U.S. patent application number 10/475141 was filed with the patent office on 2005-05-12 for method and apparatus for measurement of evoked neural response.
Invention is credited to Daly, Christopher Newton, Eder, Helmut, Nygard, Tony Mikeal.
Application Number | 20050101878 10/475141 |
Document ID | / |
Family ID | 25646658 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050101878 |
Kind Code |
A1 |
Daly, Christopher Newton ;
et al. |
May 12, 2005 |
Method and apparatus for measurement of evoked neural response
Abstract
The invention provides a method of electrical artefact
compensation in measurement of a neural response. The neural
response is evoked by a first stimulus, after which a compensatory
stimulus is applied in order to counteract a stimulus artefact
caused by the first stimulus. The invention also provides for short
circuiting the stimulating electrode subsequent to the first
stimulus. A system for implementing such steps is also provided.
The invention may be of application in measurement of physiological
responses, including neural responses and in particular a neural
response of the auditory nerve.
Inventors: |
Daly, Christopher Newton;
(Bilgola Plateau, AU) ; Nygard, Tony Mikeal;
(Kariong, AU) ; Eder, Helmut; (Lane Cove,
AU) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
PO BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
25646658 |
Appl. No.: |
10/475141 |
Filed: |
October 17, 2003 |
PCT Filed: |
April 18, 2002 |
PCT NO: |
PCT/AU02/00500 |
Current U.S.
Class: |
600/559 ;
600/554 |
Current CPC
Class: |
A61B 5/4041 20130101;
A61N 1/36038 20170801; A61B 5/7217 20130101; A61B 5/24
20210101 |
Class at
Publication: |
600/559 ;
600/554 |
International
Class: |
A61B 005/00; A61B
005/05 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 18, 2001 |
AU |
PR 4462 |
Aug 17, 2001 |
AU |
PR 7111 |
Claims
1. A method of electrical artefact compensation in measurement of a
neural response, the neural response evoked by a first stimulus,
the method comprising the step of: subsequent to the first
stimulus, applying a compensatory stimulus in order to counteract a
stimulus artefact caused by the first stimulus.
2. The method of claim 1 wherein the first stimulus comprises: a
first phase during which an electrical stimulus of first polarity
is applied; and a second phase, subsequent to the first phase,
during which an electrical stimulus of second polarity opposite to
the first polarity is applied.
3. The method of claim 2 wherein the first phase and second phase
are charge balanced.
4. The method of claim 2 or claim 3 wherein the compensatory
stimulus is of the first polarity.
5. The method of claim 1 wherein the compensatory stimulus is of
controlled profile and duration.
6. The method of claim 1 wherein the compensatory stimulus is
adaptive, whereby characteristics of the compensatory stimulus are
chosen in order to optimise the extent of cancellation of the
artefact which exists following the first stimulus.
7. The method of claim 6 further comprising the step of:
determining characteristics of the compensatory stimulus from a
measured effectiveness of a previously applied compensatory
stimulus.
8. The method of claim 7 wherein the step of determining
characteristics of the compensatory stimulus is performed in an
iterative manner, to provide an ongoing optimisation of the
compensatory stimulus, based on the effectiveness of one or more
previously applied compensatory stimuli.
9. The method of claim 7 or claim 8 wherein the measured
effectiveness of the previously applied compensatory stimulus is
determined by: obtaining at least one neural response measurement
of an actual performance of the previously applied compensatory
stimulus; and comparing the actual performance against a target
performance, in order to determine an error between the actual
performance and the target performance.
10. The method of claim 9 wherein the step of measuring the actual
performance of the previously applied compensatory stimulus
comprises: (i) applying the first stimulus and the previously
applied compensatory stimulus, and subsequently obtaining a
measurement comprising a first plurality of temporally spaced
neural samples from a sensor.
11. The method of claim 10 wherein the step of measuring the actual
performance of the previously applied compensatory stimulus further
comprises: (ii) repeating step (i) in order to obtain a second
plurality of measurements.
12. The method of claim 11 wherein the step of measuring the actual
performance of the previously applied compensatory stimulus further
comprises: (iii) discarding an initial number of said second
plurality of measurements to allow the sensor and other measurement
components to settle.
13. The method of claim 12 wherein the step of measuring the actual
performance of the previously applied compensatory stimulus further
comprises: (iv) taking an average of the remaining non-discarded
measurements of the second plurality of measurements, to obtain an
averaged measurement.
14. The method of claim 13 wherein the step of measuring the actual
performance of the previously applied compensatory stimulus further
comprises: (v) determining a stimulus artefact from the averaged
measurement, and assessing the performance of the previously
applied compensatory stimulus with reference to the determined
stimulus artefact.
15. The method of claim 10 wherein step (i) comprises obtaining
substantially 64 neural samples at substantially 48 .mu.s
intervals.
16. The method of claim 11 wherein step (ii) comprises obtaining
substantially 20 measurements.
17. The method of claim 16 wherein step (iii) comprises discarding
substantially the first 10 of said measurements.
18. The method of claim 14 wherein the step of determining the
stimulus artefact from the averaged measurement comprises
determining a deviation of the averaged measurement from a desired
response.
19. The method of claim 7 wherein the step of determining
characteristics of the compensatory stimulus from the measured
effectiveness of the previously applied compensatory stimulus is
performed by: determining an incremental change to be made to
characteristics of the previously applied compensatory stimulus in
order to reduce the error between the actual performance and the
target performance; and deriving the characteristics of the
compensatory stimulus by altering the characteristics of the
previously applied compensatory stimulus in accordance with the
incremental change.
20. The method of claim 19 wherein the incremental change is
determined so as to maximise a rate of convergence of the actual
performance to the target performance.
21. The method of claim 19 or claim 20 wherein the incremental
change is determined so as to minimise oscillation or overshoot of
the actual performance relative to the target performance.
22. The method of claim 1 wherein the compensatory stimulus
comprises a substantially rectangular pulse, whereby the amplitude
and duration of the pulse define an amount of charge to be inserted
by the pulse.
23. A method of electrical artefact compensation in measurement of
a neural response, the neural response evoked by a first stimulus,
the method comprising: subsequent to the first stimulus, applying a
compensatory stimulus in order to counteract a stimulus artefact
caused by the first stimulus; the compensatory stimulus comprising
a substantially rectangular pulse, whereby the amplitude and
duration of the pulse define an amount of charge to be inserted by
the pulse; determining characteristics of the compensatory stimulus
from the measured effectiveness of the previously applied
compensatory stimulus by: determining an incremental change to be
made to characteristics of the previously applied compensatory
stimulus in order to reduce the error between an actual performance
and the target performance; and deriving the characteristics of the
compensatory stimulus by altering the characteristics of a
previously applied compensatory stimulus in accordance with the
incremental change; and wherein the incremental change defines a
change in the amount of charge to be inserted by the pulse which is
required in order to improve the actual performance of the
compensatory stimulus compared to the target performance.
24. The method of claim 23 wherein, for a given charge to be
applied by the compensatory stimulus, the incremental change
further defines whether a relatively narrow pulse of relatively
large amplitude should be applied or whether a relatively broad
pulse of relatively small amplitude should be applied in delivering
the required amount of charge by the compensatory stimulus.
25. The method of claim 24 wherein the compensatory stimulus has a
fixed duration and a variable amplitude.
26. The method of claim 1 wherein the compensatory stimulus is
completed prior to an expected time of commencement of an
electrically evoked compound action potential.
27. The method of claim 1 wherein the neural response is the
auditory nerve neural response.
28. The method of claim 27 wherein the first stimulus is applied by
an auditory prosthesis.
29. The method of claim 28 wherein the auditory prosthesis
comprises a cochlear implant having an intra-cochlear electrode
array including electrodes used as stimulus electrodes and/or as
sense electrodes.
30. The method of claim 29 wherein the first stimulus is applied by
one or more stimulus electrodes of the array.
31. The method of claim 30 wherein the compensatory stimulus is
applied by the one or more stimulus electrodes.
32. The method of claim 30 wherein the compensatory stimulus is
applied by one or more electrodes in the array other than the
stimulus electrodes.
33. The method of claim 32 wherein additional compensatory stimuli
are applied by electrodes in close physical proximity to the
stimulating electrodes, substantially simultaneously with
application of the compensatory stimulus.
34. A method of electrical artefact compensation in measurement of
a neural response, the neural response evoked by a first stimulus
applied by at least one stimulating electrode, the method
comprising the step of: subsequent to the first stimulus, short
circuiting at least each stimulating electrode to an electrode
reference voltage.
35. The method of claim 34 wherein the at least one stimulating
electrode is an electrode of an auditory prosthesis for stimulus of
the auditory nerve.
36. The method of claim 35 wherein the auditory prosthesis
comprises an electrode array having a plurality of electrodes.
37. The method of claim 36 wherein the electrode array comprises
between 22 and 30 intra-cochlear electrodes.
38. The method of claim 36 or claim 37 wherein the method further
comprises the step of shorting electrodes in the array other than
the stimulating electrodes.
39. The method of claim 38 wherein the step of shorting electrodes
in the array other than the stimulating electrodes comprises
shorting electrodes in physical proximity to the stimulating
electrodes.
40. (canceled)
41. A neural stimulus system operable to apply a first stimulus in
order to evoke a neural response, the neural stimulus system
comprising means for electrical artefact compensation in
measurement of the neural response, the means for electrical
artefact compensation being operable to apply a compensatory
stimulus in order to counteract a stimulus artefact caused by the
first stimulus.
42. The neural stimulus system of claim 41 wherein measurement of
the neural response is performed by detection of a signal present
on designated sense electrodes.
43. The neural stimulus system of claim 42 wherein the sense
electrodes are distinct from the stimulus electrodes.
44. The neural stimulus system of any one of claims 41 to 43
wherein the neural stimulus system is an auditory prosthesis.
45. The neural stimulus system of claim 44 wherein the auditory
prosthesis is a cochlear implant.
46. The neural stimulus system of claim 44 wherein the response
measured is the evoked compound action potential of the auditory
nerve.
47. The neural stimulus system of claim 44 wherein the auditory
prosthesis comprises an array of intra-cochlear and extra-cochlear
electrodes.
48. The neural stimulus system of claim 41 wherein the neural
stimulus system is operable to apply the first stimulus such that
the first stimulus comprises: a first phase during which an
electrical stimulus of first polarity is applied; and a second
phase, subsequent to the first phase, during which an electrical
stimulus of second polarity opposite to the first polarity is
applied.
49. The neural stimulus system of claim 48 wherein the first phase
and second phase are charge balanced.
50. The neural stimulus system of claim 48 or claim 49 wherein the
means for electrical artefact compensation is operable to apply the
compensatory stimulus such that the compensatory stimulus is of the
first polarity.
51. The neural stimulus system of claim 41 wherein the means for
electrical artefact compensation is operable to control a profile
and a duration of the compensatory stimulus.
52. The neural stimulus system of claim 41 wherein the means for
electrical artefact compensation is operable to control
characteristics of the compensatory stimulus in order to optimise
the extent of cancellation of the stimulus artefact.
53. The neural stimulus system of any one claim 41 further
comprising means for measuring an effectiveness of a previously
applied compensatory stimulus.
54. The neural stimulus system of claim 53 wherein the means for
measuring an effectiveness of a previously applied compensatory
stimulus comprises means to obtain at least one neural response
measurement of an actual performance of the previously applied
compensatory stimulus, and comprises means to compare the actual
performance against a target performance, in order to determine an
error between the actual performance and the target
performance.
55. The neural stimulus system of claim 54 wherein the means to
obtain at least one neural response measurement of an actual
performance of the previously applied compensatory stimulus is
operable to obtain a measurement comprising a first plurality of
temporally spaced neural samples from a sensor.
56. The neural stimulus system of claim 55 wherein the means to
obtain at least one neural response measurement of an actual
performance of the previously applied compensatory stimulus is
operable to obtain a measurement comprising substantially 64 neural
samples spaced at substantially 48 .mu.s intervals.
57. The neural stimulus system of claim 41 wherein the means for
electrical artefact stimulation is operable to apply the
compensatory stimulus such that the compensatory stimulus comprises
a substantially rectangular pulse.
58. The neural stimulus system of claim 57 wherein the means for
electrical artefact stimulation is operable to apply the
compensatory stimulus such that the compensatory stimulus comprises
a substantially rectangular pulse having a relatively narrow width
and a relatively large amplitude, and is also operable to apply the
compensatory stimulus such that the compensatory stimulus comprises
a substantially rectangular pulse having a relatively broad width
and a relatively small amplitude.
59. The neural stimulus system of claim 41 wherein the neural
response is the auditory nerve neural response.
60. The neural stimulus system of claim 41 wherein the neural
stimulus system comprises an auditory prosthesis.
61. The neural stimulus system of claim 60 wherein the auditory
prosthesis comprises a cochlear implant having an intra-cochlear
electrode array including electrodes used as stimulus electrodes
and/or as sense electrodes.
62. The neural stimulus system of claim 61 wherein the first
stimulus is applied by the one or more stimulus electrodes.
63. The neural stimulus system of claim 62 wherein the compensatory
stimulus is applied by the one or more stimulus electrodes.
64. The neural stimulus system of claim 62 wherein the compensatory
stimulus is applied by one or more electrodes in the array other
than the stimulus electrodes.
65. The neural stimulus system of claim 41 further operable to
apply one or more additional compensatory stimuli.
66. A neural stimulus system comprising at least one stimulating
electrode operable to apply a first stimulus in order to evoke a
neural response, the neural stimulus system being operable to
compensate for electrical stimulus artefacts by short circuiting at
least each stimulating electrode to an electrode reference
voltage.
67. The neural stimulus system of claim 66 wherein the neural
stimulus system is an auditory prosthesis for stimulus of the
auditory nerve.
68. The neural stimulus system of claim 67 wherein the auditory
prosthesis comprises an electrode array having a plurality of
electrodes.
69. The neural stimulus system of claim 68 wherein the electrode
array comprises between 22 and 30 intra-cochlear electrodes.
70. The neural stimulus system of claim 68 or claim 69 wherein the
neural stimulus system is further operable to short electrodes in
the array other than the stimulating electrodes.
71. (canceled)
72. A method of electrical artefact compensation in measurement of
a physiological response, the physiological response evoked by a
first stimulus, the method comprising the step of: subsequent to
the first stimulus, applying a compensatory stimulus in order to
counteract a stimulus artefact caused by the first stimulus.
73. The method of claim 34 wherein the step of short circuiting
occurs subsequent to the step of: delivering a compensatory
stimulus in order to counteract a stimulus artefact caused by the
first stimulus.
74. The method of claim 73 wherein the step of short circuiting
extends for a relatively short time period immediately following
the compensatory stimulus.
75. The method of claim 74 wherein the step of short circuiting
extends for a period of substantially 1 .mu.s.
76. The neural stimulus system of claim 66 wherein the system is
further operable to apply a compensatory stimulus in order to
counteract a stimulus artefact caused by the first stimulus.
77. The neural stimulus system of claim 76 wherein the system is
operable to apply the compensatory stimulus after application of
the compensatory stimulus.
78. The neural stimulus system of any one of claims 66, 76 or 77
wherein the system is operable to short circuit at least each
stimulating electrode to the electrode reference voltage for a
relatively short period of time.
79. The neural stimulus system of claim 78 wherein the system is
operable to short circuit at least each stimulating electrode to
the electrode reference voltage for substantially 1 .mu.s.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method and apparatus for
measuring the evoked responses of nerves to electrical stimulation,
and more particularly to a system and apparatus to assist recovery
of such data from an auditory prosthesis.
DESCRIPTION OF THE PRIOR ART
[0002] Cochlear implants have been developed to assist people who
are profoundly deaf or severely hearing impaired, by enabling them
to experience hearing sensation representative of the natural
hearing sensation. In most such cases, these individuals have an
absence of or destruction of the hair cells in the cochlea which
naturally transduce acoustic signals into nerve impulses which are
interpreted by the brain as sound. The cochlear implant therefore
bypasses the hair cells to directly deliver electrical stimulation
to the auditory nerves with this electrical stimulation being
representative of the sound.
[0003] Cochlear implants have traditionally consisted of two parts,
an external speech processor unit and an implanted
stimulator/receiver unit. The external speech processor unit has
been worn on the body of the user and its main purpose has been to
detect the external sound via a microphone and convert the detected
sound into a coded signal through an appropriate speech processing
strategy.
[0004] This coded signal is then sent to the receiver/stimulator
unit which is implanted in the mastoid bone of the user, via a
transcutaneous link. The receiver/stimulator unit then processes
this coded signal into a series of stimulation sequences which are
then applied directly to the auditory nerve via a series of
electrodes positioned within the cochlea, proximal to the modiolus
of the cochlea.
[0005] As the implant is surgically implanted within the recipient
there is a need to obtain data about the actual performance of the
electrode array following implantation as well as the response of
the auditory nerve to stimulation. Such data collection enables
detection and confirmation of the normal operation of the device,
and allows the stimulation parameters to be optimised to suit the
needs of the patient.
[0006] Typically, following the surgical implantation of the
cochlear implant, the recipient must have the implant fitted or
customised to conform with the specific recipient demands. This
procedure collects and determines patient specific parameters such
as threshold levels (T levels) and maximum comfort levels (C
levels) for each stimulation channel. Essentially, this is manually
performed by applying stimulation pulses for each channel and
receiving an indication from the implant recipient as to the level
and comfort of the resulting sound. For implants with a large
number of channels for stimulation, this process is quite time
consuming and rather subjective as it relies heavily on the
recipients subjective impression of the stimulation rather than any
specific measurement. This aspect is further complicated in the
case of children and prelingually or congenitally deaf patients who
are unable to supply an accurate impression of the resultant
hearing sensation, and hence fitting of the implant may be
sub-optimal. In such cases an incorrectly fitted implant may result
in the recipient not receiving optimum benefit from the implant and
in the cases of children may directly hamper the speech and hearing
development of the child.
[0007] Therefore, as previously mentioned, there is a need to
obtain objective measurements of patient specific data especially
in cases where an accurate subjective measurement is not
possible.
[0008] One proposed method of interrogating the performance of the
implanted device and making objective measurements of patient
specific data such as T and C levels is to directly measure the
response of the auditory nerve to an electrical stimulus. The
measurement of Electrically Evoked Compound Action Potentials
(ECAPs) provides an objective measurement of the response of the
nerves to electrical stimulus. Following electrical stimulation,
the neural response is caused by the superposition of single neural
responses at the outside of the axon membranes. The ECAP can then
be measured in response to various stimulations and from this the
performance of the implant can be assessed and patient parameters
can be interpolated.
[0009] Indeed, there is a need to measure the response of nerves to
electrical stimulation in many applications, and not just in the
area of cochlear implants. The measurement of ECAPs has proven to
provide a useful objective measurement in many such applications.
By measuring the ECAP in response to a stimulation, the
effectiveness of the stimulation can be assessed in relation to the
neural response evoked by the stimulation.
[0010] A number of ECAP measurement methods and apparatus have been
developed which attempt to measure the response of the nerves to
electrical stimulus. In the area of cochlear implants where
electrical stimulus is delivered to the nerve cells within the
cochlea, such systems have essentially attempted to use the
electrodes implanted within the cochlea to both deliver stimulation
and to detect the responses of the nerves to such stimulation.
[0011] U.S. Pat. No. 5,758,651 describes one system and apparatus
for recovering ECAP data from a cochlear implant. This system
measures the neural response to the electrical stimulation by using
the stimulus array to not only apply the stimulation but to also
detect and receive the response. In this system the array used to
stimulate and collect information is a standard intra-cochlear
and/or extra-cochlear electrode array. Following the delivery of a
stimulation pulse via chosen stimulus electrodes, all electrodes of
the array are open circuited for a period of time prior to and
during measurement of the induced neural response. The purpose of
open circuiting all electrodes during this period is to reduce the
detected stimulus artefact measured with the ECAP nerve
response.
[0012] Whilst prior art systems of this type have proven useful in
capturing and investigating evoked neural responses in the cochlea,
there are still a number of intrinsic limitations associated with
such systems, which have affected the quality of the measurements
of the neural response. In the main this has been due to the
presence of stimulus artefacts in the measurement detected,
resulting in a measurement being taken which is not necessarily a
true indication of the actual ECAP response present.
[0013] The process of distinguishing the actual ECAP from stimulus
artefacts has presented considerable difficulties, including
problems such as the fact that the signals that are to be measured
are extremely low level signals (down to the order of 10 uV). In
cochlear implant applications in particular, an intracochlear
electrode usually delivers a stimulus pulse with an amplitude
typically in the range of 1V to 10V, which is many orders of
magnitude greater than the ECAP response that is to be measured
resulting from this stimulation.
[0014] Providing for a system that is firstly able to deliver a
stimulus of sufficient amplitude and also to detect the elicited
response of the nerves to that particular stimulation has therefore
been problematic. Due to the nature of the neural response, the
sensing system must be ready to record this response within a short
delay (preferably less then 50 us) after completion of the
stimulus. In order to properly resolve the very small neural signal
a large amplifler gain is required (typically of about 60 dB to 70
dB), however the neural signal is often superimposed on a much
larger artefact which makes it difficult to extract the neural
signal of interest due to the finite dynamic range of the amplifier
and the need for high gain to resolve the signal.
[0015] Prior to the present invention, the only way useful
measurements have been able to be obtained from the associated
artefacts has been through the use of extensive post processing
techniques. These techniques have attempted to apply complicated
mathematical algorithms to the associated measurements in an
attempt to cancel out the presence of the artefacts in the
measurements. Such a system does not provide immediate results
which can be acted upon, as the measured results often require time
consuming analysis before they can be used. With the need to use
such results immediately to adjust patient T and C levels, existing
methods are not satisfactory.
[0016] Any discussion of documents, acts, materials, devices,
articles or the like which has been included in the present
specification is solely for the purpose of providing a context for
the present invention. It is not to be taken as an admission that
any or all of these matters form part of the prior art base or were
common general knowledge in the field relevant to the present
invention as it existed in Australia before the priority date of
each claim of this application.
SUMMARY OF THE INVENTION
[0017] According to a first aspect, the present invention provides
a method of electrical artefact compensation in measurement of a
neural response, the neural response evoked by a first stimulus,
the method comprising the step of:
[0018] subsequent to the first stimulus, applying a compensatory
stimulus in order to counteract a stimulus artefact caused by the
first stimulus.
[0019] It has now been realised that stimulus artefacts in a nerve
stimulus system arise due to a number of different mechanisms in
the system and the surrounding tissue. The present invention, in
addressing such artefacts at the time of attempting to measure
evoked neural responses, allows for some reduction or compensation
for the effects of stimulus artefacts, which can reduce or remove
the need to resort to post-measurement processing.
[0020] In particular, it has been realised that stimulus artefacts
arise due to charging of the tissue during stimulation. The first
aspect of the present invention provides a method whereby
compensation may be made for such artefacts. That is, application
of the compensatory stimulus may prove effective in counteracting a
residue charge distribution in the tissue caused by the first
stimulus.
[0021] In preferred embodiments of the first aspect of the
invention, the first stimulus comprises:
[0022] a first phase during which an electrical stimulus of first
polarity is applied; and
[0023] a second phase, subsequent to the first phase, during which
an electrical stimulus of second polarity opposite to the first
polarity is applied.
[0024] Further, the first phase and second phase may be charge
balanced.
[0025] In such embodiments, it has been realised that the tissue is
charged in accordance with the first polarity during the first
phase, and is charged in accordance with the second polarity during
the second phase, and a residual charge may remain in the tissue
following the second phase, for example due to spatial charge
redistribution in the tissue during the stimulus. The residual
charge contributes to the interface stimulus artefact. Accordingly,
in such embodiments of the first aspect of the invention, the
compensatory stimulus is preferably of the first polarity, and is
preferably of controlled profile and duration, such as to
compensate for the interface stimulus artefact. In such
embodiments, the compensatory stimulus may be considered as a third
phase stimulus.
[0026] Preferably, the compensatory stimulus is adaptive, in that
characteristics of the compensatory stimulus are chosen in order to
optimise the extent of cancellation of the artefact which exists
following the first stimulus. Accordingly, the method of the first
aspect of the invention preferably comprises the further step
of;
[0027] determining characteristics of the compensatory stimulus
from a measured effectiveness of a previously applied compensatory
stimulus.
[0028] Such a step is preferably performed in an iterative manner,
to provide an ongoing optimisation of the compensatory stimulus,
based on the effectiveness of one or more previously applied
compensatory stimuli. By performing an iterative determination of
the characteristics of the compensatory stimulus, such embodiments
of the present invention allow the compensatory stimulus to be
adaptive.
[0029] In such embodiments, the measured effectiveness of the
previously applied compensatory stimulus is preferably determined
by:
[0030] obtaining at least one neural response measurement of an
actual performance of the previously applied compensatory stimulus;
and
[0031] comparing the actual performance against a target
performance, in order to determine an error between the actual
performance and the target performance.
[0032] In more detail, the step of measuring the actual performance
of the previously applied compensatory stimulus may comprise one or
more of the following steps:
[0033] (i) applying the first stimulus and the previously applied
compensatory stimulus, and subsequently obtaining a measurement
comprising a first plurality of temporally spaced neural samples
from a sensor;
[0034] (ii) repeating step (i) in order to obtain a second
plurality of measurements;
[0035] (iii) discarding an initial number of said second plurality
of measurements to allow the sensor and other measurement
components to settle;
[0036] (iv) averaging the remainder of the second plurality of
measurements to obtain an averaged measurement; and
[0037] (v) determining a stimulus artefact from the averaged
measurement.
[0038] Step (i) may comprise obtaining 64 neural samples at 48
.mu.s intervals (which is around 20 kHz), step (ii) may comprise
obtaining 20 measurements, while step (iii) may comprise discarding
the first 10 of said measurements, such that the averaged
measurement is obtained from the remaining 10 measurements. The
step of determining the stimulus artefact from the averaged
measurement may comprise determining a deviation of the averaged
measurement from a desired response.
[0039] The step of determining characteristics of the compensatory
stimulus from the measured effectiveness of the previously applied
compensatory stimulus is preferably performed by:
[0040] determining an incremental change to be made to
characteristics of the previously applied compensatory stimulus in
order to reduce the error between the actual performance and the
target performance; and
[0041] deriving the characterstics of the compensatory stimulus by
altering the characteristics of the previously applied compensatory
stimulus in accordance with the incremental change.
[0042] Preferably, the incremental change is determined so as to
(a) maximise a rate of convergence of the actual performance to the
target performance, and (b) minimise oscillation or overshoot of
the actual performance relative to the target performance.
[0043] In many applications of the present invention, the
compensatory stimulus will comprise a substantially rectangular
pulse, the amplitude and duration of the pulse defining an amount
of charge to be inserted by the pulse. Accordingly, in such
embodiments, the incremental change preferably defines a change in
the amount of charge which is required in order to improve the
actual performance of the compensatory stimulus compared to the
target performance. Further, for a given charge to be applied by
the compensatory stimulus, the incremental change may further
define whether a relatively narrow pulse of relatively large
amplitude should be applied or whether a relatively broad pulse of
relatively small amplitude should be applied in delivering the
required amount of charge by the compensatory stimulus.
[0044] In preferred embodiments of the invention, the compensatory
stimulus may be limited to having a variable amplitude only, and
having a fixed duration. Such embodiments allow measurements
subsequent to the compensatory stimulus to commence at a known
time, regardless of an amount of charge to be delivered by the
compensatory stimulus.
[0045] Preferably, the compensatory stimulus is completed prior to
an expected time of commencement of an electrically evoked compound
action potential.
[0046] In preferred embodiments of the first aspect of the
invention, the neural response is the auditory nerve neural
response, and the first stimulus is applied by an auditory
prosthesis. The auditory prosthesis may comprise a cochlear implant
with an intra-cochlear electrode array, including electrodes used
as stimulus electrodes and/or as sense electrodes.
[0047] In embodiments where the auditory prosthesis is a cochlear
implant comprising an electrode array, the first stimulus will
typically be applied by one or more electrodes of the array,
designated as stimulating electrodes. In embodiments of the first
aspect of the invention, the compensatory stimulus will typically
be applied by those same stimulating electrodes. However, it is to
be appreciated that a compensatory stimulus may alternately or
additionally be applied by other electrodes in the array. For
example, application of additional, simultaneous compensatory
stimuli may be appropriate in those electrodes in close physical
proximity to the stimulating electrodes, due to the physical charge
distribution caused by the first stimulus. The characteristics of
such additional stimuli may be chosen responsive to an expected
charge distribution in tissue adjacent to those different
electrodes, caused by the first stimulus.
[0048] According to a second aspect, the present invention provides
a method of electrical artefact compensation in measurement of a
neural response, the neural response evoked by a first stimulus
applied by at least one stimulating electrode, the method
comprising the step of:
[0049] subsequent to the first stimulus, short circuiting at least
each stimulating electrode to an electrode reference voltage.
[0050] It is preferable that the short circuiting of at least each
stimulating electrode to an electrode reference voltage occurs for
a short period of time, approximately 1 us, and preferably occurs
immediately following the delivery of a compensatory stimulus such
as is provided for by the first aspect of the present
invention.
[0051] It is again noted that it has now been realised that
stimulus artefacts in a nerve stimulus system arise due to a number
of different mechanisms in the system and the surrounding tissue.
The present invention, in addressing such artefacts at the time of
attempting to measure evoked neural responses, allows for some
reduction or compensation for the effects of stimulus artefacts,
which can reduce or remove the need to resort to post-measurement
processing.
[0052] Further, it has now been recognised that artefacts may arise
in a stimulus system itself following a stimulus, and in particular
may arise in electrodes of the system used for applying stimuli
and/or electrodes of the system used for sensing a neural response.
The second aspect of the present invention provides a method for
compensating for such artefacts in the stimulating electrodes, by
shorting the stimulating electrodes following application of the
stimulus or a compensatory stimulus. Connection of the stimulating
electrodes directly to the electrode array reference voltage
quickly returns the stimulating electrodes to that voltage.
Additionally, it has been realised that simply allowing passive
tissue load settling, that is leaving all electrodes open circuited
after a stimulus, increases the likelihood that the tissue voltage
will stray from the electrode array reference voltage. In this
event, subsequent connection of sensing electrodes for measurement
of the neural response can occur at a time when a significant
voltage difference exists between the actual tissue voltage and the
electrode array reference voltage. At the time of connection of the
sensing electrodes, such a voltage can give rise to significant
charge injection into the sensing electrodes, potentially causing
measurement of the actual neural response to be inaccurate or
impossible and adding another source of artefact.
[0053] Preferably, the stimulus system is an auditory prosthesis
for stimulus of the auditory nerve. It is anticipated that an
auditory prosthesis in relation to which the method of the second
aspect of the invention is used, will comprise an electrode array
having multiple electrodes. For example, where the auditory
prosthesis is a cochlear implant, the electrode array may comprise
22 to 30 intra-cochlear electrodes. In some embodiments of the
invention, shorting of electrodes in the array other than the
stimulating electrodes may be appropriate. For example, those
electrodes in physical proximity to the stimulating electrodes may
be influenced by charge build-up caused by the stimulating
electrodes, and may therefore benefit from being short circuited
for a brief settling period following the first stimulus, or
following delivery of a compensatory stimulus. However it is
possible that short circuiting a large number of electrodes in the
electrode array following the first stimulus may lead to larger
than acceptable current injection between the tissue and the
electrodes. Hence, in many embodiments of the second aspect of the
invention, only the stimulating electrodes are short circuited
after application of a stimulus.
[0054] It will be appreciated that the method of the first aspect
of the invention and the method of the second aspect of the
invention may both be implemented to assist in reducing stimulus
artefacts for a single measurement of neural response. In
particular, the step of the second aspect of the invention may be
performed after the step of the first aspect of the invention, and
prior to commencement of measurement of the evoked neural
response.
[0055] According to a third aspect, the present invention provides
a neural stimulus system operable to apply a first stimulus in
order to evoke a neural response, the neural stimulus system
comprising means for electrical artefact compensation in
measurement of the neural response, the means for electrical
artefact compensation being operable to apply a compensatory
stimulus in order to counteract a residue charge distribution
caused by the first stimulus.
[0056] According to a fourth aspect, the present invention provides
a neural stimulus system comprising at least one stimulating
electrode operable to apply a first stimulus in order to evoke a
neural response, the neural stimulus system being operable to
compensate for electrical stimulus artefacts by short circuiting at
least each stimulating electrode to an electrode reference
voltage.
[0057] Preferably, measurement of the neural response is performed
by detection of a signal present on designated sense electrodes.
Preferably, the sense electrodes are different to the stimulus
electrodes.
[0058] Preferably, the neural stimulus system is an auditory
prosthesis, such as a cochlear implant. Preferably the response
measured is the evoked compound action potential of the auditory
nerve. The auditory prosthesis preferably comprises an array of
intra-cochlear and extra-cochlear electrodes.
[0059] By compensating for stimulus artefacts, embodiments of the
invention may assist in enabling high resolution neural response
measurements to be acquired.
[0060] It has further been realised that application of a
compensatory stimulus may prove to be of assistance generally when
a first stimulus is applied to physiological tissue with a
capacitive characteristic, where a response of the physiological
tissue to the first stimulus is desired to be measured.
[0061] Accordingly, in a fifth aspect, the present invention
provides a method of electrical artefact compensation in
measurement of a physiological response, the physiological response
evoked by a first stimulus, the method comprising the step of:
[0062] subsequent to the first stimulus, applying a compensatory
stimulus in order to counteract a stimulus artefact caused by the
first stimulus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] Examples of the invention will now be described with
reference to the accompanying drawings in which:
[0064] FIG. 1 illustrates a sequence of stimulus, artefact
compensation and evoked neural response measurement in accordance
with the present invention;
[0065] FIG. 2 illustrates a typical evoked neural response;
[0066] FIG. 3 is a circuit diagram illustrating an electrode array
of an auditory prosthesis in accordance with the present
invention;
[0067] FIG. 4a illustrates a stimulus artefact present following a
bipolar stimulation, while FIG. 4b illustrates the effect of a
compensatory pulse;
[0068] FIG. 5 is a flow chart illustrating the implementation
algorithm of the present invention;
[0069] FIG. 6a illustrates the manner in which an MFD is obtained
according to the present invention, while FIG. 6b illustrates the
manner in which the polarity of a measured MFD may be
determined;
[0070] FIG. 7 illustrates the manner in which the compensatory
pulse is varied in accordance with preferred embodiments of the
invention;
[0071] FIG. 8 illustrates possible compensatory phase shapes which
may be applied in alternate embodiments of the present invention;
and
[0072] FIG. 9 shows equivalent pulse shape lines from FIG. 8 and
their effect on pulse shapes of varying charge.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0073] The following discussion will be made on the basis of the
present invention being implemented in a cochlear implant, such as
is discussed in U.S. Pat. No. 4,532,930, the contents of which are
incorporated herein by reference. However, it is to be appreciated
that the present invention may have application in other types of
auditory prostheses and indeed may have application in measurement
of neural responses, or in general, a physiological response to an
electrical stimulation.
[0074] FIG. 1 illustrates the occurrence over time of a first
stimulus 20, artefact compensation period 30 and evoked neural
response measurement phase 40 in accordance with the present
invention.
[0075] The first stimulus 20 includes a first phase 21 during which
an electrical stimulus of negative polarity is applied by an
intra-cochlear active stimulus electrode to the auditory nerve.
Subsequently, a second phase 22 of positive polarity is applied by
the active electrode. As will be appreciated, an intra-cochlear
reference stimulus electrode may simultaneously apply complementary
pulses. Alternatively, stimulation may occur with reference to all
other electrodes of the electrode array, or with reference to an
extra-cochlear electrode. It is to be noted that in this embodiment
the first phase 21 and second phase 22 are of equal duration and
amplitude and are therefore charge balanced. It is also envisaged
that more than two phases of stimulation could be applied within
the scope of the present invention. The first phase 21 and the
second phase 22 could, for example, have a duration of the order of
15-50 .mu.s, with an amplitude of up to around 10V. Further, first
phase 21 and second phase 22 could be differently shaped, for
example shaped as half sines, or as stepped half-sine
approximations.
[0076] Subsequent to the stimulus 20, a third phase compensatory
stimulus 23 is applied, in the present embodiment by the same
active stimulating electrode, in order to counteract a residue
charge distribution caused by the first stimulus 20. As noted
above, it has been realised that the tissue in a cochlear implant
system is charged by the negative pulse during the first phase 21,
and is charged by the positive pulse during the second phase 2, and
a relatively significant charge may remain following the second
phase 22, for example due to charge redistribution during the
stimulus. The remaining charge contributes to the tissue stimulus
artefact. Accordingly, the compensatory stimulus 23 is of negative
polarity.
[0077] The tissue artefact is mainly comprised of the effect of
charge in the tissue which is left over from the stimulation and is
being redistributed. The tissue artefact is most severe at the
stimulus electrodes, but it will also couple to surrounding
electrodes, dependant mainly on their proximity. For this reason
the sense electrodes, to be used during measurement phase 40, are
preferably selected to be different from the stimulus
electrodes.
[0078] The amplitude and duration of phase 23 have been chosen in
order to compensate for the interface stimulus artefact. In
accordance with the invention, the compensatory stimulus 23 is
adaptive, in that characteristics of the compensatory stimulus 23
are chosen depending on characteristics of the first stimulus 20.
This is discussed further with reference to FIGS. 4 to 9 in the
following.
[0079] Following compensatory stimulus 23, a load settling period
24 is applied, during which the stimulating electrodes are short
circuited in accordance with the second aspect of the invention.
Load settling period 24 is applied for approximately 1 us, however
this period in FIG. 1 is not to scale with respect to the remainder
of the Figure.
[0080] Load settling period 24 assists in compensating for
artefacts which may arise in the stimulus electrodes of the implant
by substantially restoring the tissue voltage and all internal
stimulus and sensing circuit nodes back to the electrode array
reference voltage. By shorting the stimulating electrodes following
application of the stimulus 20 and compensatory pulse 23, the
stimulating electrodes quickly return to the electrode array
reference voltage. This method provides an alternative to allowing
passive tissue load setting, in which all electrodes are left open
circuited after a stimulus, which can increase the likelihood that
the tissue will stray from the electrode array reference voltage.
In such passive load settling, subsequent connection of sensing
electrodes for measurement of the neural response can occur at a
time when a significant voltage difference exists between the
actual tissue voltage and the electrode array reference voltage. At
the time of connection of the sensing electrodes, such a voltage
can give rise to significant charge injection into the sensing
electrodes, potentially causing measurement of the actual neural
response to be inaccurate or impossible.
[0081] Programmable initial delay period 50 assists in dealing with
cases whereby the tissue artefact is too large and exceeds the
dynamic range of the amplifer. This delay period holds the
amplifier in a reset state until start of measurement is desired.
The delay period is usually set to the minimum value consistent
with capturing the neural response signal.
[0082] The stimulus artefact caused by stimulus 20 is typically
time-varying, and so the portion of signal present during
measurement due to the artefact changes over time. Hence, the
stimulus artefact introduces an artefact slew to measurements of
the neural response. The third phase 23 or compensatory phase is
preferably adjusted to minimise the artefact slew seen at the
sensing electrodes, which may be either the same electrodes as
those used for stimulation or which may be any of the other
electrodes on the array. The parameters of this third or
compensatory phase may be different for different sensing electrode
positions, for example in response to space/time differences in the
charge field. The system of the present invention is therefore
capable of delivering a wide variety of stimulus waveforms which
could be used dependant on the goals of the measurement and/or the
manner in which the measurement is carried out.
[0083] The present embodiment of the invention manipulates the
stimulus waveform applied prior to or during the measurement
process in such a way as to minimise the artefacts associated with
the measurement. As previously mentioned, in the present embodiment
this is essentially done via the application of a smaller
programmable third phase stimulus pulse immediately after the
balanced biphasic stimulus, resulting in the system delivering an
unbalanced triphasic stimulus.
[0084] The purpose of delivering this programmable third stimulus
phase following the standard balanced biphasic stimulus pulse is to
minimise or cancel the tissue artefact that would otherwise be
present in the measurement, making it much easier to capture the
neural signal at high amplifier gains, thereby allowing a higher
resolution measurement of an ECAP response to be obtained. As the
tissue artefact is mainly the effect of residual tissue potential
(voltage) in the tissue which is left from the stimulation that is
being redistributed, the third phase compensates for this.
Preferably, characteristics of the third or compensatory phase are
determined by taking into consideration the time-varying charge
recovery nature of the tissue.
[0085] It is possible that the stimulus pulse could also be an
unbalanced biphasic stimulus or a balanced triphasic stimulus, with
the introduction of a compensatory phase performing the same
function as is described above. Also, it is possible that the
compensatory pulse may precede the stimulus and be non-rectangular
or complex in shape and polarity. The invention resides in the
provision of a compensatory stimulus to negate the effects of the
tissue artefact and as such there are a number of different ways
such a scheme may be implemented, all of which would fall within
the scope of the present invention.
[0086] FIG. 2 illustrates a typical evoked neural response, which
may arise in response to a stimulus 30 as depicted in FIG. 1.
Period 70 in FIG. 2 illustrates a stimulation period, during which
a stimulus is applied to an auditory nerve. The neural response 72
typically commences approximately 100 us after the onset of the
stimulus phase 70, as indicated by period 71. The duration of the
more significant features of the response is around 1000 us, as
indicated by period 73, while the response measurement period or
window is usually around 1.5 to 3 milliseconds.
[0087] Turning now to FIG. 3, a circuit diagram of an electrode
array of a cochlear implant in accordance with the present
invention is represented, by which the first stimulus 20, artefact
compensation period 30 and evoked neural response measurement phase
40 may be applied. The electrode array is operable to apply the
first stimulus 20 in order to evoke a neural response, and is
operable to apply a compensatory stimulus 23 in order to counteract
a residue tissue potential caused by the first stimulus 20. The
electrode array is further operable to compensate for stimulus
artefacts by short circuiting at least each stimulating electrode
to an electrode array reference voltage, following application of a
stimulus.
[0088] FIG. 3 illustrates four of the 24 electrode output switch
networks and is merely an illustrative example of the system, to
enable the operation of the circuit in its switching and sensing
modes to be understood. Each of the four electrodes are indicated
as A, B, C, D. As can be seen, each electrode can be connected to
either the reference V.sub.dd.sub..sub.--.sub.OS line or the
10-line via switches S1 and S2 respectively, for each electrode.
V.sub.dd.sub..sub.--.sub.OS represents the positive supply voltage
rail for the output switches, V.sub.ss represents the negative
supply rail and the IO-line represents the current source. The
operation of these switches for the generation of charge balanced
biphasic pulses is detailed in the applicants own U.S. Pat. No.
4,532,930.
[0089] For the present invention, each electrode has two additional
switches (S3 and S4) to allow for connection to the sensing
amplifier. In the quiescent state, all electrodes are usually short
circuited to V.sub.dd.sub..sub.--.sub.OS to ensure long term charge
recovery and no DC current. However, during the measurement period
only the stimulating electrodes are shorted, for a very short time
interval, in order to minimise current flow to the sensing
electrodes which may introduce an unwanted stimulus artefact into
the measurement.
[0090] The switch configuration of the present invention also
includes another switch S.sub.IPG which is used in conjunction with
an internal load resistor R.sub.INT to allow the IO-line to settle
to a voltage close to V.sub.dd.sub..sub.--.sub.OS before the start
of the stimulus phases. The purpose of allowing the IO-line to
settle is to minimise the amplitude of the voltage spikes on the
leading edge of the stimulus pulse due to the discharging of the
IO-line capacitance into the load.
[0091] At the end of the third phase 23, there is a short load
"settling period" 24, to restore the electrodes and the internal
circuitry nodes back to as close to V.sub.dd.sub..sub.--.sub.OS as
possible. In practice this load settling period can take a duration
of .about.1 us, and during this time the stimulation electrodes are
shorted to V.sub.dd.sub..sub.--.sub.OS by closing both the S1
switches. During this load settling period S.sub.DAC is also
opened.
[0092] Following the load settling period the stimulation
electrodes are opened leaving all electrodes in the open circuit
state. The V.sub.dd.sub..sub.--.sub.OS switch is then opened
thereby changing the circuit from the stimulation mode to the
sensing mode. Following the above steps, measurement period 40 may
proceed, after any appropriate initial delay 50. Importantly,
measurement period 40 preferably commences prior to an onset of the
neural response, so that obtained measurements record a leading
edge of the neural response. The characteristic ECAP signal
typically occurs approximately 100 us after the onset of the
stimulus pulse 20 (as indicated by onset period 60 in FIG. 1) and
usually has a duration of approximately 1000 us. It has been found
that the signal's amplitude grows with the increasing number of
nerve fibres captured as the amplitude of the stimulus increases
above the threshold limit.
[0093] Measurement of the neural response is performed by detection
of a signal present on designated sense electrodes of the implant.
Preferably, the sense electrodes are different to the stimulus
electrodes. By compensating for stimulus artefacts, embodiments of
the invention may assist in enabling high resolution neural
response measurements to be acquired.
[0094] FIG. 4a illustrates a bipolar stimulation signal consisting
of a first stimulation phase of negative polarity followed by a
second stimulation phase of positive polarity, applied by an active
electrode. Following the bipolar stimulation, a stimulus artefact
is present, As can be seen, a significant stimulus artefact remains
after the final charge has been delivered at the end of a standard
two phase stimulus. This artefact has the effect of a gradual
slewing or decay towards V.sub.DD which can be as large as of the
order of millivolts at the relevant time and can take several
hundred microseconds before becoming negligible. Given the
extremely low level of neural response ECAP signals, down to the
order of 10 microvolts, this significant remaining artefact can
obscure the actual ECAP response, and can saturate the ECAP
measurement system.
[0095] FIG. 4b illustrates application of a compensatory stimulus,
comprising a substantially rectangular pulse, of negative polarity.
As opposed to FIG. 4a, at the time measurements commence,
significant cancellation of the stimulus artefact has been achieved
such that neural measurements are of the order of microvolts. As
can be seen, application of the compensatory stimulus significantly
hastens settling of the stimulus artefact, enabling a neural
response measurement system or circuits to commence operation
significantly more quickly following the bipolar stimulus, without
saturation of the measurement system being caused by residual
stimulus artefacts.
[0096] In applying a compensatory stimulus, it is important that
the shape and characteristics of this stimulus are such that it
does provide effective compensation for stimulus artefacts and that
it does not worsen such artefacts. Consequently, the
characteristics of the compensatory stimulus should be carefully
determined. Given the detailed knowledge required of both the
stimulus system and the patient physiology in order to perform such
optimisation of the compensatory stimulus, manual adjustment of the
parameters of the third phase would be a tedious and complicated
process, which would not easily lend itself to clinical
applications. Hence, in accordance with preferred embodiments of
the invention, a method is proposed in order to address this
problem, allowing the user to adjust parameters of the third phase
without requiring extensive prior knowledge of the electrical
workings of the electrode/tissue interface, thereby allowing
convenient clinical use of such a system.
[0097] Turning to FIG. 5, a flow chart is shown which reveals a
manner of implementing an algorithm in accordance with the present
invention, to optimise the parameters of the compensatory phase so
as to appropriately cancel or counteract the effects of the
stimulus artefact.
[0098] Firstly an OK Samples counter is set which determines the
number of "correct" MFD's (Measurements for Decision) which must be
obtained by the algorithm before the process can be considered to
be complete. An MFD is considered to be correct when the error of
the measurement is less than the tolerance set by the user, and
this will be discussed in more detail below. In a preferred
embodiment the counter would be set to require that four correct
MFD's must be obtained before the adjustment of the compensatory
means can be considered completed.
[0099] Subsequently, a first MFD is obtained, without the use of
any compensatory phase. As the polarity of a measured MFD may swap
depending on the manner in which sense electrodes are connected to
the input of an amplifier, obtaining such an MFD allows the
polarity of the measured MFD (denoted by variable P) and hence the
manner in which the sense electrodes are connected to the input of
the amplifier to be determined. FIG. 6b illustrates an output of a
neural response detection amplifier for both positive and negative
polarity situations. P takes a value of +1 or -1, and is
subsequently used to influence whether an increase or decrease in
charge applied by the compensatory phase should occur in response
to a given error, as discussed further below.
[0100] The next step requires the MFD to be obtained, which in the
present embodiment involves averaging 10 measurements, each
comprising 64 collected telemetry samples in the manner as shown in
FIG. 6a.
[0101] Following the delivery of the stimulus, including the
compensatory stimulus, a series of telemetry measurements are taken
of the evoked neural response. In a preferred embodiment 10
telemetry measurements are taken, each comprising 64 telemetry
samples. It has been found that, following stimulation, the initial
telemetry measurements tend to differ only slightly from the steady
state telemetry measurements, however whilst there is only a very
small settling period for the telemetry measurements it is wise to
discard some initial telemetry measurements due to this phenomena.
A preferred embodiment may be to discard the first 10 telemetry
measurements.
[0102] It has also been found that to eliminate the effects of
noise and telemetry measurement inaccuracies it is desirable to
extract the MFD from an average of a number of telemetry
measurements. In a preferred embodiment, 10 collected telemetry
measurements are averaged from which the MFD is extracted.
[0103] The MFD that is obtained for example from a single
measurement or from the average of a plurality of measurements is
referred to as the "measurement for decision" (MFD) and it is this
MFD that is used in the algorithm to decide upon the appropriate
parameters of the compensatory phase. FIG. 6 illustrates how an MFD
is obtained.
[0104] Once the MFD has been obtained an error is determined for
the MFD, in order to determine whether the compensatory phase is
appropriately adjusted. In the present embodiment, the error of a
given MFD is determined by examining each point along the MFD to
determine variations of the MFD from the target response. The
variations at each point along the MFD are summed, and when the sum
is equal to zero, that is when the MFD conforms to the target
response or exhibits positive and negative excursions from the
target response which sum to zero, the MFD is considered "correct".
That is, the measured neural response substantially conforms to a
target response, indicating that the stimulus artefact has been
substantially cancelled. In accordance with such embodiments of the
invention, the error of the MFD could be determined in accordance
with the following algorithm:
Error (.mu.s)=.SIGMA.[Sample n (.mu.s)-First Sample (.mu.s)-Target
Offset (converted to .mu.s)]
[0105] An alternate method in which the error of a given MFD may be
determined is to simply calculate the difference between the first
and last telemetry sample of the MFD. The difference between the
first and last telemetry samples represents the amount by which the
stimulus artefact has settled or decayed during the telemetry
sample period, which is around three milliseconds in the present
embodiment. As previously mentioned, in a preferred embodiment 64
telemetry measurements are used for this algorithm, to ensure that
the majority of elements of the artefact and response have subsided
from the measurement by the time the final sample is taken. Thus,
where substantial subsidence of both the neural response and the
stimulus artefact have occurred by the time the final sample is
taken, the difference between the first sample and the final sample
will be representative of the actual amplitude of the stimulus
artefact present at the time the first sample is taken. This
alternate method of determining the error of the MFD, could for
example determine the error in .mu.s in accordance with the
following formula;
Error (.mu.s)=Last Sample (.mu.s)-First Sample (.mu.s)-Target
Offset (converted to .mu.s)
[0106] It should be appreciated that this is a very simplistic
description of how the error is determined, for example, such a
method assumes that there is a flat neural response, which is not
usually the case. It is envisaged that other methods of determining
the error in artefact cancellation could also be employed which do
not make such simplifying assumptions of the present embodiment of
the invention, such as by taking into account non-flat neural
responses.
[0107] In both the preferred embodiment and in the latter alternate
embodiment of determining the error of an MFD, the "Target Offset"
variable indicates a deviation of the measurement from the desired
target, The Target Offset is specified in .mu.V and in a preferred
embodiment this value should be set to 0 .mu.V. In calculating the
error the Target Offset variable is converted to .mu.s.
[0108] Returning to FIG. 5, once the error of the MFD has been
determined, an assessment is made as to whether the error of the
MFD is acceptable. In the present embodiment, in assessing whether
the MFD is acceptable the following criteria must be met:
.vertline.Error (.mu.s).vertline..ltoreq.Target Tolerance
(.mu.s)
[0109] The "Target Tolerance" variable essentially tightens or
relaxes the criteria which determines whether or not an MFD is
considered to be correctly adjusted or not. This variable is in
units of microseconds and determines the tolerance in the pulse
widths of the measured telemetry pulse in comparison to an ideal
value. In a preferred embodiment this variable would alternate
between a "relaxed" and a "normal" setting, with the "relaxed"
setting typically being 1.0 .mu.s and the "normal" setting being
0.2 .mu.s. It is envisaged that the recommended default setting
would be "normal".
[0110] If it is decided that the MFD is not acceptable and the
error is outside the target tolerance, the compensatory phase is
adjusted in order to bring the MFD to within acceptable limits. In
essence, the amount of charge delivered by the compensatory phase
is adjusted. This process is a two-step process with the first step
involving the calculation of the new charge to be delivered by the
compensatory phase, and the next step determining and calculating
the corresponding values of phase width and phase current of the
compensatory phase.
[0111] The new charge to be delivered by the compensatory phase is
determined as follows:
New Charge (pC)=Old Charge (pC)+.DELTA.Charge (pC)
Whereby
.DELTA.Charge (pC)=Stepratio (nC/V)*Error (converted to V)*P
[0112] The "Stepratio" variable gives the rate at which the
algorithm adjusts the compensatory phase charge in order to meet
the required target measurement and describes the change in charge
applied by the compensatory phase as a result of a given target
error, the calculation of which is discussed above. In a preferred
embodiment this variable is initially set at 1000 nC/V and any
increase of this value will cause the algorithm to converge to the
target faster but has the possibility of causing overshoot and
oscillations, whilst any reduction in the value will cause slower
but more stable convergence to the target measurement. P is the
variable determined previously, relating to the polarity of
connection of sense electrodes to the input of the amplifier.
[0113] In preferred embodiments of the invention, the compensatory
stimulus comprises a rectangular pulse of fixed duration t.sub.c
and variable amplitude, as depicted in FIG. 7. To commence the
adaptive optimisation of the compensatory pulse, an initial value
of charge delivered by the compensatory pulse could be chosen
arbitrarily, for example to be 20% of the charge delivered by one
phase of the initial bi-phasic stimulus. As discussed above, the
charge to be delivered by the compensatory stimulus is altered in
accordance with the "stepratio" variable, which is implemented by
altering the amplitude of the pulse, illustrated in an exaggerated
manner in FIG. 7 by .DELTA.A. By providing a compensatory stimulus
of fixed duration, such embodiments of the invention allow the
measurement period t.sub.m to commence at a known time, rather than
requiring the measurement period to be delayed until completion of
a variable duration compensatory pulse. FIG. 7 further illustrates
the neural response to be measured, which is not to scale with
respect to the compensatory stimulus.
[0114] Once the new charge to be delivered by the compensatory
phase is determined, derivation of the 3rd phase current is a
simple matter (bearing in mind that the compensatory pulse is of
fixed duration), as follows:
3rd Phase Current (.mu.A)=[3rd Phase Charge (pC)]/[3rd Phase Width
(.mu.s)].
[0115] The width of the third phase could be arbitrarily chosen,
and for example could be of the order of 10 .mu.s.
[0116] In alternate embodiments, should it be desired to use a
compensatory stimulus of both varying width and amplitude, the
corresponding values of phase width and phase current level to be
delivered by the compensatory phase can be derived as follows:
Compensatory Charge (pC)=Compensatory Phase Width (.mu.s)*
Compensatory Phase Current (.mu.A)
[0117] The relationship of the Compensatory Phase Width to the
Compensatory Phase Current can be better understood by what is
termed the Phase Shape variable. This variable is used to determine
the "shape" of the compensatory phase. When selecting the shape of
the compensatory phase it is possible to select almost any shape,
however in the embodiment shown in FIGS. 8 and 9 a choice is made
between a relatively narrow pulse, a relatively normal width pulse
and a shallow pulse shape, to deliver a given charge. As can be
seen in FIG. 8, a very narrow pulse has a much higher current/time
compared to a low current/time characteristic of a shallow pulse,
however the charge (l*t) delivered by all three shapes is the same.
Therefore, whilst all these three shapes deliver the same charge,
they have different ratios of l/t.
[0118] With this in mind, FIG. 9 shows equivalent pulse shape lines
(l/t constant) and their effect on pulse shapes of varying charge.
In essence for each of the three pulse shapes, narrow, normal and
shallow, there is one value of l/t. In a preferred embodiment the
values may be: Narrow -80 A/s, Normal -40 A/s and Shallow 20
A/s.
[0119] Therefore the following relationship can be established: 1
Compensatory Phase Current ( A ) Compensatory Phase Width ( s ) =
Pulse Shape ( A / s )
[0120] From which the following relationship can be derived:
[Compensatory Phase Current (mA)].sup.2=Compensatory Phase Charge
(pC)*Pulse Shape (A/s)
[0121] Therefore from this the Compensatory Phase Width and
Compensatory Phase Current can be determined and adjusted
accordingly so that a new MFD can be obtained and assessed for
error. However, it is to be appreciated that alternate embodiments
of this type may prevent commencement of sampling at a common time
for all values of pulse width, as commencement of sampling should
not occur until after conclusion of the compensatory pulse.
[0122] Referring again to FIG. 5, if it is decided that the MFD is
acceptable and the error is within the target tolerance, the OK
Samples counter is incremented and the counter is then interrogated
to determine whether the measurement process is complete and the
desired number of correct MFD's has been measured.
[0123] If the number of correct MFD's is less than the desired
number then the above measurement and error determination process
is continued until this criteria has been satisfied.
[0124] When a desired number of correct MFD's have been obtained, a
Time-out variable is applied to the algorithm which stops the
algorithm from further adjustments after a specified time delay. In
a preferred embodiment this time delay may be 5 seconds. Following
this time the compensatory phase is set to zero and the process is
completed As can be seen, the described embodiments provide for
adjustment of the characteristics of the compensatory phase without
need for detailed user involvement. To provide differing levels of
user input, the system may provide for user input of variables such
as the number of telemetry measurements obtained per MFD, the
Stepratio variable, and/or the pulse shape.
[0125] Throughout this specification the word "comprise", or
variations such as "comprises" or "comprising", will be understood
to imply the inclusion of a stated element, integer or step, or
group of elements, integers or steps, but not the exclusion of any
other element, integer or step, or group of elements, integers or
steps.
[0126] It will be appreciated by persons skilled in the art that
numerous variations and/or modifications may be made to the
invention as shown in the specific embodiments without departing
from the spirit or scope of the invention as broadly described. The
present embodiments are, therefore, to be considered in all
respects as illustrative and not restrictive.
* * * * *