carbamazepine)
Antidepressants
Tricyclic (Elavil, Pamelor)
Others (Prozac, etc.)
Lithium
Chemotherapeutic agents
Cisplatin
Diuretics
Lasix
Ethacrynic acid
Anitpsychotic
Haldol
Industrial solvents
Xylene, Toluene
Trichloroethylene, Styrene
Vestibular dysfunction: positional nystagmus; brainstem-cerebellar dysfunction
pattern
Vestibular dysfunction: chronic and irreversible vestibular hypofunction, can
cause total loss of labyrinthine response
Brainstem-cerebellar dysfunction: vertical or oblique up-beating nystagmus;
saccadic intrusions (i.e., cogwheeling) in pursuit pattern
Central sedation pattern; static positional nystagmus; internuclear
ophthalmoplegia; slow peak saccade velocity; opsoclonus; partial or total
gaze paresis. Brainstem-cerebellar dysfunction pattern
Vestibular dysfunction: chronic and irreversible vestibular hypofunction, can
cause total loss of labyrinthine response
Vestibular dysfunction: temporary labyrinthine hypofunction
Opsoclonus
Brainstem-cerebellar dysfunction pattern; vestibular dysfunction; central
positional nystagmus; exaggerated
VOR; impaired VOR fixation suppression
Marijuana Brainstem-cerebellar dysfunction pattern
Methadone Brainstem-cerebellar dysfunction pattern; saccadic undershoot
Opiates Brainstem-cerebellar dysfunction pattern; downbeat vertical nystagmus
Quinine Vestibular dysfunction: positional nystagmus
Salicilates (aspirin) Vestibular dysfunction: transient labyrinthine hypofunction
Stimulants
Amphetamine
Cocaine
Ritalin (methyphendiate)
Sedatives
Barbituates
Tobacco, nicotine Upbeat nystagmus; square-wave jerks
Tranquilizers
Benzodiazepines (Valium,
Ativan, Xanax)
Vestibular suppressants
Meclizine, Scopolamine,
Phenergan, Benadryl
Source: Adapted from and reprinted with permission from Cass and Furman (1993, Nov). Medications and their effects on vestibular function testing. In ENG Report (pp. 81–84). Chicago, ICS Medical.
Impaired gaze convergence
Shortened saccade reaction time
“
Rippled” saccade waves with decreased amplitude. Saccadic abnormalities
Brainstem-cerebellar dysfunction pattern
Central positional nystagmus
Central sedation pattern
Brainstem-cerebellar dysfunction pattern
Downbeat nystagmus, impaired optokinetic response
Central sedation pattern
Reduced peak velocity; optokinetic nystagmus;
Decreased slow component velocity (VOR) in caloric and rotational chair testing
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hold it securely against the headrest and prevent any
movement of the head in either the vertical or horizontal direction. This is the most critical element to securing your patient in the rotational chair, as the degree
of head movement will be inferred directly from the
degree of chair movement. Any difference between the
two will decrease the signal-to-noise ratio and reduce
the reliability and accuracy of your results, which can
often be subtle in nature.
Instructions to the patient should be straightforward and simple. Placing the two-way talk-back communication headphones on your patient will further
alleviate any patient anxiety and provide an excellent way to reduce any test anxiety, further instruct
the patient throughout the testing protocols, and provide mental tasking during required portions of the
test. Communication with your patient is imperative,
as good feedback is essential in recording quality eye
movements. Periodic reminders to maintain an “eyes
open” posture in the dark are absolutely critical to
obtaining quality recordings. There is a simple relationship here; good quality and clean recordings will
always produce the best possible analyses from your
data. Conversely, poor ocular recordings secondary
to noisy insufficient eye tracking, excessive blink artifact, and/or a complete loss of data due to drooping
(sleepy) eyelids cannot be recovered or improved even
with the best post-hoc analysis filters. Without a doubt,
obtaining the “cleanest” eye movement tracking with
video goggles is the most critical goal when performing any vestibular assessment. Simply put, analyzing
noise is no fun. Moreover, analyzing missing data is
impossible. The most important responsibility during the
assessment is to constantly maintain good eye tracking data
at all costs. This often means online monitoring of the
pupil tracking software to ensure good tracking of
the eyes rather than focusing one’s visual inspection on
the recorded eye movement data (Figure 13–21). Periodic adjustment of the IR threshold for pupil detection
may be required throughout testing, as ocular position
in the orbit, combating eye detection “jitter” between
pupil and makeup contrast, and even very lightly colored pupils can often cause transient problems with the
IR software “locking on” the pupil with good reliability. I have often found poor pupil tracking to be one
of the primary shortcomings when performing rotational tests; particularly when conducting rotational
testing for the first time. For various reasons, visual
attention by the clinician (online visual inspection of
the real-time data) is usually directed to the recorded
eye tracings (i.e., the ensuing VOR response data)
rather than the VOG data and “real-time” eye-tracking
data (Figure 13–21). This is similar to directing one’s
visual attention to the cumulative-averaged response
of an auditory brainstem response (ABR) rather than
inspecting or monitoring the online EEG data for noise.
The primary difference between ABR recordings and
VOG recordings, however, is that once the moment of
eye-tracking data has passed and a portion of the VOG
data is lost (due to a loss of eye tracking or eye closure),
there is no going back or extending the recording time
to collect more “averaged” eye-tracking data (i.e., similar to extending the ABR runtime to collect more ABR
sweep data). This is largely because rotational protocols have a predetermined time domain for data collection. There is no “extension” of the immediate test
rotation past the predetermined number of chair rotations if data are lost. The analysis is either performed
with missing data (never easy) or the specific rotation must be repeated, which can significantly extend
the overall test session time (not to mention potential
patient discomfort).
Calibration Test
Prior to performing any test that involves the presentation and recording of stimuli (i.e., eye movements in
our case), it is vital to validate the degree (i.e., size) of
eye movements before performing any vestibular test
protocols. For vestibular testing, this often means validating the size and direction of eye movements relative
to the size and direction of known target movements.
This validation is known as calibration and should
always be the first test performed before any test protocols are administered. Although it is understandable
that we all may not be experts on detailing the exact
calibration standards for vestibular testing, it is important that we all have a thorough appreciation for why
we calibrate and how an inaccurate calibration can
negatively impact our test results.
Calibration of the VOG response involves the
precise measurement of the change in pupil position
within a video image against a known target visual
stimulus. Specifically, calibration measures the change
in number of pixels the eye image travels as it moves
from one position on the video screen to another in
relation to a known change in stimulus (target) position. For example, if the eye were to move 200 pixels
in relation to a 30° visual stimulus, then the calibration
for eye movement would be 200 (pixels) divided by 30
(degrees) or 6.67 pixels for every degree of eye movement. Once calibration has been performed in both the
horizontal and vertical directions, the computer software uses this pixel per degree of eye movement to
calculate the degree of eye displacement (i.e., degree of
A
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B
figure 13–21. Examiner screen views offer one viewing image showing real-time data acquisition (A) similar to that
of an EEG response during acquisition of an auditory brainstem response. Horizontal and vertical data for each
eye are captured in real-time and traced in the graphs (a). Quality eye tracking is essential for the acquisition
of high value data and should constantly be monitored for any eye tracking problems including pupil detection
noise (e.g., excessive mascara), loss of eye tracking (disappearance of crosshairs), excessive blink artifact, eye lash
obstruction, ptosis, eye closure, and any significant gaze deviation from primary center. Data for each eye response
are then recorded directly from the acquisition screen and stored for analysis (B). Stimulus feedback is provided
for monitoring of chair velocity (b).
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nystagmus) in relation to any eye displacement tracked
by the video goggles during the assessment. In doing
so, the degree of eye movement can be determined for
any eye deflection (even torsional eye movements).
It is important that calibration be performed for
each patient, as small differences in facial features
can produce small to moderate changes in the depth
between the goggle lens and the pupil, which can create
large differences in the pixel per degree calculation. This
can create significant differences in how the algorithm
determines the degree of nystagmus from one patient
to another. Although the direction of movement will be
the same for every patient (positive sloping eye changes
for left-beating nystagmus and negative sloping eye
changes for right-beating nystagmus), the specific degree
of nystagmus will not be valid unless patient-specific calibration is performed. The same principle applies if the
video goggles are moved on the patient’s head midway
during an assessment. Such changes may be minimal,
but even small changes can produce surprisingly large
changes in the number of pixels per degree. The same is
true for using a default calibration, which can introduce
measurement errors of up to 25% from the actual value
(Miles & Zapala, 2015). Although the use of a default
calibration is unavoidable at times (e.g., pediatrics), it is
vital to realize that any eye movements recorded during a vestibular assessment (ocular motor movements,
VOR or vestibular nystagmus) and subsequently analyzed using a default calibration can be interpreted only
qualitatively; not quantitatively.
Ocular Motor Testing
Following calibration and prior to conducting any VOR
rotational assessment, it is vital to have each patient
perform a complete ocular motor assessment. Because
the VOR is measured through the ocular motor system,
it is critical to identify any potential deficits secondary
to central or ocular motor lesion(s). Such deficits will
invariably manifest themselves in any VOR recordings
during rotational assessments. Therefore, it is important to identify any ocular motor abnormalities prior to
conducting any rotational test in order to avoid inappropriately assigning an abnormal rotational test result
as a vestibular lesion (when in actuality, the abnormal
result is secondary to an ocular motor palsy/paresis, or
central visual disorder).
Rotational Test Protocols
When performing rotational testing, the primary test
most often performed is SHA testing. Sinusoidal accel-
eration testing provides critical data regarding the
responsiveness of the vestibular system across a much
broader frequency range than the much more common VOR test performed during videonystagmography (VNG) testing or than caloric testing (see Figure 13–1). In particular, SHA testing provides valuable
insight into the responsiveness of the vestibular system for frequencies that are closer to those stimulated
during daily life activities (see Figure 13–1). This is
essential when considering the role of rotational testing in a comprehensive vestibular assessment. Initially,
vestibular lesions often impact the effectiveness of
the vestibular system for detecting and transducing
low frequency accelerations. Because of this, a bilateral vestibular lesion will often yield absent caloric
responses even though residual (and even normal)
vestibular reactivity can objectively be measured for
higher frequency stimuli like those elicited during SHA
testing.
Velocity step testing (VST) is also often included
in a routine rotational assessment, as it complements
the SHA data and can also assist in the lateralization of peripheral vestibulopathies. VST is the oldest
of all rotational tests, initially introduced to the vestibular clinic by Robert Bárány (Chapter 1). It consists
of a quick acceleration of the chair to a sustained
constant velocity rotation before an equally quick
deceleration of the chair brings the chair back to a full
stop. Depending on the intensity of the acceleration
stimulus, the VST can either give unique insight into
the central functioning of the vestibular system or offer
valuable insight into lateralizing a peripheral vestibular lesion.
Collectively, SHA and VST are often performed
together. Jointly, their data can provide invaluable
diagnostic insight into peripheral and central vestibular function that no other test can equally deliver. However, SHA and VST are not without their shortcomings,
and each test has its own advantages and limitations.
The remaining additional tests that can be performed
(VOR suppression, visual-vestibular enhancement,
etc.) offer additional unique insights into vestibular
physiology and should be performed when clinical
needs indicate their use. Table 13–2 provides a quick
summary of the various rotational tests. The use and
clinical indications for these ancillary and specialized
tests often lies heavily in the judgment of the clinician.
In 2008, Jacobson and colleagues wrote that the “clinician’s end goal is to be ‘one-step ahead’ during the
assessment, and that the hypothesis generating process should continue as each bit of new information is
acquired through quantitative testing.” This is true on a
smaller scale during rotational assessment. The administration of certain tests like high frequency VOR sup-
13. ROTATIONAL VESTIBULAR ASSESSMENT 311
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Table 13–2. Rotational Test Protocols
Test Protocol Stimuli Parameters
Calibration 10° gaze right, left, up, and down.
Ocular Motor Routinely performed Saccades, smooth pursuit, gaze, optokinetic
Sinusoidal Harmonic
Acceleration (
Velocity (Trapezoidal)
Step (VST)
VOR Suppression Routinely performed 0.08, 0.16, 0.32, 0.64 Hz (no higher than 1.0 Hz)
Visual-Vestibular
Enhancement
Unilateral Centrifugation Perform when clinically indicated
Off-Vertical Axis Rotation
OVAR)
(
Note. *Minimum of 2 cycles performed for 0.01–0.02 Hz, 4 cycles for 0.04–0.08 Hz, and 8–10 cycles for 0.16–0.64 Hz. †Currently
limited by the FDA to IRB approved research only.
SHA)
Routinely performed 0.01, 0.02, 0.04, 0.08, 0.16, 0.32, 0.64 Hz*
Routinely performed Low (60°) Velocity Step Testing
Perform when clinically indicated
(central/cerebellar/migraine)
(otolith)
Perform when clinically indicated
(otolith)
†
pression testing can be (and should be) supplemented
Smooth pursuit between gaze positions or
saccades.
pursuit, spontaneous testing
(higher frequencies optional)
High (240°)
0.08, 0.16, 0.32, 0.64 Hz (no higher than 1.0 Hz)
SVV or SVH during static, On-Center Rotation,
UCF-R, and UCF-L eccentric rotations;
generally high velocity 300°/sec.
VOR Gain, VOR Phase, VOR Symmetry
Velocity Step Testing
SINUSOIDAL HARMONIC ACCELERATION
to the “standard” rotational battery of SHA and VST
when results indicate the need for further exploration
of an abnormal finding. Table 13–2 details the clinical
test protocols that should be performed during a standard or routine rotational assessment, as well as the
typical stimuli that should be conducted for each test.
In addition, various specialized rotational test protocols are listed, which should be performed (or recommended) when clinically indicated.
The principal test protocol that is essential to the
most basic, or routine, clinical rotational vestibular
assessment is sinusoidal harmonic acceleration (SHA)
testing. Additional rotational tests are available but
may not be routinely performed during a standard
clinical assessment. Such additional tests include VST,
VOR fixation suppression testing, visual-vestibular
enhancement testing, subjective visual vertical and
subjective visual horizontal performed during unilateral centrifugation (UCF), and off-vertical axis rotation (OVAR) (see Table 13–2). This chapter will focus
on the principal assessment protocol of SHA testing.
In Appendix IV of this text, secondary assessment
protocols are discussed, specifically VST, VOR suppression testing, and dynamic subjective visual vertical (SVV) assessment during unilateral centrifugation
testing.
When performing rotational testing, the primary test
most often performed is SHA testing. Sinusoidal
acceleration testing provides critical data regarding
the responsiveness of the vestibular system across a
broad frequency range (see Figure 13–1). This is essential when considering the role of the rotational testing
in a comprehensive vestibular assessment, insomuch
that vestibular lesions often impact the effectiveness
of the vestibular system for detecting and transducing
low-frequency movements (<0.1 Hz). Because of this, a
bilateral vestibular lesion will often yield absent caloric
responses (0.003 Hz), even though residual and even
normal vestibular reactivity can objectively be measured for higher frequency stimuli (>0.1 Hz) like those
conducted during SHA testing.
During SHA testing, a vertically upright individual is rotated (or oscillated) back and forth in the
horizontal (or yaw) plane, while the responsiveness of
the vestibular system to such rotations (i.e., the VOR)
is determined across a known frequency range of stimuli. Rotational stimuli are most commonly presented as
sinusoids, with the chair first accelerating in one direction until a peak target velocity is achieved (usually
50°/sec or more commonly 60°/sec), after which the
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chair slows and reverses to the same peak velocity in
the opposite direction. This periodic motion, whereby
the chair (head) oscillates about a centric position, is
known as sinusoidal acceleration testing. Provided the
rotational frequencies being administered are simple
harmonics of one another, this form of stimulation is
known as SHA testing (Stockwell & Bojrab, 1997). The
frequencies most commonly administered are harmonic or octave frequencies between 0.01 and 0.64 Hz,
specifically, 0.01, 0.02, 0.04, 0.08, 0.16, 0.32, and 0.64 Hz.
Each sinusoidal frequency has discrete stimulus characteristics with respect to cycle duration, chair acceleration, angular displacement, and chair revolutions.
These data are detailed in Table 13–3, as well as inclusion of the acceleration predicted by the caloric stimulus for comparison.
NORMAL PHYSIOLOGICAL
RESPONSE DURING SHA TESTING
In response to chair rotations, specifically chair acceleration and deceleration, an intact vestibular system
will generate a slow compensatory eye movement
(i.e., the VOR) in the opposite direction to that of chair
rotation. Due to the constant varying acceleration and
deceleration stimuli during SHA testing, the cupulae
are subjected to omnipresent (acceleration and deceler-
ation) deflection forces that are commensurate with the
target velocity stimulus. This is eloquently described
by the pendulum model of cupular deflection, which
offers the reasoning for the alternating nystagmus provoked during SHA testing. According to the pendulum
model, the degree of cupular deflection is dependent
upon the frequency of angular rotation (acceleration
force), and the resulting VOR nystagmus will similarly
crescendo and decrescendo in relation to the acceleration and deceleration of the angular stimulus.
It is the constant–varying cupular deflection that
creates the ever-changing excitatory and inhibitory
peripheral afferent response that is integrated by the
central vestibular nuclei, cerebellum, and various brainstem structures. Central efferent signals are sent to the
appropriate efferent ocular motor neurons, resulting in
the compensatory slow-phase vestibular nystagmus
that is generated in the opposite direction of chair rotation in accordance with the vestibular-ocular reflex.
The vestibular slow phase occurs until the eye
reaches an eccentric position within the orbit and
is then followed by a fast resetting of the eyes back
to the primary position (fast phase), which is initiated by the brainstem. This pattern continues as long
as the chair is accelerating or decelerating in a single
direction. The resultant nystagmus (named for the
fast phase) is therefore always in the same direction of
chair rotation; right-beating with rightward rotation
and left-beating with leftward rotation (Figure 13–22).
Table 13–3. Relative Rotational Properties by Frequency of Rotation
Cycle
Stimulus
(Hz)
0.004 250 ~ 1.2°/sec
0.01 100 60 2.4°/sec
0.02 50 60 4.8°/sec
0.04 25 60 9.6°/sec
0.08 12.5 60 19.2°/sec
0.16 6.25 60 38.4°/sec
0.32 3.125 60 76.8°/sec
0.64 1.5625 60 153.6°/sec
1.28 0.78125 60 307.2°/sec
2.0 0.5 60 480°/sec
Duration
(s)
Velocity
(ω
)
Acceleration
(a)
2
2
2
2
2
2
2
2
2
2
Angular
Displacement
(½ cycle)
Caloric
Stimulus
3000°
1500°
750°
375°
187.5°
93.75°
46.875°
23.4375°
11.71875°
Cycles of
Oscillation
(# of rotations)
1
8
⁄3
1
4
⁄6
1
2
⁄12
1
1
⁄24
1
~
⁄2
1
~
⁄4
1
~
⁄8
1
~
⁄16
1
~
⁄32
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figure 13–22. Relationship between direction of chair rotation versus slow and fast phase of nystagmus. Raw tracing of nystagmus (top plot
chair rotation, respectively.
As the chair decelerates back to a velocity of 0° per
second (full stop), it immediately reverses direction
and begins accelerating and then decelerating in the
opposite direction. The nystagmus switches directions
in response to the change in the direction of chair rotation. This process is repeated until the predetermined
number of oscillations has been completed for each
respective frequency. The observed nystagmus is captured and “traced” or “charted” in real time through
a computer software program. A series of rightward
and leftward chair oscillations and the respective rightbeating and left-beating nystagmus can be seen in Figure 13–22. Notice how the nystagmus increases in its
slope as the rotation of the chair increases to its peak
target velocity, at which time the slope of the nystagmus begins to diminish as the chair decelerates back to
a velocity of 0°/sec.
From the resulting nystagmus, software programs
delete the fast-phase components of the VOR response,
leaving only the slow-phase vestibular components
) shows right-beating and left-beating nystagmus in relation to rightward and leftward
remaining (Figure 13–23). The reason for this is simple
but should, nevertheless, be stated. It is the vestibular response that is of interest during rotational testing
and therefore only the slow phase that is of primary
concern. After the slow phases are parsed from the
response, the degree of each slow phase is measured
and plotted in relation to chair oscillation. Because the
plot of slow-phase velocity of vestibular nystagmus is
in the opposite direction to chair rotation and the degree
of nystagmus increases and decreases with the acceleration and deceleration of the chair rotation, the
resultant nystagmus velocity and chair velocity data
always appear as opposing (or mirror) sinusoids
— that
is, 180 degrees out of phase from one another (Figure 13–24). The peak velocity of the slow-phase component of vestibular nystagmus will generally increase
and peak near the same time the chair reaches peak
velocity (point of maximum acceleration prior to beginning deceleration; see Figure 13–24). Once the degree
of slow-phase nystagmus has been plotted, various
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