Apparatus and method for automatically determining stimulation parameters

Disclosed is an arrangement allowing for automatic calculation of stimulation parameters, for example dynamic ranges for stimulation, in an auditory prosthesis, for example a multichannel cochlear implant. The arrangement includes, in a preferred form, an electrode 12 for detecting activity of the stapedius muscle, and uses the electrode array 5 to sense neural response to stimulation, so that a maximum comfortable stimulation level and threshold level for each channel can be determined. The process may be initiated by the implantee, avoiding the requirement for external equipment and extensive audiological testing.

TECHNICAL FIELD
 The present invention relates to auditory prostheses, particularly but not
 exclusively cochlear implants.
 BACKGROUND ART
 The present invention relates generally to auditory prostheses, but will be
 principally described in relation to multi-channel cochlear implants. Such
 an implant conventionally consists of three components--an implanted
 electrode array, an implanted receiver/stimulator unit (RSU) and an
 externally worn speech processor. The speech processor receives sound
 signals, for example via a microphone, processes them so as to produce a
 set of signals corresponding to stimuli, then communicates these signals
 to the RSU. Communication between the speech processor and the RSU may be
 by an inductive link, a direct cable, or any other suitable means. The
 RSU, in accordance with the received signals, provides electrical
 stimulation signals to the electrode array.
 For each implantee, it is necessary to set the dynamic range of the
 stimulus pulses presented by the electrode array in order to optimally and
 comfortably enhance speech perception by the implantee. The dynamic range
 is generally set between two parameters--the threshold level (T), being
 the minimum amount of electrical stimulation that is required to elicit a
 perceived sound from the implantee, and the comfort (C) level, defined as
 the maximum amount of electrical stimulation which can be applied before
 the patient reports discomfort. The T and C levels typically vary for each
 channel in a multichannel implant.
 Conventional setting of the dynamic range uses an elaborate audiometric
 process, heavily reliant upon patient responses, to set T & C levels. A
 particular difficulty exists in relation to children, who are often unable
 to provide meaningful indications as to their perceptions and responses to
 various stimuli. Moreover, it would be desirable to allow patients to
 reset the dynamic range using an automatic process, as required, so that
 physiological variations in their perception can be accounted for. Some of
 these variations are routine--for example, commonly dynamic range will
 vary during a woman's menstrual cycle, or may vary with medication or
 illness. The present system for dynamic range setting requires the
 services of a trained audiologist in a clinic, and hence cannot provide
 routine resetting when required by the patient.
 Various workers have examined the use of either the stapedius reflex or
 various evoked action potentials with a view to objectively setting speech
 processors.
 The stapedius muscle, when contracted, acts as a dampening mechanism on the
 ossicular chain within the ear. In the normally functioning ear,
 contraction of the stapedius attenuates the vibration transmitted through
 the malleus, incus and stapes to the oval window, so as to prevent
 overstimulation of the auditory system. A survey of the prior art shows
 that the general approach to measuring the stapedius reflex has been to
 use an acoustic probe, placed in the ear contralateral to the applied
 stimulation, in order to measure the muscle's response via the mechanical
 impedance of the tympanic membrane. This approach allows for accurate
 measurement of the response of the stapedius but is not appropriate for
 implementation in an implanted device.
 For example, Battmer et al. (Electrically Elicited Stapedius Reflex in
 Cochlear Implant Patients--Ear and Hearing Vol. 11, No. 5, 1990),
 investigated the use of stapedius reflex evaluations for objective setting
 of cochlear implant speech processors. In contrast to the present
 invention they recorded the stapedius muscle's response to electrical
 stimulation of the cochlea by means of a contralateral acoustic impedance
 meter. The level of contraction of the stapedius muscle was used to
 determine both the T and C level.
 In a paper by Stephan et al ("Acoustic Reflex in Patients with Cochlear
 Implants" American Journal of Otology Vol 12, Supplement 1991) the authors
 indicated that psychoacoustic tests relying on evoked auditory responses
 and electrically elicited acoustic stapedius reflexes were of use in
 setting the patient's speech processor. However, that paper taught that
 acoustic reflex testing using contralateral detection is recommended over
 electrophysiologic methods because of the difficulties associated with
 overcoming the difficulties presented by artefacts in the
 electrophysiological methods.
 In a paper by Jerger et al, in Ear and Hearing, vol 9, No 1 (1988),
 entitled "Prediction of dynamic range for the stapedius reflex in cochlear
 implant patients", amplitude growth functions for an electrically-elicited
 stapedius reflex were compared with behavioural estimates of dynamic
 range. This paper concluded that comfort levels are typically greater than
 or equal to the saturation or plateau, level of stapedius response. The
 stapedius reflex, whilst electrically elicited, was measured using an
 external acoustic probe arrangement.
 In a 1990 paper by Shallop et al ("Electrically Evoked Auditory Brainstorm
 Responses (EABR) and Middle Latency Responses (EMLR) Obtained from
 Patients with the Nucleus Multi-Channel Cochlear Implant" Ear and Hearing
 Vol 11, No. 1) the technique of using EABR measurements to set dynamic
 range was investigated. The author's conclusion was that EABR and EMLR
 measurements correlate better with comfort levels than with threshold
 levels. In a paper by Shallop et al ("Prediction of Behavioural Threshold
 and Comfort Values for Nucleus 22 Channel Implant Patients from Electrical
 Auditory Brain Stem Response Test Results", Annals of Otology, Rhinology,
 & Laryngology, vol 100, No 11 (Nov 91)) the authors again discussed and
 investigated prediction of behavioural threshold and comfort level values
 using EABR procedures. In both of these papers, the neural response is
 obtained via a second monitoring mechanism not associated with the
 implant, and later correlated. The authors state that they are "cautious"
 about inferring T and C levels to be expected from speech from EABR and
 EMLR recordings.
 None of these papers disclose an arrangement, in which the parameter is
 electrically measured, and this measurement is directly input to the
 receiver stimulator unit for use in deriving dynamic range. Moreover,
 these papers do not disclose any arrangement which could automatically
 adjust dynamic range without input from skilled personnel.
 It is an object of the present invention to provide an arrangement in which
 at least one of the dynamic range parameters are automatically derived and
 processed, without the necessity for the implantee's perceptions to be
 subjectively assessed. It is a further object of the present invention to
 provide an auditory prosthesis arrangement in which the dynamic range
 parameters are able to be automatically reset by the implantee without the
 need for specialised external equipment and personnel.
 SUMMARY OF THE INVENTION
 According to a first aspect the present invention provides an auditory
 prosthesis including processing means for providing electrical stimulus
 signals to a stimulation means, said prosthesis including a sensor means
 adapted to sense physiological response to applied stimulation, said
 sensor means communicating with said processing means, and memory means
 communicating with said processing means to provide values for stimulation
 parameters to said processing means so that said processing means can
 define appropriate stimulus signals, wherein signals from said sensor
 means are processed by said processing means in accordance with a
 predetermined algorithm, so as to determine at least one stimulation
 parameter for at least one stimulation mode of said device, said value
 being stored in said memory means.
 According to another aspect the present invention provides an auditory
 prosthesis including processing means for providing electrical stimulus
 signals to a stimulation means, said prosthesis including a sensor means
 adapted to sense neural response correlating to an acoustic percept, said
 sensor means communicating with said processing means, and memory means
 communicating with said processing means to provide values for stimulation
 parameters to said processing means so that said processing means can
 define appropriate stimulus signals, wherein signals from said sensor
 means are processed by said processing means in accordance with a
 predetermined algorithm, so as to define a threshold stimulation level for
 at least one stimulation mode of said device, said value being stored in
 said memory means. Preferably, the neural response sensed is the EAP
 response of the basilar membrane.
 According to a further aspect, the present invention provides an auditory
 prosthesis including processing means for providing electrical stimulus
 signals to a stimulation means, said prosthesis including sensor means
 adapted to sense activity of the stapedius muscle, said sensor means
 communicating with said processing means, and memory means communicating
 with said processing means to provide values for stimulation parameters to
 said processing means so that said processing means can define appropriate
 stimulus signals, wherein signals from said sensor means are processed by
 said processing means in accordance with a predetermined algorithm, so as
 to define a maximum comfortable stimulation level for at least one
 stimulation mode of said device, said value being stored in said memory
 means.
 Preferably, the sensor means are arranged so as to electrically sense
 activity of the stapedius muscle. The sensor may be an electrode on or
 adjacent to the stapedius muscle.
 According to another aspect, the present invention comprises an auditory
 prosthesis adapted to automatically derive threshold and maximum
 comfortable stimulation levels so as to determine a dynamic range for
 electrical stimuli, said prosthesis including processing means for
 providing electrical stimulus signals to a stimulation means, first sensor
 means adapted to sense activity of the stapedius muscle, second sensor
 means adapted to sense a neural response correlating to an acoustic
 percept, and memory means communicating with said processing means to
 provide values for stimulation parameters to said processing means so that
 said processing means can define appropriate stimulus signals, said first
 and second sensor means communicating with said processing means, wherein
 signals from said sensor means are processed by said processing means in
 accordance with a predetermined algorithm, so as to define a threshold
 stimulation level and a maximum comfortable stimulation level for at least
 one stimulation mode of said device, said value being stored in said
 memory means.
 The present invention further relates to the methods for setting parameters
 in relation to the dynamic range of auditory prostheses, and to systems
 incorporating these methods.
 In its broadest form, the present invention is concerned with providing an
 auditory prosthesis which includes sensors communicating with a processing
 means so that the stimulation parameters of the prosthesis can be modified
 in response to the sensed response to the stimuli presented. It is
 envisaged that various stimulation parameters could be controlled in this
 way, avoiding the need for subjective, labour intensive adjustment of the
 parameters, and allowing the patient to select when these parameters need
 adjustment and perform the adjustment on demand.
 In a preferred implementation, the inventors proposes the use of
 electrically measured neural responses as a direct input to stimulation
 processor so as to define the dynamic range of an auditory prosthesis for
 a given patient. In a preferred aspect, the inventive device uses a
 combination of evoked neural potentials and electrically measured activity
 of the stapedius muscle to determine dynamic range without subjective
 assessment. Various workers have examined the use of either the stapedius
 reflex or various evoked action potentials with a view to objectively
 setting speech processors. This work has not contemplated providing an
 automatic system for use by the patient alone. The prior art cited above
 shows that the general approach to measuring the stapedius reflex has been
 to use an acoustic probe, placed in the ear contralateral to the applied
 stimulation, in order to measure the muscle's response via the mechanical
 impedance of the tympanic membrane. This approach allows for accurate
 measurement of the response of the stapedius but is not appropriate for
 the portability and convenience facilitated by the present invention.

DETAILED DESCRIPTION
 The present invention is described in the context of a multichannel
 cochlear implant. However, the principle of the present invention is
 applicable to related devices, including totally implanted devices, direct
 neural stimulation, and other auditory prostheses which are intended to
 produce a neural response to stimulation. Similarly, other or more
 stimulation parameters than dynamic range could be controlled using the
 principle of the present invention. Alternative sensors could be used to
 the stapedius activity and evoked response measurement via the electrode
 array which are proposed--for example, a separate evoked response array.
 The illustrative embodiment of the present invention makes use of an
 extracochlear electrode from a conventional receiver stimulator unit of a
 cochlear implant to monitor stapedius muscle activity. The intracochlear
 electrodes are used to monitor the electrical status of the auditory
 nerve. Both evoked action potential (EAP) of the auditory nerve and
 stapedius reflex information are telemetered back from the receiver
 stimulator to the wearable speech processor. The speech processor includes
 integral hardware and software to test for comfort and threshold setting
 levels by using the telemetered information, and applying a predefined
 algorithm, which will be discussed below. This enables levels to be set
 automatically by the patient at the press of a button. It will be
 appreciated that whilst this division between the processing functions of
 the receiver stimulator unit and the speech processor is convenient in
 terms of current cochlear implant technology, alternative implementations
 could be used, for example in the case of a fully implantable device. The
 location of the processing step is not critical to the general principles
 of the present invention.
 Referring to FIG. 1, the relevant anatomical features of the ear are
 illustrated. In the normally functioning ear, the tympanic membrane 1
 vibrates in response to ambient sound, and via the ossicular chain 2 the
 vibration is transferred to the oval window 3. The stapedius muscle 4
 operates in the normal ear to contract and hence damp mechanically the
 transmission of vibrations to the oval window 3. An electrode array 5 is
 shown implanted via conventional surgical procedures, inserted within the
 scala tympani 6 via the round window 7, and connected to the implanted
 receiver stimulator unit 8. Receiver-stimulator unit 8 communicates via an
 RF link with RF coil 9 and hence the speech processor 10. A microphone 11,
 illustratively mounted behind the pinna 25, provides sound signals to the
 speech processor. The implant described to this point is essentially a
 conventional arrangement.
 A further stapedius monitoring electrode 12 is attached to the stapedius
 muscle 4. This provides signals indicative of stapedius reflex activity.
 It may be attached either to the belly of the muscle or to the tendon
 which is a surgically easier point of attachment, or to any suitable site
 which enables a signal indicative of stapedius activity to be detected.
 According to the preferred implementation of the present invention, the
 neural response of the auditory nerve 26 and basilar membrane 27 evoked by
 stimulation may be monitored using the implanted electrode array 5. Thus,
 the implanted array 5 is used both to provide stimuli, and to measure the
 response to such stimuli during the period between stimuli. Such a
 monitoring arrangement and telemetering arrangement is described in
 Australian patent application No. 56898/94 by the present applicant, the
 disclosure of which is hereby incorporated by reference.
 The stimulations are delivered by means of a number of "channels". For
 example, the delivery of a stimulation current between two particular
 electrodes of the array may be defined as a stimulation via channel 1.
 Similarly other combinations of electrodes involved in stimulation
 delivery will also define other stimulation channels. Extra-cochlear
 electrode 13, which is also used in some conventional arrangements, is
 used as the reference electrode in measuring the evoked action potential
 of the auditory nerve and the electrical activity of the stapedius.
 The EAP response, detected by the electrode array 5, and the response of
 the stapedius, monitored by the stapedius monitoring electrode 12, are
 detected by the receiver stimulator unit 8 relative to the reference
 electrode, and then telemetered back to the speech processor. As in known
 arrangements speech processor 10 sends signals via the RF link to receiver
 stimulator unit 8, which then provides stimulus pulses via the electrode
 array in accordance with the commands sent by speech processor 10.
 T&C switch 14 is pressed by the patient to initiate the T&C level setting
 procedure. FIG. 2 shows the components of the device in block form,
 including microphone 11, audio pre-processing 25, central processing unit
 (CPU) 22, and transcutaneuous link 15.
 With reference to FIG. 2 the operation of the present invention will now be
 described. On pressing the T&C switch 16 the CPU 22 is directed to
 automatically calculate the patient's required T and C levels. Initially
 the Automatic T&C Level Program 17 is retrieved from program storage
 memory 28. The CPU then steps through the program. Firstly the system is
 put into a telemetry mode whereby the response of the auditory nerve to
 stimulation can be monitored. The CPU transmits the code for a stimulus
 pulse via the data transmitter 19 and transcutaneous link 15. The
 transmission contains information as to which electrodes are to deliver
 the stimulation and the stimulation amplitude and duration which are
 retrieved from the patient data storage memory 24. The received
 transmission is decoded by the receiver-stimulator 20 and the prescribed
 stimulation is applied. The evoked action potential of the auditory nerve
 in response to the stimulation is monitored by the receiver-stimulator and
 telemetered back to the telemetry receiver 21 via the transcutaneous link
 15. This procedure is repeated several times and the recorded data is
 conditioned and tested for significance as will be explained subsequently.
 At the end of this procedure a figure is reached for the EAP response
 derived threshold level of the implantee. It has been found experimentally
 that the stimulus level which elicits a definite EAP response is
 significantly higher than the T level derived by subjectively testing
 patients. Accordingly the final T level value is derived from the final
 stimulation level after suitable adjustment and then stored as an entry in
 the patient data storage T&C level table 23. The entire procedure is then
 repeated for all stimulation channels.
 Once the T levels have been calculated for each stimulation channel those
 levels are used as a starting point for calculating the C levels. In the
 previously described manner the CPU transmits the code for a stimulus
 pulse via the data transmitter 19 and transcutaneous link 15. The first
 stimulation pulse is transmitted with a stimulation level equal to the T
 level for the stimulation channel. The electrical activity of the
 stapedius muscle is measured both when there is and when there is not
 application of stimulation and by a method which will shortly be described
 in more detail the C level for each stimulation channel is determined.
 These levels are stored as entries in the patient data storage T&C level
 table 23.
 The overall operation of the invention which has now been described is
 depicted as a flow chart in FIG. 3. After startup 31 the system enters
 telemetry mode 32 as the information regarding the electrical activity of
 the auditory nerve and the stapedius muscle are to be sent to the speech
 processor. The T levels are then calculated for each channel and stored in
 the T&C level table at step 33. Using the T levels as a starting point the
 C levels are then derived for each channel and similarly stored in the T&C
 level table 34. The cochlear implant then returns to normal operation 36
 using the newly defined dynamic range. The T&C level setting program then
 ends 37.
 The details of box 33 will now be described. The steps involved in the
 process of determining the T levels are shown diagrammatically in FIG. 4
 which is a flowchart of the process. Before entering a first loop relevant
 stimulation parameters including pulse width and inter-phase gap duration
 are retrieved from memory. The number of the channel whose T level is to
 be derived is set to one and the stimulation level that is to be applied
 is set to a minimum level which has been empirically found to be below
 that capable of evoking an auditory nerve response. Alternatively the
 minimum current level could simply be set to zero.
 A single stimulus pulse is then delivered at the minimum amplitude by
 channel 1. Any response to the stimulus pulse is telemetered back to the
 CPU 22 according to 44. The procedure then cycles through blocks 45, 43
 and 44 until several responses have been measured. At the end of this
 process the average of the readings is stored according to 46. The values
 stored at 46 represent the EAP response to the stimulation but said
 response is also heavily affected by an artefact due to the evoking
 stimulation. This artefact must be removed in order to gain an accurate
 value for the EAP response.
 At 47 the program undertakes signal conditioning procedures in order to
 lessen the effects of said artefact. One previously published way of
 performing said conditioning is the `double pulse` method which will be
 described shortly.
 The amplitude of the EAP response is evaluated at 48 and stored in variable
 Delta. Decision box 49 tests Delta for significance against a preset
 value. If Delta is found to be insignificant then no T level is deemed to
 have been detected and so the current level of the applied stimuli is
 increased at 50. The process then loops until the current level is
 sufficiently high to enable the "Delta&gt;Preset" threshold condition of
 decision block 49 to be met. In that case a stimulation level at which a
 significant EAP response is elicited is deemed to have been reached for
 the stimulation channel under test. However, as has been discussed
 previously it has been found experimentally that said stimulation level is
 significantly higher than the optimal T level and so the T level is found
 by reducing the value Stim_level either by means of known algorithms or by
 an empirically determined amount. In the embodiment of FIG. 4 the level is
 set at box 51 to 80% of the above threshold stimulation level. The final
 value of the T level is then stored as a table entry in the T&C Patient
 Data Storage table 23. The decision box 53 tests whether or not the T
 level has been found for all channels and if not then the previously
 described process is repeated until completion.
 In order to further clarify the previous procedure the steps involved in
 measuring the nerve's EAP response to stimulation, items 43-48 will now be
 described with references to FIGS. 5-11. As stated in box 43 a stimulation
 is applied to the auditory nerve. In response to the applied stimulation a
 response of the form shown in FIG. 5 is elicited and data from said
 waveform is measured and telemetered to the speech processor 10. FIG. 5
 shows that the response is obscured by noise. Accordingly the experiment
 is performed a number of times, indicated by the integer n in the present
 embodiment, and an average graph, as shown in FIG. 6, corresponding to the
 instructions of item 46 is obtained in order to reduce the obscuring
 effects of random noise. An example of the signal conditioning referred to
 in box 47 will now be explained.
 Two successive stimulus pulses are applied about 1 ms apart. The patient's
 response is measured after the application of the second of the successive
 stimulations. The first pulse recruits the nerve so that the recording
 after the second pulse produces only the artefact with no neural response
 component present. The average waveform that is derived from repeating
 this procedure several times is depicted in FIG. 7. FIG. 8 is a graph of
 the difference between the data depicted in FIG. 7 and that of FIG. 6.
 That is, it is the result of subtracting the recorded artefact from the
 data representing the combined EAP response and artefact. In practice,
 even after this subtraction, there remains a small though significant
 amount of artefact superimposed on the neural response. The artefact
 consists of an exponentially decaying low frequency signal. The signal is
 further conditioned to enhance the fidelity of the EAP signal by twice low
 pass filtering the combined signal depicted in FIG. 8. The first filtering
 is shown in FIG. 9 and is conveniently achieved by taking a seven point
 moving average of the data presented in FIG. 8. Similarly the second
 filtering shown in FIG. 10 is simply the seven point moving average of the
 data in 9. Thus the signal depicted in FIG. 10 consists largely of the
 residual artefact. This artefact signal is subtracted from the combined
 EAP response and residual artefact of FIG. 8 and the resulting EAP
 response to the stimulation is plotted in FIG. 11. This method of
 extracting the EAP response from the combined response and amplitude
 corresponds to the step described as box 47 of the flowchart depicted in
 FIG. 4. Apart from the "double pulse" method other signal conditioning
 known in the art could also be used at box 47. The standard deviation of
 the data is calculated where the neural response has the greatest range,
 that is, across the range indicated by the double headed arrow 60. This
 value is proportional to the size of the EAP response. Determination of
 this value corresponds to the value Delta of box 48.
 The previously described procedure of calculating Delta is repeated with
 increasing stimulation levels as indicated by box 50 until Delta is deemed
 to be greater than an empirically measured threshold. Said threshold is
 derived by testing a population of cochlear implant patients and is
 factory set and stored in the system memory 24. As previously described it
 has been found that the current level at which the first significant EAP
 response is detected is usually higher than the patient's actual T level
 and so the T level is determined to an adjusted value of Delta. This
 adjustment is shown at box 51 and an example is given there of simply
 setting the T level to 20% below the stimulation level that was found to
 generate a significant EAP response. Other transformations could also be
 used for this step and are known in the prior art, see for example Parkins
 and Colombo Hearing Research, 31 (1987) pp267-286.
 Once the T level has been determined and recorded for each stimulation
 channel the procedure for calculating the C level is embarked upon. The
 steps for doing this are shown in the block diagram of FIG. 12. Boxes 72
 to 78 describe a method for determining the magnitude of the muscle's
 response to the application of a current of amplitude set by the variable
 "Stim_level" delivered by means of stimulation channel "Stim_channel". The
 steps dictated by each of those boxes will be described with reference to
 FIGS. 13 and 14.
 Initially from commencement time up to the first half second no stimulation
 is applied in accordance with box 72. This waiting period is included to
 ensure that the muscle has had sufficient time to emerge from any
 refractory period, Throughout the next 0.5 s the electrical activity of
 the stapedius muscle 4 is monitored via an extra-cochlear electrode 12
 placed either on, in, or near to the muscle. During this period no
 stimulation is applied. The activity of the muscle is frequently sampled
 at periods of t.sub.s secs and the average of each of the samples taken in
 that time are used to form a set of envelope values A.sub.off1 . . .
 A.sub.offn as shown in box 73. These values are represented as crosses in
 the A.sub.off range 91. Subsequently stimulation 94 is applied at time
 =0.5 s up until time =1.5 s as determined by boxes 75 and 76. The
 stimulation consists of high frequency biphasic current pulses, typically
 of the form depicted by item 95 which is intended to be an enlargement of
 two cycles of stimulation 94. In response to application of the
 stimulation the electrical activity of the stapedius muscle is as shown in
 sections 96, 97, 98. Section 96 exhibits behaviour in accordance with the
 "onset" effect of the stimulation whereby the electrical activity of the
 muscle "ramps-up" to the plateau of section 97. Upon cessation of
 stimulation at time=1.5 s there is a decay of muscle activity 98 until a
 lower plateau region 99 is reached. In order to detect the C level the
 envelope of the recorded voltage 90 is detected and plotted at intervals
 as crosses 100. The portion of the envelope prior to cessation of the
 stimulation applied during period 94 is defined as the "A.sub.on range"
 92. The average of the envelope values 100 during the A.sub.on range 92 is
 defined as
 ##EQU1##
 Similarly the portion of the envelope 100 during the period prior to
 application of a burst of stimulation 91, is defined as the "A.sub.off
 range" 91. The average of the A.sub.off range of values is defined as
 ##EQU2##
 In order to find the amplitude 114 of the stimulation 94 that must be
 applied to elicit muscle activity indicative as being in response to the
 patients C level, the amplitude 114 of the applied stimulation is
 gradually increased until the difference 115 between
 ##EQU3##
 is found to exceed a preset threshold. Said preset threshold is an
 empirically determined value, which can be determined from studies on a
 population of cochlear implant patients. Once the current level at which
 this significance criterion is met has been found then a transformation is
 applied by which the stimulation current level is increased by a small
 amount. It has been found that without this transformation the patient's C
 level is set significantly less than at an optimal level.
 Items 72-78 comprise the steps of calculating a value
 ##EQU4##
 for the average level of muscle activity during the period of no
 stimulation 91 and the average level of muscle activity,
 ##EQU5##
 during the period of stimulation 92 at an intensity of stimulation given by
 `Stim_level`. The stimulation level is tested at box 79 for significance
 by comparing
 ##EQU6##
 If the difference of the two is less than the empirically derived preset
 threshold then the parameter Stim_level is increased at box 80 and the
 procedure is repeated until the Stim_level reaches a magnitude where the
 muscle activity, represented by
 ##EQU7##
 in response to stimulation is above the preset threshold. In that case the
 stimulation level transformation is applied at box 81. In the present
 embodiment the transformation comprises setting the C_level to 10% above
 the first significant value of Stim_level. C_level is recorded as the C
 level for the stimulation channel under test and stored as part of the T&C
 level table in memory 23 of the processor. The whole procedure is then
 repeated until the C level has been derived and stored for all channels.
 The system as described so far facilitates the automatic recalibration of
 T&C levels for all channels. The time taken to perform said recalibration
 is of the order of twenty minutes. It may be though that the patient
 desires recalibration only of the T levels or of the C levels but not
 both. Furthermore it may be that only some stimulation channels require
 recalibration and that most are operating between comfortable and
 detectable levels of stimulation. Therefore, a further aspect of the
 invention is that the user may at his or her option request calibration of
 only certain selected channels and either T or C levels. By reducing the
 extent of the recalibration the time taken to perform the operation is
 reduced.
 The channels to be recalibrated may be designated by the user by means of a
 simple selection system. For example, on pressing T&C switch 14 the speech
 processor 10 may produce a sequential stimulation at each channel. The
 user could then again press switch 14 in order to request recalibration of
 the T and C levels for that channel. If the user did not press the switch
 within a short time frame then the processor would quickly move on to the
 next channel so that only selected channels would be recalibrated and the
 time taken for the overall procedure would be limited to only that needed
 to adjust problematic levels.
 It will be appreciated that the algorithms used are merely illustrative,
 and alternative techniques may be used within the general concept of
 electrically evoked and measured parameters being used as a basis for
 automated level setting.
 It will also be understood that the present invention contemplates either
 the T or C levels only being automatically set as described, with
 alternative techniques being used for the other of C and T levels.
 Preferably, however, both T and C levels are determined as set out above.
 It will be further understood that the present invention contemplates that
 the automatic procedures may be customised further by an audiologist or
 physician, for example to manually alter levels, fix levels for some
 channels independent of the automatic procedure, or utilise special rules
 for certain implantees.