U.S. patent application number 09/906537 was filed with the patent office on 2002-02-28 for galvanic vestibular stimulation system and method.
Invention is credited to Collins, James J., Inglis, J. Timothy.
Application Number | 20020026219 09/906537 |
Document ID | / |
Family ID | 22276006 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020026219 |
Kind Code |
A1 |
Collins, James J. ; et
al. |
February 28, 2002 |
Galvanic vestibular stimulation system and method
Abstract
A system and method of altering the output of a vestibular
system including providing a source of time-varying galvanic
current, transcutaneously delivering time-varying galvanic current
to vestibular afferents associated with the vestibular system in
order to modulate firing level of the vestibular afferents, and
inducing a coherent time-varying sway response that counteracts
postural sway. In an alternative embodiment there is provided a
galvanic vestibular stimulation system including a source which
transcutaneously delivers time-varying galvanic current to
vestibular afferents in order to modulate the firing level of the
vestibular afferents, a monitor which monitors postural sway
thereby providing indication of necessary galvanic current to be
delivered. The system induces a coherent time-varying sway response
that counteracts the monitored postural sway.
Inventors: |
Collins, James J.; (Newton
Centre, MA) ; Inglis, J. Timothy; (Vancouver,
CA) |
Correspondence
Address: |
Samuels, Gauthier & Stevens, LLP
Suite 3300
225 Franklin Street
Boston
MA
02110
US
|
Family ID: |
22276006 |
Appl. No.: |
09/906537 |
Filed: |
July 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09906537 |
Jul 16, 2001 |
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09392186 |
Sep 9, 1999 |
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6219578 |
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60099651 |
Sep 9, 1998 |
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Current U.S.
Class: |
607/2 |
Current CPC
Class: |
A61N 1/3655
20130101 |
Class at
Publication: |
607/2 |
International
Class: |
A61N 001/20 |
Goverment Interests
[0002] This invention was made with government support under
contract no. DC03484-01 awarded by the National Institutes of
Health. The government has certain rights in the invention.
Claims
What is claimed is:
1. A method of altering the output of a vestibular system
comprising: providing a source of time-varying galvanic current;
transcutaneously delivering time-varying galvanic current to
vestibular afferents associated with said vestibular system in
order to modulate firing level of said vestibular afferents; and
inducing a coherent time-varying sway response that counteracts
postural sway.
2. The method of claim 1, wherein said firing level is increased in
response to delivering cathodal galvanic currents.
3. The method of claim 1, wherein said firing level is decreased in
response to delivering anodal galvanic currents.
4. The method of claim 1, wherein said postural sway is
self-generated.
5. The method of claim 1, wherein said postural sway is
externally-generated.
6. The method of claim 1 further comprising monitoring postural
sway in order to determine the characteristics of the galvanic
current to be delivered.
7. The method of claim 1, wherein said coherent time-varying sway
reduces said postural sway.
8. The method of claim 1, wherein said coherent time-varying sway
eliminates said postural sway.
9. A galvanic vestibular stimulation system comprising: a source of
time-varying galvanic current; and a delivery module which
transcutaneously delivers time-varying galvanic current to
vestibular afferents associated with said vestibular system in
order to modulate firing level of said vestibular afferents; and
means for inducing a coherent time-varying sway response that
counteracts postural sway.
10. The system of claim 9, wherein said firing level is increased
in response to delivering cathodal galvanic currents.
11. The system of claim 9, wherein said firing level is decreased
in response to delivering anodal galvanic currents.
12. The system of claim 9, wherein said postural sway is
self-generated.
13. The system of claim 9, wherein said postural sway is
externally-generated.
14. The system of claim 9 further comprising a monitor that
monitors postural sway in order to determine the characteristics of
the galvanic current to be delivered.
15. The system of claim 9, wherein said coherent time-varying sway
reduces said postural sway.
16. The system of claim 9, wherein said coherent time-varying sway
eliminates said postural sway.
17. A galvanic vestibular stimulation system comprising: a
time-varying galvanic current source which transcutaneously
delivers time-varying galvanic current to vestibular afferents in
order to modulate the firing level of said vestibular afferents: a
monitor which monitors postural sway thereby providing indication
of necessary galvanic current to be delivered; and means for
inducing a coherent time-varying sway response that counteracts the
monitored postural sway.
18. A method of altering the output of a vestibular system
comprising: providing a source of time-varying anodal binaural
galvanic current; and transcutaneously delivering time-varying
anodal binaural galvanic current to vestibular afferents associated
with said vestibular system in order to reduce firing level of said
vestibular afferents, thus reducing or eliminating the
functionality of said vestibular afferents.
19. A method of altering the output of a vestibular system
comprising: providing a source of time-varying cathodal binaural
galvanic current; and transcutaneously delivering time-varying
cathodal binaural galvanic current to vestibular afferents
associated with said vestibular system in order to increase firing
level of said vestibular afferents, thus enhancing the
functionality of said vestibular afferents.
Description
PRIORITY INFORMATION
[0001] This application claims priority from provisional
application Ser. No. 60/099,651 filed Sep. 9, 1998.
BACKGROUND OF THE INVENTION
[0003] The invention relates to the field of vestibular
stimulation, and in particular to a galvanic vestibular stimulation
system and method.
[0004] Galvanic vestibular stimulation has proven to be a valuable
technique for studying the role played by vestibular information in
the control of stance and balance. With this technique,
small-amplitude galvanic current is delivered transcutaneously to
the vestibular afferents that lie directly below the mastoid bones.
This serves to modulate the continuous firing level of the
peripheral vestibular afferents. Specifically, cathodal (negative)
currents increase the firing rate of vestibular afferents, whereas
anodal (positive) currents decrease the firing rate of vestibular
afferents. Thus, constant bipolar galvanic current produces a tonic
vestibular asymmetry. This effect causes a standing subject to lean
in different directions depending on the polarity of the current
and the direction of the subject's head. In general, a subject will
tend to lean toward the anodal stimulus (in the direction of the
vestibular apparatus with reduced afferent activity levels) and/or
away from the cathodal stimulus (away from the vestibular apparatus
with increased afferent activity levels).
[0005] A considerable number of studies have examined the body-sway
response to constant galvanic stimulation of the vestibular system.
One study, for instance, used monopolar monaural constant galvanic
stimulation and demonstrated that the amplitude of the body-sway
response increases linearly with increasing stimulus current (from
0.2 mA to 1.0 mA). Another study used bipolar binaural constant
galvanic stimulation and showed that the direction of the evoked
sway is approximately in the direction of the intermastoid line.
Thus, with bipolar binaural constant galvanic stimulation, lateral
sway is produced if a subject's head is facing forward, whereas
anteroposterior sway is produced if a subject's head is turned to
the left or right (over the left or right shoulder).
[0006] A limited number of studies have shown that the application
of sinusoidally varying bipolar galvanic currents to the vestibular
system can lead to sinusoidally-varying postural sway. With
sinusoidal galvanic stimulation, as with constant galvanic
stimulation, the body tends to sway towards the positive stimulus
and away from the negative stimulus. For low-frequency stimulation,
the frequency of the evoked body sway matches the frequency of the
stimulus, whereas the amplitude of the evoked body sway varies from
subject to subject.
SUMMARY OF THE INVENTION
[0007] The invention provides a methodology and system for altering
the output of the human vestibular system in a controlled and
systematic manner. The invention is based on galvanic vestibular
stimulation. With galvanic vestibular stimulation, galvanic current
is delivered transcutaneously to the vestibular afferents that lie
directly below the mastoid bones. This serves to modulate the
continuous firing level of the peripheral vestibular afferents, and
causes a standing subject to lean in different directions depending
on the polarity of the current and the direction of the subject's
head.
[0008] The invention utilizes time-varying galvanic vestibular
stimulation as the basis for an artificial vestibular control
system to reduce or eliminate certain types of pathological
postural sway. Such a system can include sensors, e.g., lightweight
accelerometers, for monitoring an individual's postural sway, and a
galvanic-stimulation control system. In such an arrangement, the
sensor output would be used as input to the galvanic-stimulation
control system.
[0009] A methodology and system of this sort can be used to improve
balance control in elderly individuals, who are often predisposed
to falls. In addition, patients with vestibular paresis, who have
lost some of their hair cells and therefore have a decreased
response from the vestibular system during head movement, could
also benefit from such a methodology and system. The hair cells,
which are responsible for indicating head tilt and acceleration,
transmit their information to the vestibular nuclei via the
8.sup.th nerve. Galvanic vestibular stimulation acts directly on
the 8.sup.th nerve and thus the stimulation technique can be
implemented as a vestibular prosthesis to operate in place of the
lost hair cells. The invention also utilizes galvanic vestibular
stimulation to eliminate or enhance the function of the vestibular
system. The former application of the invention can be of use to
astronauts, pilots, and sailors. The latter application of the
invention can be of use to individuals requiring heightened balance
function.
[0010] With galvanic vestibular stimulation, small-amplitude
galvanic current is delivered transcutaneously to the vestibular
afferents that lie directly below the mastoid bones. This serves to
modulate the continuous firing level of the peripheral vestibular
afferents. Specifically, cathodal (negative) currents increase the
firing rate of vestibular afferents, whereas anodal (positive)
currents decrease the firing rate of vestibular afferents. Thus,
constant bipolar galvanic current produces a tonic vestibular
asymmetry. This effect causes a standing subject to lean in
different directions depending on the polarity of the current and
the direction of the subject's head. In general, a subject will
tend to lean toward the anodal stimulus (in the direction of the
vestibular apparatus with reduced afferent activity levels), and/or
away from the cathodal stimulus (away from the vestibular apparatus
with increased afferent activity levels).
[0011] With the invention, time-varying binaural galvanic
vestibular stimulation is used to produce coherent time-varying
postural sway. The galvanic stimulation can be monopolar or
bipolar. This application of the invention is based on the
aforementioned motor control effects of galvanic vestibular
stimulation. With the invention, time-varying galvanic vestibular
stimulation is used as the basis for an artificial vestibular
control system to reduce or eliminate certain types of pathological
postural sway. Such a system would consist of sensors, e.g.,
lightweight accelerometers, for monitoring an individual's postural
sway, and a galvanic-stimulation control system. In such an
arrangement, the sensor output would be used as input to the
galvanic-stimulation control system.
[0012] In addition, with the invention, time-varying monopolar
(anodal) binaural galvanic vestibular stimulation is used to
eliminate or reduce the function of the vestibular system. This
application of the invention is based on the finding that anodal
(positive) currents decrease the firing rate of vestibular
afferents. Similarly, with the invention, time-varying monopolar
(cathodal) binaural galvanic vestibular stimulation is used to
heighten or enhance the function of the vestibular system. This
application of the invention is based on the finding that cathodal
(negative) currents increase the firing rate of vestibular
afferents.
[0013] For each of these applications, at least two surface
electrodes are placed on the mastoid bones of each subject, one
behind each ear, in order to apply the galvanic vestibular
stimulation. The appropriate stimulation signals are generated on a
microprocessor, e.g., a computer chip, and transmitted to the
electrodes via a D/A system and isolation unit.
[0014] One advantage of the invention is that it utilizes and
exploits the features of existing sensory neurons via non-invasive
means. In particular, it uses galvanic stimulation signals to alter
the firing behavior of peripheral vestibular afferents. In this
manner, the invention can modify the dynamics of the human postural
control system.
[0015] The invention as described includes the utilization of
galvanic vestibular stimulation. A possible future modification of
this methodology would be to utilize other forms of stimulation,
such as mechanical vibration, to alter the output of the human
vestibular system.
[0016] The invention provides a non-invasive means for altering the
output of the human vestibular system in a controlled and
systematic manner. Accordingly, the invention can be used to alter
an individual's postural sway in a controlled and systematic
manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic diagram of an exemplary embodiment of
a galvanic vestibular stimulation system in accordance with the
invention;
[0018] FIG. 2 is a graph providing plots of the coherency between
the 0-2 Hz stochastic vestibular stimulation signal and the
resulting mediolateral COP time series for a single 60 s trial from
one subject;
[0019] FIG. 3A is a graph showing a plot of the 0-2 Hz stochastic
vestibular stimulation signal and the resulting mediolateral COP
time series for a single 60 s trial from one subject; FIG. 3B is a
graph showing a plot of the coherency between the vestibular
stimulation signal and the COP time series in FIG. 3A;
[0020] FIGS. 4A-4C are graphs showing plots of the coherency
between the stochastic vestibular stimulation signal and the
resulting mediolateral COP time series for each trial from the
subject of FIGS. 3A and 3B;
[0021] FIGS. 5A-5C are graphs showing the average coherency values
between the respective vestibular stimulation signals and the
resulting mediolateral COP time series for the significant coherent
trials;
[0022] FIG. 6A is a graph showing a plot of the 0-2 Hz stochastic
vestibular stimulation signal and the resulting anteroposterior COP
time series for a single 60 s trial from one subject; FIG. 6B is a
graph showing a plot of the coherency between the vestibular
stimulation signal and the COP time series in FIG. 6A;
[0023] FIGS. 7A-7C are graphs with plots of the coherency between
the stochastic vestibular stimulation signal and the resulting
anteroposterior COP time series for each trial from the subject of
FIGS. 3A and 3B; and
[0024] FIGS. 8A-8C are graphs showing the average coherency values
between the respective vestibular stimulation signals and the
resulting anteroposterior COP time series for the different trials
from each of the nine subjects.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Galvanic vestibular stimulation serves to modulate the
continuous firing level of the peripheral vestibular afferents. It
has been shown that the application of sinusoidally varying bipolar
galvanic currents to the vestibular system can lead to sinusoidally
varying postural sway. The invention results from testing the
hypothesis that stochastic galvanic vestibular stimulation can lead
to coherent stochastic postural sway.
[0026] In accordance with the invention, nine healthy young
subjects (6 females and 3 males, aged 18-30 years; height:
1.63-1.91 m, mean 1.71 m; body weight: 43.1-86.2 kg, mean 62.8 kg)
were included in a study. The subjects had no evidence or history
of a neurological, gait, postural, or skeletal disorder.
[0027] FIG. 1 is a schematic diagram of an exemplary embodiment of
a galvanic vestibular stimulation system 100 in accordance with the
invention. Postural sway was evaluated by using a Kistler 9287
multicomponent force platform 102 to measure the displacements of
the COP under a subject's feet. Each subject was instructed to
stand upright on the platform in a standardized stance. The
subject's feet were separated mediolaterally by a distance of 1-2
cm. During the testing, the subjects stood barefoot with their arms
crossed in front and their head facing forward. Subjects were
required to close their eyes and wear headphones to block out
visual and auditory cues, respectively. Subjects were instructed to
relax during the tests and to allow their body to react to the
vestibular stimulus.
[0028] Two flexible, carbon-rubber, surface electrodes 104 were
placed on the mastoid bones of each subject, one behind each ear,
in order to apply the galvanic vestibular stimulation. A conductive
adhesive gel was used to ensure proper conduction between the skin
and the electrodes and to keep the electrodes in place. The
electrodes were approximately 9 cm.sup.2 in area and kidney-shaped
to fit comfortably behind the ears. Stochastic current stimuli were
applied binaurally and bipolarly to each subject. The anodal
electrode was positioned behind the right ear of each subject, and
the cathodal electrode was positioned behind the left ear.
[0029] The stochastic stimulus was formed digitally on a computer
106. The stimulus was transmitted via a D/A board to an isolation
unit 108 (BAK Electronics, Model BSI-1), which was connected to the
electrodes via a current-limiting device 110. The feedback from the
platform was fed to the computer via a filter 112.
[0030] The stimulus amplitude for individual subjects was
determined using the following protocol. Each subject was
galvanically stimulated using a sine wave (1-2 Hz) and the
amplitude of the stimulus was gradually increased until: (1) the
subject felt a mild but not uncomfortable tingling on their skin
under the stimulating electrodes, (2) the subject reported a mild
sensation of disorientation, and (3) periodic sway at the input
frequency was observable. The subject's stimulation level (range
0.4 mA to 1.5 mA, peak-to-peak) was then used as the maximum
amplitude limit during the stimulation trials for that subject.
[0031] The stimulus x(t) used for galvanic vestibular stimulation
was a realization of a stochastic process, given by the first-order
autoregressive difference equation
x(t)=.alpha.x(t-1)+.epsilon.(t),
.epsilon.(t).apprxeq.N(0,.sigma..sup.2) (1)
[0032] From a physical standpoint, this process describes a
relaxator that is driven by white noise .epsilon.(t), with variance
.sigma..sup.2. The relaxation time .tau. of the process can be
written in terms of the parameter .alpha. as .tau.=-1/log(.linevert
split..alpha..linevert split.). In the exemplary study, .tau.=100
was used.
[0033] The spectrum of this process is continuous (it contains all
frequencies) and its power is distributed such that it is inversely
related to frequency. The second-order spectral properties of this
process are thus similar to those of quiet-standing COP data, the
power spectra of which decrease with increasing frequencies. This
process is, therefore, a more natural choice for a stochastic
posture stimulus than a white noise signal, whose power is
distributed equally over all frequencies.
[0034] The autoregressive process was filtered, using a simple
Fourier filter, to create three stimulation signals, each with a
different frequency content: 0-1 Hz, 1-2 Hz, and 0-2 Hz. Each of
the signals contained a part of the continuous spectrum, e.g., the
0-1 Hz stimulus contained the entire frequency band from 0-1 Hz,
with the shape of the aforementioned autoregressive process. Each
of the three stimulation signals (duration: 60 sec) was used in
five different trials. Each trial was 60 sec in duration and
subjects were galvanically stimulated throughout each trial. In
addition to the stimulation trials, five 60 sec quiet-standing
trials, without galvanic stimulation, were conducted on each
subject. Thus, in total, 20 trials were conducted on each subject;
15 stimulation trials and five control (no stimulation) trials. The
presentation order of the stimulation and control trials was
randomized. The displacements of the COP during each trial were
measured with the force platform. To prevent anti-aliasing effects,
the COP data were low-pass filtered with filter 112 at 30 Hz during
data acquisition. All data were sampled at 100 Hz and stored on the
computer for off-line analysis.
[0035] The cross-spectrum CS(.omega.), where .omega. is frequency,
of two stationary, zero-mean time series x(t) and y(t) is defined
as the Fourier transform (FT) of the cross-correlation function
CCF(t')=<x(t)y(t-t')&- gt;, where <.cndot.> denotes
expectation. The coherency spectrum Coh(.omega.) is defined as the
modulus of the normalized cross-spectrum CS(.omega.) 1 Coh ( ) = CS
( ) S x ( ) S y ( ) ( 2 )
[0036] where S.sub.x(.omega.) and S.sub.y(.omega.) denote the power
spectra of x(t) and y(t), respectively, the FT of the respective
autocorrelations. The coherency can be interpreted as a measure of
linear predictability; it equals one whenever x(t) is a linear
function of y(t).
[0037] The estimation of the power and cross spectra is achieved by
a direct spectral estimation, based on the discrete FT of the
recorded data. The periodogram, which is the squared modulus of the
discrete FT, is smoothed by a window function W.sub.j to obtain a
consistent estimator of the spectra. The simplest form of such a
procedure is a sliding average. A triangular window was chosen (the
so-called Bartlett estimator) to calculate the spectra because its
statistical properties are superior to those of a sliding average.
The coherency is then estimated by replacing the spectra in Eq. 2
with their respective estimated quantities.
[0038] For each trial, the coherency between the stochastic
vestibular stimulation signal x(t) and the resulting COP time
series (mediolateral and anteroposterior, respectively) was
investigated. It is possible, however, that estimation bias due to
misalignment results in an underestimation of coherency. To control
for this effect, all time series, i.e., x(t) and the resulting COP
time series, were realigned using an iterative procedure. In short,
all calculations were performed using x(t-d) instead of x(t), since
it is expected that the COP time series lags x(t) by a certain
delay d. The delay d was estimated using the phase spectra
.PHI.(.omega.) defined by the relationship
CS(.omega.)=.linevert split.CS(.omega.).linevert split. exp
(i.PHI.(.omega.)) (3)
[0039] FIG. 2 is a graph providing plots of the coherency between
the 0-2 Hz stochastic vestibular stimulation signal and the
resulting mediolateral COP time series for a single 60 sec trial
from one subject. The results are shown for the two time series
without and with realignment. The dashed line indicates the level
of significance, s, for .alpha.=0.95. It can be seen that
realignment resulted in a significant increase in the amount of
coherency found between the two time series.
[0040] To test each output trial for linear independence from the
input stimulus, the power spectra and cross-spectra were estimated
by using a direct spectral estimator. The critical value s for the
null hypothesis of zero coherency for a given significance level
.alpha. is 2 s = 1 - 2 v - 2 , ( 4 )
[0041] where .nu. is the so-called equivalent number of degrees of
freedom, which depends on the direct spectral estimator, i.e., on
W.sub.j and the tapering used.
[0042] To determine whether two series are uncorrelated, it is not
sufficient to consider simply the value s. The reason is that the
derivation of the underlying statistics that lead to a test based
on Eq. 4 assumes that the cross spectrum is approximately constant
over the width of the window function W.sub.j used in the direct
spectral estimation. Asymptotically, this assumption is always true
given the required properties of a valid smoothing window function
W.sub.j. If, however, a cross spectrum of a finite series exhibits
a high curvature, then the confidence interval is no longer valid.
To overcome this problem, investigators commonly use a technique
known as "prewhitening", in which one (or two) of the series is
linearly filtered so that the cross spectrum of the resulting,
filtered series is flat. This can be done because a linear filter
applied to one or both of the signals does not modify the
coherency. In the exemplary studies, the stochastic vestibular
stimulation signal was prewhitened before the coherency was
calculated. Since the parameter .alpha., was known in Eq. 1, x(t)
is able to be prewhitened simply by inverting the filter of Eq.
1.
[0043] In addition to the above tests, an average coherency between
the respective vestibular stimulation signals and the significantly
dependent COP time series was also determined for each subject. The
average was taken for all values within the broadest contiguous
frequency band of significant coherency. If the contiguous
frequency band showing significant coherency was smaller than 0.5
Hz (which was the width of the spectral estimator W.sub.j), then
the bandwidth of the stochastic stimulation signal (0-1 Hz, 1-2 Hz,
or 0-2 Hz) was taken by default.
[0044] The 0-2 Hz stochastic vestibular stimulus and the resulting
mediolateral COP time series for a single 60 s trial from one
subject are shown in the graph of FIG. 3A. The figure demonstrates
the difficulty in determining by visual inspection whether there is
a relationship between the two time series. The coherency plot for
the two series in FIG. 3A is shown in the graph of FIG. 3B. The
dashed line indicates the level of significance, s, for
.alpha.=0.95 (see Eq. 4). It can be seen that there is significant
coherency between the vestibular stimulus and the mediolateral COP
time series at frequencies less than 2.0 Hz, i.e., at frequencies
less than the upper limit of the filtered input stimulus.
[0045] FIGS. 4A-4C provide the coherency results for the three
different stochastic vestibular stimulation signals, i.e., signals
that were bandlimited between 0-1 Hz (FIG. 4A), 1-2 Hz (FIG. 4B),
and 0-2 Hz (FIG. 4C), for the subject in FIGS. 3A-3B. It can be
seen that for each single trial, there is significant coherency
between the vestibular stimulus and the mediolateral COP time
series at frequencies less than the upper limit of the filtered
input stimulus. In addition, it can be seen that the coherency
results for each stimulation signal were highly reproducible from
trial to trial (FIGS. 4A-4C), i.e., the coherency plots for the
five trials for a given stimulus have similar shapes. The dashed
line indicates the level of significance, s, for .alpha.=0.95 (see
Eq. 4).
[0046] As expected, the position of the maximum coherency varied
with the frequency band of the different stimulation signals and
was observed within the respective frequency band. These general
results were found in eight of the nine subjects tested. In
particular, significant coherency between the stochastic vestibular
stimulation signal and the resulting mediolateral COP time series
was found in 12-15 trials (out of a possible 15) for each of these
subjects. The ninth subject only exhibited significant coherency in
six trials; this reduced level of coherency might have occurred
because the subject did not appear to relax during the testing, as
instructed.
[0047] FIG. 5 is a graph showing the average coherency values
between the respective vestibular stimulation signals and the
resulting mediolateral COP time series for the significant coherent
trials from each of the nine subjects. Shown are the results for
the (a) 0-1 Hz (FIG. 5A), (b) 1-2 Hz (FIG. 5B), and (c) 0-2 Hz
(FIG. 5C) vestibular stimulation signals. The number of points
plotted for each subject corresponds to the number of significant
coherent trials for that subject. Note that the values plotted in
FIGS. 5A-5C are slightly lower than the peak values, e.g., see
FIGS. 4A-4C, since they correspond to an average over a frequency
band. It should also be noted that for each subject the average
coherency for a given stimulation signal was consistent from trial
to trial. Moreover, in general, the highest degree of coherency was
found for the 1-2 Hz stochastic vestibular stimulation signal.
[0048] The 0-2 Hz stochastic vestibular stimulus and the resulting
anteroposterior COP time series for a single 60 s trial from one
subject are shown in the graph of FIG. 6A. The corresponding
coherency plot for that trial is shown in the graph of FIG. 6B. The
dashed line indicates the level of significance, s, for
.alpha.=0.95 (see Eq. 4). It can be seen that there is no
significant coherency between the vestibular stimulus and the
anteroposterior COP time series.
[0049] Similar results were obtained for all subjects. FIGS. 7A-7C
are graphs with plots of the coherency between the stochastic
vestibular stimulation signal and the resulting anteroposterior COP
time series for each trial from the subject of FIGS. 3A-3B. The
graphs show the results for the (a) 0-1 Hz (FIG. 7A), (b) 1-2 Hz
(FIG. 7B), and 0-2 Hz (FIG. 7C) vestibular stimulation signals.
Five trials were conducted for each stimulation signal. The dashed
line indicates the level of significance, s, for .alpha.=0.95 (see
Eq. 4).
[0050] FIGS. 8A-8C are graphs showing the average coherency values
between the respective vestibular stimulation signals and the
resulting anteroposterior COP time series for the different trials
from each of the nine subjects. The graphs show the results for the
(a) 0-1 Hz (FIG. 8A), (b) 1-2 Hz (FIG. 8B), and 0-2 Hz (FIG. 8C)
vestibular stimulation signals. The mean values and standard
deviations of the average coherency for the control (no
stimulation) trials are also given in each plot.
[0051] In accordance with the invention, it has been demonstrated
that in subjects who are facing forward, bipolar binaural
stochastic galvanic stimulation of the vestibular system leads to
coherent stochastic mediolateral postural sway. Specifically,
significant coherency between the stochastic vestibular stimulation
signal and the resulting mediolateral COP time series has been
found in the majority of trials in 8 of the 9 subjects tested. The
coherency values obtained were up to 0.8 for several trials.
[0052] It was also found that in subjects who are facing forward,
bipolar binaural stochastic galvanic stimulation of the vestibular
system does not lead to coherent stochastic anteroposterior
postural sway. This result is consistent with the conventional
findings that show that with bipolar binaural constant galvanic
vestibular stimulation, the direction of the evoked sway is
approximately in the direction of the intermastoid line. Thus, it
is possible that coherent stochastic anteroposterior sway could be
produced with bipolar binaural stochastic galvanic vestibular
stimulation if the subject's head is turned to the left or right
(over the left or right shoulder).
[0053] Other conventional studies have shown that if a subject's
head is facing forward, monopolar binaural constant galvanic
stimulation of the vestibular system can be used to induce
anteroposterior sway in the subject. Thus, it is also possible that
coherent stochastic anteroposterior postural sway could be produced
with monopolar binaural stochastic galvanic vestibular
stimulation.
[0054] Previous studies have suggested that the role of the
vestibular system is to modulate the amplitude of the body's
postural response. The results of the study in accordance with the
invention support this notion. In particular, it has been shown
that time-varying galvanic vestibular stimulation can continuously
modulate mediolateral postural sway. In addition, by utilizing
stochastic stimulation signals, the subjects could not predict a
change in the vestibular stimulus. Thus, the findings indicate that
subjects can act as "responders" to galvanic vestibular
stimulation.
[0055] The findings in accordance with the invention indicate that
time-varying galvanic vestibular stimulation could be used as the
basis for an artificial vestibular control system to reduce or
eliminate certain types of pathological postural sway. Such a
system could consist of light-weight accelerometers for monitoring
an individual's postural sway, and a galvanic-stimulation control
system. In such an arrangement, the accelerometer output could be
used as input to the galvanic-stimulation control system.
[0056] A system of this sort could be used to improve balance
control in elderly individuals, who are often predisposed to falls.
In addition, patients with vestibular paresis, who have lost some
of their hair cells and therefore have a decreased response from
the vestibular system during head movement, could also benefit from
such a system. The hair cells, which are responsible for indicating
head tilt and acceleration, transmit their information to the
vestibular nuclei via the 8.sup.th nerve. Galvanic vestibular
stimulation acts directly on the 8.sup.th nerve and the stimulation
technique of the invention could be implemented as a vestibular
prosthesis to operate in place of the lost hair cells.
[0057] In addition, in accordance with the invention, time-varying
monopolar (anodal) binaural galvanic vestibular stimulation is used
to eliminate or reduce the function of the vestibular system. This
application of the invention is based on the finding that anodal
(positive) currents decrease the firing rate of vestibular
afferents. Similarly, with the invention, time-varying monopolar
(cathodal) binaural galvanic vestibular stimulation is used to
heighten or enhance the function of the vestibular system. This
application of the invention is based on the finding that cathodal
(negative) currents increase the firing rate of vestibular
afferents.
[0058] Although the present invention has been shown and described
with respect to several preferred embodiments thereof, various
changes, omissions and additions to the form and detail thereof,
may be made therein, without departing from the spirit and scope of
the invention.
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