AU2005203696A1 - Cochlear implant fitting - Google Patents

Description

AUSTRALIA

Patents Act 1990 COCHLEAR LIMITED COMPLETE SPECIFICATION STANDARD PATENT Invention Title.

Cochlear implant fitting The following statement is a full description of this invention including the best method of performing it known to us:- 2 Reference to Related Application The present application claims the benefit of Australian Patent Applications No.

2005201999 and No. 2004902462, the content of each of which is incorporated herein by reference.

Technical Field The present invention relates to fitting of cochlear implants, and in particular to an electrode array fitting which allows the auditory nerve to be stimulated by both acoustic and electrical stimuli.

Background Art Cochlear implants have been developed to assist people who are profoundly deaf or severely hearing impaired, by enabling them to experience a hearing sensation representative of the natural hearing sensation. For most such individuals the hair cells in the cochlea, which normally function to transduce acoustic signals into nerve impulses which are interpreted by the brain as sound, are absent or have been partially or completely destroyed. The cochlear implant therefore bypasses the hair cells to directly deliver electrical stimulation to the auditory nerve with this electrical stimulation being representative of the sound.

Cochlear implants have traditionally consisted of two parts, an external speech processor unit and an implanted receiver/stimulator 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 using a microphone and convert the detected sound into a coded signal through an appropriate speech processing strategy.

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 processes the coded signal into a series of stimulation sequences which are then applied directly to the auditory nerve via a series or an array of electrodes positioned within the cochlea, proximal to the modiolus of the cochlea. One such cochlear implant is set out in US Patent No. 4,532,930, the contents of which are incorporated herein by reference.

With improvements in technology it is possible that the external speech processor and implanted stimulator unit may be combined to produce a totally implantable cochlear implant unit that is capable of operating, at least for a period of time, without the need for any external device. In such a device, a microphone would be implanted within the body of the user, for example in the ear canal or within the stimulator unit, and sounds would be detected and directly processed by a speech processor within the stimulator unit, with the subsequent stimulation signals delivered without the need for any transcutaneous transmission of signals. Such a device would, however, still have the capability to communicate with an external device when necessary, particularly for program upgrades and/or implant interrogation, and if the operating parameters of the device required alteration.

Much effort has gone into developing stimulation strategies to provide for device customisation in order to produce the best available percepts for the prosthesis recipient. Nevertheless it is acknowledged in the cochlear implant field that the percepts produced by pulsatile electrical stimulation often sound unnatural and somewhat harsh. Although many patients adapt to this sound and, after some time, even find it natural, this is not always the case and some patients may experience difficulties. In some instances in the past, for potential implant recipients having some amount of residual natural hearing, the expectation of harsh and/or unnatural sounding percepts produced by cochlear implants has been less attractive than simply persisting with residual hearing, usually assisted by an acoustic hearing aid.

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 before the priority date of each claim of this application.

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.

Summary of the Invention According to a first aspect the present invention provides a method of electrically and acoustically stimulating a cochlea, the method comprising: providing an acoustic signal delivery path to the cochlea; providing an electrical signal delivery path to the cochlea, for processing a first frequency sub-range of a detected sound signal for electric stimulation of the cochlea; imposing a delay on at least one of the acoustic signal delivery path and the electrical signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

According to a second aspect, the present invention provides a method of fitting a cochlear prosthesis to a cochlea having residual acoustic hearing capability such that the cochlea can be electrically stimulated via an electrical signal delivery path and acoustically stimulated via an acoustic signal delivery path, the method comprising: configuring a delay to be imposed on at least one of the acoustic signal delivery path and the electrical signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

According to a third aspect the present invention provides a cochlear prosthesis comprising: an electrical signal delivery path to the cochlea, for processing a first frequency sub-range of a detected sound signal for electric stimulation of the cochlea; and delay means for imposing a delay on at least one of the electrical signal delivery path and an acoustic signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

According to a fourth aspect the present invention provides a speech processor for a cochlear prosthesis, the speech processor for processing a first frequency subrange of a detected sound signal for electric stimulation of the cochlea via an electrical signal delivery path, the speech processor comprising delay means for imposing a delay on at least one of the electrical signal delivery path and an acoustic signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

Providing an acoustic signal delivery path may comprise simply a passive acoustic delivery path along which acoustic sound travels from the outer ear through the middle ear to the cochlea, by ensuring no acoustic obstructions are positioned along the normal acoustic hearing path. In such embodiments, the delay will be imposed upon the electrical signal delivery path.

Alternatively the acoustic signal delivery path may comprise an acoustic sound processor for processing and/or amplifying a second frequency sub-range of the detected sound signal, and acoustically delivering the processed acoustic sound signal to the cochlea. In such embodiments the delay may be imposed upon either or both of the electrical signal delivery path and the acoustic signal delivery path. The second frequency sub-range may correspond to a residual natural hearing capability of the cochlea. The acoustic processing may comprise a plurality of acoustic channels. A channel-specific delay may be imposed upon each acoustic channel. A channelspecific gain may be applied to each acoustic channel.

The delay may be substantially equal to a difference between the time taken for a signal to travel along the electrical signal delivery path and the time taken for a signal to travel along the acoustic signal delivery path, such that the cochlea is substantially simultaneously stimulated electrically and acoustically in response to the detected sound signal. In such embodiments, depending on the magnitude of the time taken for a signal to travel along the electrical signal delivery path and the magnitude of the time taken for a signal to travel along the acoustic signal delivery path, the delay may be imposed on either the electrical signal delivery path or the acoustic signal delivery path in order to achieve substantially simultaneous electrical and acoustic stimulation. A non-zero delay may be imposed upon both the electrical signal delivery path and the acoustic signal delivery path. Preferably, the delay is kept sufficiently small that a user does not perceive undesirable effects such as a perceivable delay between another person's lips moving during speech, and signals corresponding to the speech being delivered to the cochlea.

In further embodiments of the invention, the electrical signal delivery path may comprise a plurality of channels, wherein the first frequency sub-range of the detected sound signal is divided into a plurality of frequency bands, with one frequency band processed in each channel. In such embodiments, a channel-specific delay may be applied to each channel. The channel-specific delay of each channel may be configured to delay low frequency electrical channels for a time longer than that by which high frequency electrical channels are delayed, to mimic the time taken for sound to travel from a basal region of the cochlea tonotopically corresponding to the high frequency channels to a more apical region of the cochlea tonotopically corresponding to the low frequency channels.

The first frequency sub-range may comprise a portion or portions of the audible frequency spectrum in respect of which active stimulus electrodes have been tonotopically positioned proximal to the cochlea. For example the first frequency subrange may comprise a high frequency portion of the audible frequency spectrum, where active electrodes have been positioned proximal to a basal portion of the cochlea.

In preferred embodiments of the invention, the imposed delay is configurable.

The imposed delay may be clinically configurable such that a clinician may optimise the configuration of the delay applied. Such embodiments enable the delay to be optimised to recipient-specific characteristics, such as the dimensions of the outer, middle and inner ear of the specific recipient, variations caused by surgical implantation outcome, the respective tonotopic location of the first frequency sub-range and the second frequency sub-range along the cochlea, and psychophysiological characteristics of the specific recipient.

Additionally or alternatively, the imposed delay may be field configurable, such that an implant recipient or associated person may perform ongoing configuration of the delay to suit personal preference or to adapt to differing sound environments.

Configuration of the delay, whether clinical configuration or field configuration, may be performed on a channel-by-channel basis in multi-channel devices. Sensing of a neural response, for example in accordance with the teachings of International Patent Publication No. WO 02/082,982, the content of which is incorporated herein by reference, may be used to assist or implement delay configuration.

Brief Description of the Drawings By way of example only, preferred embodiments of the invention will be described with reference to the accompanying drawings, in which: Figure 1 is a pictorial representation of a cochlear implant system; Figure 2 is a block diagram of a device for applying acoustic stimulation and electrical stimulation to a cochlea at a controlled time relative to each other; Figure 3 is a block diagram of a device for applying electrical stimulation to a cochlea at a controlled time relative to normal acoustic stimulation; Figure 4 illustrates the impact of surgical implantation upon residual hearing of a cochlea; Figure 5a is a schematic drawing of an implementation of intra-operative determination of insertion depth of an electrode array to a cochlea having residual acoustic hearing capability; Figures 5b and 5c are charts of measured ECAP responses in the implementation of Figure Figure 5d is a flowchart illustrating the intra-operative process of Figure Figure 6a is a schematic drawing of post operative determination of a patient map for a cochlea having residual acoustic hearing capability; Figures 6b and 6c are charts of measured ECAP responses in the implementation of Figure 6a; Figure 6d is a flowchart illustrating the post-operative process of Figure 6a; Figure 7 is a schematic drawing of mapping a device using both electrical and acoustic modes of stimulation; and Figure 8 illustrates a system for clinical testing and/or fitting of a cochlear implant to be used to supplement the residual acoustic hearing capability of a cochlea.

Detailed Description of the Invention Before describing the features of the present invention, it is appropriate to briefly describe the construction of a cochlear implant system with reference to Fig. 1.

Cochlear implants typically consist of two main components, an external component including a sound processor 29, and an internal component including an implanted receiver and stimulator unit 22. The external component includes an onboard microphone 27. The sound processor 29 is, in this illustration, constructed and arranged so that it can fit behind the outer ear 11. Alternative versions may be worn on the body or it may be possible to provide a fully implantable system which incorporates the speech processor and/or microphone into the implanted stimulator unit. Attached to the sound processor 29 is a transmitter coil 24 which transmits electrical signals to the implanted unit 22 via an RF link.

The implanted component includes a receiver coil 23 for receiving power and data from the transmitter coil 24. A cable 21 extends from the implanted receiver and stimulator unit 22 to the cochlea 12 and terminates in an electrode array 20. The signals thus received are applied by the array 20 to the basilar membrane 8 thereby stimulating the auditory nerve 9. While the cochlea is generally spiral shaped as shown, it is convenient to describe electrode positions and the like as being "along" the cochlea between a basal end of the cochlea and an apical end of the cochlea as if the cochlea were unrolled to lie straight. The operation of such a device is described, for example, in US Patent No. 4,532,930.

The sound processor 29 of the cochlear implant can perform an audio spectral analysis of the acoustic signals and outputs channel amplitude levels. The sound processor 29 can also sort the outputs in order of magnitude, or flag the spectral maxima as used in the SPEAK strategy developed by Cochlear Ltd. Multi-channel adaptive processing may be applied, for example by use of the ADRO technique set out in United States Patent No. 6,731,767.

With the continued improvement in performance of Cochlear implants, more recipients receiving implants have appreciable residual hearing capability. The present invention provides for bimodal stimulation of the cochlea by electrical and acoustic stimulation, to take advantage of the recipient's residual natural hearing capability, while supplementing that natural hearing with electrical stimuli to convey sound information which is only partially conveyed or is not conveyed by the natural hearing of the recipient. The bimodal stimulation may be controlled by a speech processor having the ability to process a detected sound to produce both electrical stimulations for application by an electrode array and acoustic stimulations for application by a hearing aid. Alternatively, a speech processor for producing electrical stimulations may be used alone, without generation of any acoustic stimulation beyond that naturally entering the ear. Alternatively, a first speech processor for generating electrical stimulations may be used in conjunction with a second speech processor for producing acoustic stimulations.

Further, it is desirable to optimise the combination of electrical and acoustical stimulation in the fitting process, whether intra-operatively in the positioning of the electrode array, or post-operatively in establishing an optimal patient map, or both. It may further be necessary to determine which electrodes are to be active and which are to remain inactive, to determine the frequency allocation of each active electrode so as to optimise the combinatory hearing, and to apply channel specific gain to optimise a timing of delivery of stimuli by each active electrode.

Figure 2 is a block diagram of a sound processor for applying acoustic stimulation and electrical stimulation to a cochlea at a controlled time relative to each other. A microphone 200 detects sound signals and passes corresponding electrical signals to a pre-processor 202. Pre-processor 202 filters the detected signal and passes a signal component in a first frequency sub-band to an electrical signal analyser 208, and passes a signal component in a second frequency sub-band to an acoustic signal analyser 204. In this embodiment the first frequency sub-band comprises a high frequency portion of the audible frequency spectrum, which corresponds to a basal region of a cochlea adjacent to which a shortened electrode array has been positioned.

The second frequency sub-band comprises a low frequency portion of the audible frequency spectrum, which corresponds to residual natural hearing of the cochlea.

The sound processor of Figure 2 further comprises a loudspeaker 206 for acoustically stimulating the cochlea, and a cochlear implant 210 for electrically stimulating the cochlea.

In accordance with the invention acoustic signal analyser 204 and electrical signal analyser 208 comprise respective delay circuits, for delaying one or both of the electrical signals in order to ensure substantially simultaneous stimulation of the cochlea by the loudspeaker 206 and the cochlear implarit 210.

Figure 3 is a block diagram of a speech processor for applying electrical stimulation to a cochlea at a controlled time relative to normal acoustic stimulation. A sound signal 301 is detected by a microphone 300, and also passes along the natural acoustic delivery path 303 to the cochlea. Microphone 300 passes a corresponding electrical signal to pre-processor 302, which in turn passes a pre-processed electrical signal to the electrical signal analyser 304. Analyser 304 passes signals to a cochlear implant for electrical stimulation of the cochlea.

Analyser 304 further comprises a delay circuit for delaying the electrical signal in order to ensure substantially simultaneous stimulation of the cochlea by the cochlear implant 210 and by the sound passing along acoustic delivery path 303, from the outer ear, through the middle ear, to the inner ear.

Figure 4 is a circuit schematic of a processing path 400 of a signal for acoustic stimulation of the cochlea. An input signal 402, having been generated by a microphone, pre-amplified, converted from analogue to digital and passed through automatic sensitivity control circuitry, is input to an acoustic gain amplifier 410, and then passed through a 1:4 down-sampler 412, an input automatic gain controller (AGC) 414 and an AGC gain 416. A filter bank 418 divides the signal into eight frequency bands 420a 420h. Each frequency band 420 is processed by a respective AGC 422 and AGC gain 424. In accordance with the invention, each channel is then passed through a delay circuit 426, with each delay circuit 426 applying a channel specific delay. The channels are then reconstructed by a reconstructor 428, and passed through a volume control amplifier 430, a peak clipper 432 and a 1:4 up-sampler 434 before being passed to a loudspeaker (not shown) for acoustic stimulation of the cochlea.

Thus, in addition to processing path 400, an acoustic signal delivery path comprises the propagation path of the acoustic signal from the loudspeaker, through the outer ear and middle ear into the inner ear.

Embodiments of the present invention may further recognise that appropriate configuration of the delay can be assessed objectively without requiring subjective patient responses, by detecting a neural response evoked by acoustic and/or electrical stimulation. The sensing of the evoked neural response is preferably performed in accordance with the method set out in International Patent Publication Number WO 02/082,982, the contents of which are incorporated herein by reference. By thus eliminating the need for subjective patient responses in determining the respective timing of delivery of acoustic and electrical stimuli, such embodiments may be particularly advantageous where the recipient is a young child unable to indicate subjective responses to cochlea stimuli.

A further embodiment of the invention is illustrated in Figures 5a to 5d. In this embodiment, an optimal insertion depth of an electrode array is intra-operatively determined. In patients with residual hearing it is difficult to determine a suitable depth of insertion of the electrode array, due to difficulties in precisely determining the location of the electrode, or in precisely determining the location of surviving neural elements that still can respond to acoustical stimulation. In this embodiment, since residual hearing usually exists on the more apical (lower frequency) part of the cochlea; the electrode array is introduced only until the point where usable hearing begins. An advantage of this technique is that trauma is reduced to a minimum, and that the cochlear mechanics (especially at the point where residual hearing is present) are influenced as little as possible. That is, no portion of the electrode array is positioned adjacent to the apical portion of the cochlea having residual acoustic hearing capability, and the array is thus less likely to interfere with or damage the residual acoustic hearing ability of that portion of the cochlea, while providing electrical stimulation to the basal portion of the cochlea lacking in acoustic hearing capability. Accordingly, surgical trauma to the apical portion of the cochlea may be avoided or minimised by this method.

The optimal insertion depth is determined intra-operatively during the insertion of the electrode array, by investigating the interaction between electrically evoked and acoustically evoked neural responses locally at the point of insertion of the tip of the electrode array. The electrode array is advanced by increments into the cochlea, with the interaction being determined after each incremental advance. When an increase of the interaction strength is determined the insertion is halted, or the electrode may be slightly withdrawn, as the electrode tip has then reached a point where residual hearing exists.

In the present embodiment, the interaction between the electrical and acoustical interaction is evaluated as follows. Initially, the most apical electrode records a first compound action potential evoked by application of an electrical stimulus of a given amplitude. The most apical electrode is then used to record a second compound action potential evoked by application of an essentially identical electrical stimulus of the same amplitude, in the presence of a background acoustic noise, preferably a narrowband background acoustic stimulus of a frequency substantially corresponding to the position of the tip electrode, applied to the cochlea. Should the first recorded ECAP and the second recorded ECAP be substantially identical, it can be assumed that the apical electrode of the array is yet to reach the residual functional 'normal hearing' part of the cochlea. However, should the first recorded ECAP and the second recorded ECAP substantially differ, this gives an indication that the presence of masking acoustic noise is evoking a component of neural response which interferes with that evoked by the electrical stimulus. Accordingly, the portion of the cochlea proximal to the apical electrode exhibits residual acoustic hearing capability.

This procedure is schematically indicated in Figure 5a. At 20, the recipient's audiogram is displayed, with considerable residual acoustic hearing capability evident in the low frequencies. A cut-off frequency for useful hearing is indicated at 21, which also marks the place in the unrolled cochlea diagrams 24, 27 where residual hearing terminates. In the unrolled cochlea, the area 25, 28 of the cochlea that can be measurably masked by a broadband acoustical masking stimulation is indicated by a hashed background. If the electrode array 22 is inserted with the tip in the deaf part of the cochlea (cochlea diagram 24) and an ECAP is recorded at the apical electrode with and without a background acoustical masker, no difference in ECAP amplitude is expected, as indicated by the ECAP graph of Figure 5b. However, when the electrode array tip is introduced into the acoustically maskable region 28 (corresponding to the region containing residual hearing), as shown in cochlea diagram 27, the ECAP amplitude with a background acoustical masker present decreases as compared to the unmasked NRT, as indicated by the ECAP graph of Figure Figure 5d is a flowchart of the intra-operative process illustrated in Figures 5a to At 30 the process commences, after which the electrode array is incrementally advanced by a small amount into the cochlea at 31. An electrical stimulus is then applied at 32 in the absence of acoustic stimulation, and a first ECAP denoted ECAPx is recorded at 33 using the apical electrode of the array. Subsequently, another electrical stimulus substantially identical to the first stimulus is applied at 34 simultaneously with an acoustical masking stimulation. The apical electrode is again used at 35 to measure a second ECAP, denoted ECAPy. At 36 a comparison is made between ECAPx and ECAPy. Should ECAPx be substantially identical to ECAPy, this indicates that the presence of the acoustic masking signal has made no measurable difference to the evoked neural response, thus indicating that there exists no residual acoustic hearing capability at the current position of the apical electrode. Accordingly, the process returns to 31 at which the electrode array is again incrementally advanced into the cochlea and the process is repeated to determine whether residual acoustic hearing capability exists at the new location of the apical electrode. Should ECAPx be different to ECAPy,, this indicates that the presence of the acoustic masking signal has made a difference to the evoked neural response, thus indicating that residual acoustic hearing capability exists at the present position of the apical electrode. Accordingly, at 37 the insertion is halted and the process ends at 38.

In addition to merely determining a point at which residual acoustic hearing capability commences, the process may further include assessing a relative strength of the acoustic hearing capability beyond the threshold, by further inserting the electrode array during the operation for such assessment, and withdrawing the array to its desired post operative position prior to the conclusion of the operation.

In a preferred embodiment the electrode array has a number of electrodes which is significantly greater than the number of channels to be applied by the speech processing scheme to be implemented by the speech processor. Such an electrode is set out in International Publication No. WO 03/003791, the contents of which are incorporated herein by reference. Providing an increased number of electrodes from which to choose for use in applying each signal channel recognises that, depending on the surgical implantation process, some of the electrodes may be positioned adjacent the apical portion of the cochlea and thus may be inactivated, and/or some of the electrodes may be positioned outside the basal end of the cochlea. Providing an electrode array with sufficiently many electrodes ensures that a sufficient number of the electrodes are adjacent that portion of the cochlea which lacks adequate residual acoustic hearing capability. Such embodiments cater for the application of, for example, all 22 signal channels of an ACE speech processing scheme to only that portion of the cochlea which lacks adequate residual acoustic hearing capability, and provide for finer frequency resolution between signal channels over that portion of the cochlea. To enable such fine frequency resolution, the speech processing scheme implemented is preferably applied to a subset of the audible frequency range tonotopically corresponding to the portion of the cochlea lacking adequate residual acoustic hearing capability.

Alternatively, once a suitable depth of insertion has been determined, a selection may be made of a suitable length electrode to implant. That is, the electrode used for the above determination of suitable insertion depth may be withdrawn, and an electrode of suitable length may be selected and then implanted to the appropriate insertion depth to conclude the surgical procedure. However, care must be taken to avoid or limit damage to the cochlea during such an operative procedure.

Once such partial insertion is complete, a patient map should be determined to allocate suitable frequencies and amplitudes C and T levels) to each electrode of the partially inserted array. In particular, in allocating frequency bands to electrodes in the patient map, it is desirable to avoid: the tip electrode(s) applying frequencies which are heard naturally in the more apical part of the cochlea (potentially leading to a "duplicate perception" at that frequency); or the tip electrode frequency being too high and leaving a frequency "gap" which is neither heard naturally nor conveyed electrically.

Figure 6a illustrates a further embodiment of the present invention, in which the electrode array 42 is inserted fully within the cochlea, and involving a post-operative decision as to which electrodes to inactivate and which to include in the patient map.

At 40 the residual hearing capability of the recipient is illustrated, indicating significant residual hearing at low frequencies, up to a threshold 41. Threshold 41 also indicates a position along the cochlea at which the residual hearing portion 43 terminates. In this embodiment, only a subset of electrodes are active in the map. Thus the natural pathway for delivery of acoustic sound is utilised to the extent that it still exists, while electrical stimuli are provided to convey sound information which is no longer perceptible by the cochlea and/or to supplement sound information only partially perceptible by the cochlea. Accordingly, the implant recipient will receive natural sounding percepts from those portions of the cochlea having hearing capability. This technique is advantageous in that when the hearing loss progresses over time, the patient map can be adjusted accordingly, without the need for further surgical intervention.

However, once again, in order to determine an optimal patient map it is desirable to assess the interaction between electrical and acoustic stimulation along the cochlea, in order to determine the physical point 41 in the cochlea at which electrical stimulation begins to interfere with (useful) acoustical stimulation.

Figure 6d is a flowchart illustrating a process for determining a suitable patient map for the implant configuration set out in Figure 6a. At 50 the process begins, and at 51 a sense electrode is set to be a first electrode of the array. At 52 an electrical stimulus is applied in the absence of any acoustic stimulation, and at 53 the first electrode is used to sense and record a first ECAP, denoted ECAPx, evoked by the electrical stimulus applied at 52.

Subsequently, at 54 a substantially identical electrical stimulus is applied, and in addition a simultaneous acoustic stimulus is applied having at least a frequency component tonotopically corresponding to the position of electrode n. At 55 the same (first) electrode is used to sense and record a second ECAP, denoted ECAPy, evoked by the simultaneous electrical and acoustic stimuli applied at 54.

A comparison is then made at 56 of ECAPx and ECAPy. Should ECAPx be substantially identical to ECAPy, as shown in Figure 6b, then this indicates that the presence or absence of acoustic stimulation has made no measurable difference, and thus indicates that residual hearing capability does not exist proximal to the first electrode. In this event the first electrode is at 57 included in the patient map in order that the first electrode is used to apply electrical stimuli to the cochlea to convey sound information tonotopically corresponding to the position of that electrode, due to the natural hearing no longer conveying such sound information. Alternatively, should the comparison made at 56 reveal that ECAPx is not substantially identical to ECAPy, as shown in Figure 6c, then this indicates that the presence of the acoustic masking stimulus has made a difference to the evoked neural response, thus indicating that residual acoustic hearing capability exists proximal to the first electrode. In this event, electrode n is excluded from the patient map at 58. This process is repeated for all electrodes of the array by incrementing n at 59, unless all electrodes have been assessed, in which case n=nma and decision 60 causes the process to end at 61.

It is to be appreciated that the interaction between electrical and acoustic stimuli may be assessed in an alternate manner. For instance, the interaction may alternatively or additionally be assessed by applying a narrowband acoustic stimulus and recording an evoked CAP using a sense electrode tonotopically corresponding to the frequency of the narrowband stimulus.. Then a substantially identical narrowband acoustic stimulus may be applied simultaneously with a masking electrical stimulation (for example applied by an electrode adjacent to the sense electrode), and again recording an evoked CAP. This process may be repeated for acoustically applied frequencies throughout the normal hearing range with a tonotopically corresponding sense electrode for each frequency.

Once an assessment has been made of the interaction between acoustic and electrical stimuli along the cochlea an "interaction map" of the cochlea may be produced, of the type illustrated by audiogram 44 in Figure 6a. Such an interaction map may be used in conjunction with conventional audiometry to determine a suitable cut off frequency 41, to determine where in the cochlea electrical stimulation does not influence the acoustic hearing capability, to map the processor on the non interacting electrodes only, using a frequency allocation table (FAT) that only stimulates those frequencies that are not represented and conveyed by the auditory system naturally.

The audiogram shown in Figure 6a is representative of the most common type of hearing loss in which a cochlea loses hearing ability at high frequencies. However it is to be appreciated that, for a cochlea where hearing capability is deficient in alternate ways, such as the loss of low frequency capability, the embodiment of Figures 6a to 6d can still be used to provide for electrical supplementation of residual acoustic hearing capability, wherever that capability may exist in the operating frequency range.

The embodiments shown in Figures 5a to 5d and Figures 6a to 6d may each be incorporated into a bimodal device. Illustrated in Figure 7 is a bimodal device based on the embodiment of Figures 6a to 6d. Once the cut off frequency 71 (corresponding to a "cut-off electrode") has been determined, whether by the method set out in Figure 2b or Figure 3b, electrical stimuli may be applied in one mode above that frequency, and acoustic stimuli may be applied in the other mode below that frequency. At 70 the determined masking profile is displayed. Portion 73 of the cochlea is where residual acoustic hearing capability exists. Knowing the acoustic cut off frequency of that residual hearing, and knowing which electrode is located at the cut off boundary 71 in the cochlea, it is possible to separate detected sound 75 into two components. Below the cut-off frequency the sound is amplified and applied to the ear acoustically, by low pass filter 76 and transducer 77. The portion of the detected sound above the cut off frequency is processed by a high pass filter 78 and a cochlear implant speech processor 79 and mapped to those electrodes that are in the non-functional part of the cochlea. In this way the accessible frequency range is optimised, and the interaction between acoustic and electric hearing is minimized.

It is to be appreciated that electrical stimulation may also be used to supplement the application of acoustic stimulation to the portion 73 of the cochlea having partial residual hearing. Such supplemented electrical stimulation may be mapped to have a strength to complement the residual audiogram strength along the cochlea in region 73.

Such embodiments provide for a transition from purely acoustic hearing at the apical end of the cochlea to purely electrical stimulation at the basal end of the cochlea, with a central portion of the cochlea having both acoustic and electrical stimulation applied thereto and the central portion of the cochlea being relied upon to convey both acoustic and electrical stimulation. Should the residual hearing be inadequate, electrical stimulation may be applied by all electrodes of the array, with apical electrodes being mapped to have a strength which complements the residual acoustic hearing strength of the portion 73 of the cochlea.

Further, high pass filter 78 may be excluded where an appropriate patient map exists in speech processor 79 which inactivates apical electrodes of the array.

Figure 8 illustrates a system 100 for clinical testing and/or fitting of a cochlear implant 102 to be used to supplement the residual acoustic hearing capability of a cochlea. A trigger signal 101 is applied electrically via the electrode array of the cochlear implant 102 and acoustically via headphones 103. The trigger signal in the system 100 illustrated is generated by a personal computer (PC) with infrared connection via L34, a digital to analogue converter and a clinical audiometer to the headphones 103, and via L34, PCI and a Sprint speech processor to the cochlear implant 102. The PC-infrared-L34-DAC connection may alternatively be provided by a triggerable signal generator with analogue output. For example, the system 100 may be used in obtaining intra-operative NRT recordings of both electrical and acoustic stimuli to determine suitable array insertion depths.

The system 100 may further be used to determine suitable delays to be introduced in order to ensure accurate timing of the delivery of electrical stimulations relative to acoustic stimulations. Such an embodiment recognises that, due to the differing pathways for delivery of acoustic stimuli and electric stimuli to the cochlea, neural responses caused by electrical stimuli may be mis-timed relative to neural responses caused by acoustic stimuli. In certain circumstances such a mis-timing may reduce the intelligibility of speech or otherwise have an undesirable effect on sound perception by the user. Should neural responses to acoustic stimuli arise after neural responses to electrical stimuli, for instance due to the time of transmission from the outer ear, through the middle ear, into the inner ear and via the basilar membrane, an appropriate delay in applying electrical stimulation is preferably introduced. The timing of the electrical stimulations relative to the timing of the acoustic stimulations may be optimised by use of neural response measurements, and appropriate adjustments are preferably made to the delay in delivery of electrical stimuli.

Uses of embodiments of the present invention include research and auditory states monitoring, in which a bimodal speech processor could generate an acoustical stimulus and record the resulting CAP, comparing it to previous CAP recordings. If the recipient's hearing is further deteriorating and refitting is needed, this could be detected automatically by such a system. Embodiments of the present invention may further provide for high resolution audiogram imaging in combination with classical audiometry, and may provide for detection of double peaks in the neural response.

It is to be appreciated that determination of tonotopic cochlear positions in this document is not limited to a classical determination of cochlear position relative to frequency or pitch. In particular, it is to be appreciated that existing formulae to relate cochlear position to pitch, such as Greenwood's formulae Acoust. Soc. Am. Vol 87, No 6, June 1990) may not always be sufficiently accurate in relation to tonotopic cochlear positions for electrical stimuli.

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 17 described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (83)

1. A method of electrically and acoustically stimulating a cochlea, the method comprising: providing an acoustic signal delivery path to the cochlea; providing an electrical signal delivery path to the cochlea, for processing a first frequency sub-range of a detected sound signal for electric stimulation of the cochlea; imposing a delay on at least one of the acoustic signal delivery path and the electrical signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

2. The method according to claim 1, wherein the acoustic signal delivery path comprises an acoustic sound processor for processing a second frequency sub-range of the detected sound signal, and acoustically delivering the processed acoustic sound signal to the cochlea.

3. The method of claim 2 wherein the delay is imposed upon the acoustic signal delivery path.

4. The method of claim 2 or claim 3 wherein the second frequency sub-range corresponds to a residual natural hearing capability of the cochlea.

The method of any one of claims 2 to 4 wherein the acoustic processing comprises a plurality of acoustic channels.

6. The method of claim 5 wherein a channel-specific delay is imposed upon each acoustic channel.

7. The method of claim 5 or claim 6 wherein a channel-specific gain is applied to each acoustic channel.

8. The method of any one of claims 1 to 7 wherein the delay is imposed upon the electrical signal delivery path.

9. The method of any one of claims 1 to 8 wherein the electrical signal delivery path comprises a plurality of electrical channels, wherein the first frequency sub-range of the detected sound signal is divided into a plurality of frequency bands, with one frequency band processed in each electrical channel.

10. The method of claim 9 wherein a channel-specific delay is applied to each electrical channel.

11. The method of claim 10 wherein the channel-specific delay of each electrical channel is configured to delay low frequency electrical channels for a time longer than that by which high frequency electrical channels are delayed, to mimic the time taken for sound to travel from a basal region of the cochlea tonotopically corresponding to the high frequency channels to a more apical region of the cochlea tonotopically corresponding to the low frequency channels.

12. The method of any one of claims 1 to 11 wherein the first frequency sub-range comprises a portion or portions of the audible frequency spectrum in respect of which active stimulus electrodes have been tonotopically positioned proximal to the cochlea.

13. The method of claim 12 wherein the first frequency sub-range comprises a high frequency portion of the audible frequency spectrum, where active electrodes have been positioned proximal to a basal portion of the cochlea.

14. The method of any one of claims 1 to 13 wherein the delay is substantially equal to a difference between the time taken for a signal to travel along the electrical signal delivery path and the time taken for a signal to travel along the acoustic signal delivery path, such that imposing the delay provides for the cochlea to be substantially simultaneously stimulated electrically and acoustically in response to the detected sound signal.

15. The method of any one of claims 1 to 14 wherein the delay is kept sufficiently small that a user does not perceive undesirable effects.

16. The method of any one of claims 1 to 15 wherein the imposed delay is configurable.

17. The method of claim 16 wherein the imposed delay is clinically configurable.

18. The method of claim 16 or claim 17 wherein the imposed delay is field configurable.

19. The method of any one of claims 16 to 18 wherein the imposed delay is configurable on a channel-by-channel basis in multi-channel devices.

The method of any one of claims 1 to 19 further comprising sensing a neural response to determine a relative timing of delivery of electrical stimuli and acoustic stimuli.

21. The method of claim 1 wherein providing the acoustic signal delivery path comprises ensuring no acoustic obstructions are positioned along the normal acoustic hearing path, to allow a passive acoustic delivery path along which acoustic sound travels from the outer ear through the middle ear to the cochlea.

22. A method of fitting a cochlear prosthesis to a cochlea having residual acoustic hearing capability such that the cochlea can be electrically stimulated via an electrical signal delivery path and acoustically stimulated via an acoustic signal delivery path, the method comprising: configuring a delay to be imposed on at least one of the acoustic signal delivery path and the electrical signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

23. The method according to claim 22, wherein the acoustic signal delivery path comprises an acoustic sound processor for processing a second frequency sub-range of the detected sound signal, and acoustically delivering the processed acoustic sound signal to the cochlea.

24. The method of claim 23 wherein the delay is configured to be imposed upon the acoustic signal delivery path.

The method of claim 23 or claim 24 wherein the second frequency sub-range corresponds to a residual natural hearing capability of the cochlea.

26. The method of any one of claims 23 to 25 wherein the acoustic processing comprises a plurality of acoustic channels.

27. The method of claim 26 further comprising configuring a channel-specific delay to be imposed upon each acoustic channel.

28. The method of claim 26 or claim 27 wherein a channel-specific gain is applied to each acoustic channel.

29. The method of any one of claims 22 to 28 wherein the delay is configured to be imposed upon the electrical signal delivery path.

The method of any one of claims 22 to 29 wherein the electrical signal delivery path comprises a plurality of electrical channels, wherein the first frequency sub-range of the detected sound signal is divided into a plurality of frequency bands, with one frequency band processed in each electrical channel.

31. The method of claim 30 further comprising configuring a channel-specific delay to be applied to each electrical channel.

32. The method of claim 31 wherein the channel-specific delay of each electrical channel is configured to delay low frequency electrical channels for a time longer than that by which high frequency electrical channels are delayed, to mimic the time taken for sound to travel from a basal region of the cochlea tonotopically corresponding to the high frequency channels to a more apical region of the cochlea tonotopically corresponding to the low frequency channels.

33. The method of any one of claims 22 to 32 wherein the first frequency sub-range comprises a portion or portions of the audible frequency spectrum in respect of which active stimulus electrodes have been tonotopically positioned proximal to the cochlea.

34. The method of claim 33 wherein the first frequency sub-range comprises a high frequency portion of the audible frequency spectrum, where active electrodes have been positioned proximal to a basal portion of the cochlea.

The method of any one of claims 22 to 34 wherein the delay is configured to be substantially equal to a difference between the time taken for a signal to travel along the electrical signal delivery path and the time taken for a signal to travel along the acoustic signal delivery path, such that imposing the delay provides for the cochlea to be substantially simultaneously stimulated electrically and acoustically in response to the detected sound signal.

36. The method of any one of claims 22 to 35 wherein the delay is kept sufficiently small that a user does not perceive undesirable effects.

37. The method of any one of claims 22 to 36, wherein the imposed delay is field configurable.

38. The method of any one of claims 22 to 37 further comprising sensing a neural response to determine a relative timing of delivery of electrical stimuli and acoustic stimuli.

39. The method of claim 22 wherein providing the acoustic signal delivery path comprises ensuring no acoustic obstructions are positioned along the normal acoustic hearing path, to allow a passive acoustic delivery path along which acoustic sound travels from the outer ear through the middle ear to the cochlea.

A cochlear prosthesis comprising: an electrical signal delivery path to the cochlea, for processing a first frequency sub-range of a detected sound signal for electric stimulation of the cochlea; and delay means for imposing a delay on at least one of the electrical signal delivery path and an acoustic signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

41. The cochlear prosthesis according to claim 40, wherein the acoustic signal delivery path comprises an acoustic sound processor for processing a second frequency sub-range of the detected sound signal, and acoustically delivering the processed acoustic sound signal to the cochlea.

42. The cochlear prosthesis of claim 41 wherein the delay means is for imposing the delay upon the acoustic signal delivery path.

43. The cochlear prosthesis of claim 41 or claim 42 wherein the second frequency sub-range corresponds to a residual natural hearing capability of the cochlea.

44. The cochlear prosthesis of any one of claims 41 to 43 wherein the acoustic sound processor is for processing a plurality of acoustic channels.

45. The cochlear prosthesis of claim 44 wherein the delay means is for imposing a channel-specific delay upon each acoustic channel.

46. The cochlear prosthesis of claim 44 or claim 45 wherein the acoustic sound processor comprises amplifier means for applying a channel-specific gain to each acoustic channel.

47. The cochlear prosthesis of any one of claims 40 to 46 wherein the delay means is for imposing the delay upon the electrical signal delivery path.

48. The cochlear prosthesis of any one of claims 40 to 47 wherein the electrical signal delivery path comprises a plurality of electrical channels, and wherein the first frequency sub-range of the detected sound signal is divided into a plurality of frequency bands, with one frequency band processed in each electrical channel.

49. The cochlear prosthesis of claim 48 wherein the delay means is for imposing a channel-specific delay to each electrical channel.

The cochlear prosthesis of claim 49 wherein the channel-specific delay of each electrical channel is configured to delay low frequency electrical channels for a time longer than that by which high frequency electrical channels are delayed, to mimic the time taken for sound to travel from a basal region of the cochlea tonotopically corresponding to the high frequency channels to a more apical region of the cochlea tonotopically corresponding to the low frequency channels.

51. The cochlear prosthesis of any one of claims 40 to 50 wherein the first frequency sub-range comprises a portion or portions of the audible frequency spectrum in respect of which active stimulus electrodes have been tonotopically positioned proximal to the cochlea.

52. The cochlear prosthesis of claim 51 wherein the first frequency sub-range comprises a high frequency portion of the audible frequency spectrum, where active electrodes have been positioned proximal to a basal portion of the cochlea.

53. The cochlear prosthesis of any one of claims 40 to 52 wherein the delay means is for imposing a delay which is substantially equal to a difference between the time taken for a signal to travel along the electrical signal delivery path and the time taken for a signal to travel along the acoustic signal delivery path, such that imposing the delay provides for the cochlea to be substantially simultaneously stimulated electrically and acoustically in response to the detected sound signal.

54. The cochlear prosthesis of any one of claims 40 to 53 wherein the delay means is for imposing a delay which is sufficiently small that a user does not perceive undesirable effects.

The cochlear prosthesis of any one of claims 40 to 54 wherein the imposed delay is configurable.

56. The cochlear prosthesis of claim 55 wherein the imposed delay is clinically configurable.

57. The cochlear prosthesis of claim 55 or claim 56 wherein the imposed delay is field configurable.

58. The cochlear prosthesis of any one of claims 55 to 57 wherein the imposed delay is configurable on a channel-by-channel basis in multi-channel devices.

59. The cochlear prosthesis of any one of claims 40 to 58 further comprising means for sensing a neural response to determine a relative timing of delivery of electrical stimuli and acoustic stimuli.

60. A speech processor for a cochlear prosthesis, the speech processor for processing a first frequency sub-range of a detected sound signal for electric stimulation of the cochlea via an electrical signal delivery path, the speech processor comprising delay means for imposing a delay on at least one of the electrical signal delivery path and an acoustic signal delivery path, to provide for delivery of the electrical stimulation to the cochlea at a desired time relative to a time of arrival of acoustic stimuli at the cochlea.

61. The speech processor according to claim 60, wherein the acoustic signal delivery path comprises an acoustic sound processor for processing a second frequency sub-range of the detected sound signal, and acoustically delivering the processed acoustic sound signal to the cochlea.

62. The speech processor of claim 61 wherein the delay means is for imposing the delay upon the acoustic signal delivery path.

63. The speech processor of claim 61 or claim 62 wherein the second frequency sub-range corresponds to a residual natural hearing capability of the cochlea.

64. The speech processor of any one of claims 61 to 63 wherein the acoustic sound processor is for processing a plurality of acoustic channels.

The speech processor of claim 64 wherein the delay means is for imposing a channel-specific delay upon each acoustic channel.

66. The speech processor of claim 64 or claim 65 wherein the acoustic sound processor comprises amplifier means for applying a channel-specific gain to each acoustic channel.

67. The speech processor of any one of claims 60 to 66 wherein the delay means is for imposing the delay upon the electrical signal delivery path.

68. The speech processor of any one of claims 60 to 67 wherein the electrical signal delivery path comprises a plurality of electrical channels, and wherein the first frequency sub-range of the detected sound signal is divided into a plurality of frequency bands, with one frequency band processed in each electrical channel.

69. The speech processor of claim 68 wherein the delay means is for imposing a channel-specific delay to each electrical channel.

70. The speech processor of claim 69 wherein the channel-specific delay of each electrical channel is configured to delay low frequency electrical channels for a time longer than that by which high frequency electrical channels are delayed, to mimic the time taken for sound to travel from a basal region of the cochlea tonotopically corresponding to the high frequency channels to a more apical region of the cochlea tonotopically corresponding to the low frequency channels.

71. The speech processor of any one of claims 60 to 70 wherein the first frequency sub-range comprises a portion or portions of the audible frequency spectrum in respect of which active stimulus electrodes have been tonotopically positioned proximal to the cochlea.

72. The speech processor of claim 71 wherein the first frequency sub-range comprises a high frequency portion of the audible frequency spectrum, where active electrodes have been positioned proximal to a basal portion of the cochlea.

73. The speech processor of any one of claims 60 to 72 wherein the delay means is for imposing a delay which is substantially equal to a difference between the time taken for a signal to travel along the electrical signal delivery path and the time taken for a signal to travel along the acoustic signal delivery path, such that imposing the delay provides for the cochlea to be substantially simultaneously stimulated electrically and acoustically in response to the detected sound signal.

74. The speech processor of any one of claims 60 to 73 wherein the delay means is for imposing a delay which is sufficiently small that a user does not perceive undesirable effects.

The speech processor of any one of claims 60 to 74 wherein the imposed delay is configurable.

76. The speech processor of claim 75 wherein the imposed delay is clinically configurable.

77. The speech processor of claim 75 or claim 76 wherein the imposed delay is field configurable.

78. The speech processor of any one of claims 75 to 77 wherein the imposed delay is configurable on a channel-by-channel basis in multi-channel devices.

79. The speech processor of any one of claims 60 to 78 further comprising means for sensing a neural response to determine a relative timing of delivery of electrical stimuli and acoustic stimuli.

A method of electrically and acoustically stimulating a cochlea substantially as herein described and with reference to the accompanying drawings.

81. A method of fitting a cochlear prosthesis to a cochlea having residual acoustic hearing capability such that the cochlea can be electrically stimulated via an electrical signal delivery path and acoustically stimulated via an acoustic signal delivery path substantially as herein described and with reference to the accompanying drawings.

82. A cochlear prosthesis substantially as herein described and with reference to the accompanying drawings.

83. A speech processor for a cochlear prosthesis substantially as herein described and with reference to the accompanying drawings. Dated this seventeenth day of August 2005 Cochlear Limited Patent Attorneys for the Applicant: F B RICE CO