GB2061733A

GB2061733A - Hearing aids

Info

Publication number: GB2061733A
Application number: GB8030648A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-09-24
Filing date: 1980-09-23
Publication date: 1981-05-20
Also published as: FR2465474B1; CH657984A5; FR2465474A1; DE3034394A1; DE3034394C2; GB2061733B

Abstract

An audio signal 55 is split into different bands by filter 50 and the different bands processed in respective channels to modulate a transmitter 74, an implanted receiver (Figs. 1, 3,4) having a corresponding number of channels to detect the transmitted signals and thereby apply stimulating signals to electrodes implanted in the cochlea. Each of four channels in the transmitted may comprise a limiter 58, a frequency-to-voltage converter 60 and a voltage-to-frequency converter 62 giving a pulse train 63 having a prf in the range 40 to 400 Hz and related to the band pass filtered audio signal 57 in that channel - the pulse trains 63 of all four channels all being in the same 40 to 400 Hz range. In each channel a monostable 70 gives a pulse train 71 with a prf. determined by the signal 63 and a pulse width determined by the output of a log. amp. 68 which receives the outputs of filter 50 via a rectifier 66. In transmitter 74, the signals 71 from channels 1 and 3 modulate a 12 MHz carrier, and signals 71 from channels 2 and 4 modulate a 31 MHz carrier, the modulated RF signal from channels 1 and 2 being applied to coil 76 and those from channels 3 and 4 to coil 78. Each channel in the receiver has a tuned circuit with a respective pick-up coil (81 to 84, Figs. 3, 4) and passes the detected signal to electrodes located at a position in the cochlea responsive to an audio frequency in that frequency band passed by the filter 50 in the corresponding transmitter channel. The cochleal implant (Fig. 5) may comprise wires in a silicone elastomer body, each wire terminating in a ball acting on an electrode. Reference is also made to apparatus comprising a four channel implanted receiver and a single channel transmitter, and to apparatus comprising a single channel transmitter, a single channel receiver and a single channel electrode fixed at or adjacent to the round window membrane of the ear. <IMAGE>

Description

SPECIFICATION Multi-frequency system and method for enhancing auditory stimulation and the like This invention relates generally to apparatus for neural and muscle stimulation such as for facilitating hearing in the deaf, and more particularly the invention relates to a method and means for stimulating by means of electrical pulses.

The use of subcutaneously implanted hearing devices is known. U.S. Patent No. 3 209 081 discloses a device which is implanted in the mastoid bone. The receiver makes direct contact with the bone through which sound waves may be conducted to the innerear.

More recently, implanted prosthetic devices for stimulating the auditory nerve by means of electrical pulses have been disclosed. U.S. Patent No.

3 449 768 discloses the use of coded pulse trains to create an electrical gradient field to facilitate visual or audio stimulations. U.S. Patent No. 3 752 939 discloses the use of an electrode including a pair of elongated conductors for implanting in the cochlea.

Schindler et al, "Multielectrode Intracochlear Implants" Arch Otolaryngol, Vol 103, December 1977, discloses the use of spatial excitation of the cochlear nerve in cats. Clark and Hallworth, "A Multiple-Electrode Array for Cochlear Implant", J.

Laryngol, Otol, 90/7, 1976 discloses a ribbon array including a plurality of elongated flat electrodes which are positioned along the length of the cochlea for stimulating the auditory nerve. Similarly, bundles of thin wires have been employed by the Stanford Auditory prosthesis group by direct placement into the auditory nerve.

European patent application No. 78300567.1 by Foster et al. and German patent No. 2 823 793 by Hochmair and Hochmair both describe multichannel implantable hearing aidsforthe deaf containing active circuits.

According to a first aspect of the invention there is provided a multi-frequency system for electrical stimulation characterized by: transmission means for transmitting a plurality of carrier signals each of which is modulated by a signal representing a band of frequencies, multi-channel receiver means for receiving said transmitted signals with each channel responsive to one of said signals representing a band of frequencies, a multi-electrode prosthetic device, and means connecting a signal from each of said channels to at least one electrode of said prosthetic device whereby said prosthetic device provides electrical stimulation.

Similarly according to a second aspect of the invention there is provided a multi-channel receiver for subcutaneous placement characterized by a plurality of independent channels, each channel including a coil tuned to receive a transmitted signal, a detector interconnected with the coil to detect a modulation signal, and means for connecting the detected modulation signal to electrode stimulation means.

Similarly according to a third aspect of the invention there is provided a multi-electrode prosthetic device for cochlea implantation characterized by an elontated molded body of biocompatible material, a plurality of wires within said body, each wire terminating in a contact with the contact positioned at the surface of said body.

Similarly according to a fourth aspect of the invention there is provided a method of fabricating a multi-electrode prosthetic device having a molded elongated body, a plurality of wires within said body, each wire terminating in a ball with the ball positioned at the surface of said body, characterized in that the method includes the steps of a) providing a plurality of wires with contacts formed on one end thereof, b) placing said wires in a mold with said contacts maintained in position by vacuum chuck means, and c) filling said mold with biocompatible material.

Similarly according to a fifth aspect of the invention there is provided the method of enhancing auditory stimulation characterized by the steps of transmitting a plurality of mudolated signals with each signal corresponding to a frequency band in the audio range, receiving said plurality of modulated signals, detecting the modulation signals of said modulated signals, and applying each detected signals to electrode means implanted in a cochlea.

An embodiment of the present invention is an improved system for neural and muscle stimulation.

One method according to the invention is an improved method of enhancing auditory stimulation by means of multiple electrode stimulation.

Still another embodiment of the invention is elec- trode means for selectively applying electrical stimulation to the auditory nerve.

Yet another embodiment of the invention is electrode means which is readily inserted into the cochlea.

Another method according to the invention is a method of making a multi-electrode prosthetic device for cochlear excitation.

A development of the invention is the transmission of signals corresponding to bands of audio frequencies.

Another development of the invention is a receiver having independent channels for processing signals corresponding to audio frequency bands.

Still another feature of the invention is electrode means for applying signals corresponding to audio frequency bands to selected regions in the cochlea.

Briefly, in accordance with one embodimentofthe invention a multi-frequency system for enhancing audio stimulation, for example, includes a mul tichannel transmission means for transmitting a plurality of signals each of which is modulated by a signal representing a band of frequencies in the audio range, multi-channel receiver means for subcutaneous placement is provided for receiving the transmitted signals with each channel of the receiver responsive to one of the transmitted signals representing a band of frequencies. In a preferred embodiment each channel of the receiver is independent and includes a tuned receiving coil for receiving a transmitted signal and detector means for detecting the transmitted modulation signal.

A multi-electrode prosthetic device is provided for cochlea implantation with means connecting a signal from each of the receiver channels to at least one electrode pair of the prosthetic device whereby the prosthetic device provides electrical stimulation to the auditory nerve. Piacement of the electrodes in the device is chosen whereby the implanted device will stimulate the cochlea in accordance with the frequency response thereof.

The multi-electrode prosthetic device comprises a molded biocompatible body with a plurality of wires within the body. Each wire is terminated in a contact body such as a ball with the ball positioned at the surface of the body. Advantageously, each wire is wrinkled prior to the molding of the body to provide stress relief and facilitate flexing of the prosthetic device for insertion into the cochlea. The balls at the end of the wires are selectively positioned whereby the inserted prosthetic device stimulates the cochlea in accordance with the frequency response of the cochlea.

In a preferred embodiment, each channel of the transmission means includes a band pass filter for selecting and passing a band of audio frequency signals, a pulse generator, means responsive to the frequency of signals passed by the band pass filter for controlling the frequency of the pulse generator, and means responsive to the amplitude of signals passed by the band pass filter for controlling pulse width in the pulse generator. The output signal from the pulse generator is applied to modulate a carrier frequency in a transmitter.

The signal from the transmission means is transmitted to the receiver by means of a coil connected to the transmitter output. The multi-channel receiver includes a plurality of coils corresponding in a number to the number of channels of the receiver, and the transmitter coil and the receiver coils are magnetically coupled. The receiver coils may be provided in spaced apart groups with the coils in each group overlapping to minimize magnetic coupling effects of the receiver coils.

The invention and embodiments and developments thereof will be more readily apparentfrom the following detailed description and appended claims when taken with the drawings, in which: Figure 1 is a section view of a human ear illustrating the application of the present invention.

Figure 2 is an electrical schematic of a transmitter for use in the multi-frequency system for enhancing audio stimulation in accordance with one embodiment of the present invention.

Figure 3 is an electrical schematic of one embodiment of a multiple channel receiver for use in a multi-frequency system for enhancing audio stimulation in accordance with the invention.

Figure 4 is a plan view illustrating the placement of receiver coils in accordance with one embodiment of the invention.

Figure 5 is a perspective view of one embodiment of a multi-electrode prosthetic device in accordance with the invention.

Figure 6 is a schematic of a cochlea illustrating the frequency response thereof.

Figure 7 is a section view of a multi-electrode prosthetic device in accordance with the invention.

Figure 8 is a perspective view illustrating a mold useful in fabricating the prosthetic device of Figure 7.

Figure 9a is an alternative embodiment of a prosthetic device, and Figure 9b illustrates the insertion of the device of Figure 9a.

Figure 10 is a block circuit diagram of a single channel portable sound processor/transmitter.

Figure 11 is an electrical schematic circuit diagramm of one embodiment of the circuitfor dynamic range compression according of Figure 10.

Figure 12 is a schematic circuitdiagramm of a preferred embodiment of the transmitter according to Figure 10.

Referring now to the drawings, Figure 1 is a section view of a human ear illustrating the application of a multi-frequency audio stimulation system in accordance with the present invention. Normally, sounds are transmitted through the outer ear 10 to the eardrum 12 which moves the bones of the middle ear shown generally at 14 and excites the cochlea shown generally at 16. The cochlea is a long narrow duct wound spirally about its axis for approximately two and one-halfturns. The cochlea includes an upper channel 18, the scala vestibuly, and a lower channel 20, scala tympani, with the cochlear duct 22 therebetween.The fluid filled scala vestibuli and scala tympani transmit waves in response to received sounds and in cooperation with the cochlear duct 22 function as a transducer to generate electric pulses which is transmitted to the cochlear nerve 24 and thence to the brain.

In people with total sensorineural hearing loss the cochlea does not respond to sound waves to generate electrical signals which can be transmitted to the cochlear nerve. The multi-frequency stimulation system in accordance with the present invention effects direct electrical stimulation of the cochlea. The system includes a multi-frequency transmitter 30 which may be worn on the body. The transmitter is coupled to an implanted receiver. The coupling is preferably accomplished by means of coils 36 and 38 which are connected to the multi-channel transmitter 30 and coils 32 and 34 associated with the receiver. As will be described hereinbelow in detail, the transmitter 30 transmits a plurality of signals which are modulated in accordance with the signal content of a plurality of audio frequency bands. The transmitted signals are received and detected in the receiver with the detected signal connected through wires 42 and 44to a prosthetic device 46 which is implanted in the cochlea. As will be described further hereinbelow, the prosthetic device includes a plurality of electrodes which are positioned on the surface of the device to provide selective stimulation of the cochlea in accordance with the frequency response thereof.

In a preferred embodiment the multi-frequency system includes four channels corresponding to four frequency bands in the audio frequency range. Figure 2 is an electrical schematic of an embodiment of the transmitter which includes four channels co rresponding to 0,25 - 0,5 KHz, 0,5 - 1,0 KHz, 1,0 - 2,0 KHz, and 2,0 - 4,0 KHz. The circuitry for each channel is illustrated in block diagram form in channel 1 and includes a band pass filter 50 tuned for the desired frequency band (e.g. 0,25 - 0,5 KHz for channel 1).

Filter 50 receiveds and audio signal picked up by a microphone 52 and passed through a gain control amplifier 54. The signal from amplifier 54 has a wide frequency range as illustrated at 55 and after passing through the bandpass filter 50 a signal of limited frequency range is provided as shown at 57. Delay circuitry can be included in the lower frequency channels to compensate for the delay normally introduced in transmitting acoustic waves through the length of the cochlea for stimulating the lower frequency stimulation sites near the apex of the cochlea.

The signal 57 is then applied through a limiter 58 which produces a clipped output signal 59 showing the same zerocrossings as wave 57. The clipped wave 59 is applied to a frequency to voltage converter 60 which produces a time varying dc voltage that is proportional to the frequency of signal 59. The frequency to voltage converter comprises suitable sircuitry such as a monostable multivibrator which is triggered by signal 59 to generate a plurality of pulses of equal pulse width having a repetition rate at the frequency of the signal 59. The monostable multivibrator output is passed through a low pass filter to generate a time varying dc voltage which is proportional to the pulse rate.

The time varying voltage output from converter 60 is then applied to a voltage to frequency converter 62 such as a voltage controlled oscillator which generates an output signal 63 comprising a train of pulses having a fixed pulse width and a frequency corresponding to the voltage applied to the controlled oscillator. However, the frequency range of the pulse train 63 may cary in a limited range such as 40-400 Hz while the band pass filter passes a smaller or larger frequency range. As will be described further hereinbelow, the auditory nerve can detect signal pitch wherein excitation is limited to electrical pulses at frequency limited to 400 Hz applied to particular stimulation sites in the cochlea. Thus, the passband is transformed into a lower frequency range corresponding to the range of electrical stimulation frequency where pitch discrimitation can be achieved.

This range is in most cases limited to e.g. 40-400 Hz, although it might also be considerable larger in certain cases.

The signal from bandpass filter 50 is also passed through a rectifier 66 and a logarithmic amplifier 68 which produces a varying dc output voltage which is logarithmically proportional to the amplitude of the rectified signal from rectifier 66.

The signal 63 from converter 62 and the dc voltage from amplifier 68 are applied to a monostable mul tivibrator 70 which generates an output pulse train 71 with a pulse repetition rate determined by the pulse repetition rate of signal 63 and with a pulse width determined by the voltage from logarithmic amplifier 68. Signal 71 is applied to an RF transmitter 74 for modulating a carrier signal, as illustrated at 75.

The modulated carrier is then transmitted by antenna coil 76 or antenna coil 78.

Each of the channels has similar circuitry with the bandpass filters selected to pass the desired frequency band. In each of the channels the monostable multivibrator generates an output pulse train varying in frequency from about 40 to 400 Hz as this frequency range is particularly suitable for stimulating the cochlea. Thus, each channel generates a similar pulse train varying in frequency from 40 to 400 Hz and with varying pulse width, as described, which are used to modulate carrier signals in the transmitter 74. In the illustrated embodiment employing four channels, the RF transmitter includes four carrier signals with two signals being at 12 MHz and two signals being at 31 MHz. The pulse trains from channel 1 and channel 3 are employed to modulate 12 megahertz signals, respectively, and channels 2 and 4 are used to modulate 31 megahertz signals, respectively.The carrier signals modulated by signals 1 and 2 are applied to one output coil and the carrier signals modulated by channel 3 and 4 are applied to a second output coil. Because of the frequency difference in the two carrier frequencies applied to each coil, minimum cross talk results therefrom.

The use of only one transmitter coil per group of receiver coils simplifies fabrication, however, is may be of advantage for other reasons to use a separate transmitter coil for each channel.

Figure 3 is an electrical schematic of a multichannel detector in accordance with a preferred embodiment which includes four independent channels with each channel including a coil 81-84 with coils 81 and 82 magnetically coupled to transmitter coil 76 and the coils 83 and 84 magnetically coupled to the transmitter coil 78. Each of the coils 81-84 is shunted by a capacitor 85 which tunes the coil to 12 megahertz or 31 megahertz, as required for each of channels 1-4. The signal coupled to coil 81 by coil 76 passes through a detector comprising serially connected diode 86 and capacitor 87 and shunt resistor 88. By using pulse modulation and demodulation a Zener diode can be included in parallel with resistor 88 thus limiting the voltage of the detector.

Accordingly, effects of voltage variations due to coupling of the transmitter and detector coils can be minimized. The detected voltage across output terminals 90 preferably varies from 0 to 3 volts and at a frequency from 40 to 400 Hz, depending on the detected modulation signal.

Especially for tissue stimulation systems with a small number of independent channels simultaneously carrying different signals, especially 2-9 channels, the following method can be used with advantage.

In orderto reduce the space required by the plurality of receiver coils they are arranged in stacked groups. Even though each of the receiver coils is tuned to a different frequency, the mutual coupling of the two or three receiver coils would result in unacceptably high crosstalk, if the coils were just arranged on top of each other. Arranging the coils in such a way to compensate their mutual magnetic flux, their mutual inductance vanishes. Thus 2 or 3 independent channels with negligible crosstalk are obtained, using only negligibly more space than one channel. Accordingly, coils 81 and 82 are grouped together and spaced from coils 83 and 84, as illustrated in the plan view thereof in Figure 4. 85, 86 and 87 represent a group of three coils with compensated mutual inductance.Each of the coils has a diameter on the order of 1,5 to 2 centimeters and the spacing between the two groups of coils is approximately 3 centimeters to prevent crosstalk between the groups. As illustrated, coils 81 and 82 and coils 83 and 84 are overlapped to minimize crosstalk between the coils in each group. The overlapping of the coils provides offsetting flux from one coil to the otherthereby minimizing distortion or crosstalk between the two coils. Since coils 81 and 82 are tuned to different frequencies (e.g. 12 MHz and 31 MHz, respectively), each channel of the receiver receives and detects only the signal from the transmission coil to which it is coupled.

The detected signals in each of the receiver channels are connected to a multi-electrode prosthetic cevide such as the device 90 illustrated in Figure 5.

Each channel can be connected to one or more electrodes having contacts positioned on the prosthetic device to stimulate a region of the cochlea for a desired frequency response. Thus, bipolar stimulation, unipolar stimulation against a remote ground, or a common distributed ground stimulation can be employed. The device comprises an elongated molded body of a silicone elastomer such as Silastic (Registered Trade Mark) in which a plurality of wires shown generally at91 are implanted. Each wire is terminated in a ball 92 which is positioned atthe surface of the device 90. The spacing of the balls on the surface of the device 90 provided selected frequency response when the device is inserted into the cochlea.As noted in the schematic of a cochlea shown in Figure 6, the frequency response generated by the cochlea varies from a high frequency at the basal turn of the cochlea and has a progressingly lower frequency response towards the apex of the cochlea. Accordingly, by proper positioning the electrode contacts or balls 92 within the cochlea, the electrical stimulation of the cochlea provided by the prosthetic device will induce a desired frequency response. By the additional variaton of stimulation frequency a pitch continuum can be achieved.

Figure 7 is a section view of the prosthetic device of Figure 5 illustrating the positioning of wires 93 and 94 within the device. To facilitate illustration only two wires are shown. Each of the wires is wrinkled to provide stress relief and to facilitate flexing of the prosthetic device as it is inserted into the cochlea. In a preferred embodiment the wires are Teflon (Registered Trade Mark) coated platinum (90%)iridium (10%) wires having a diameter of 25 microns.

The balls at the ends of the wire are 300 microns in diameter and are formed by heating the wires in a flame until the wire tips melt. The balls are arranged in pairs in two diametrically opposed rows. In one embodiment the total diameter of the prosthetic device is 0,9 millimeter and is tapered to about 0,5 millimeter at its tip. Overall length must accommodate an insertion in the cochlea of 20-25 millimeters.

Figure 8 is perspective view of the bottom portion 96 of a suitable mold for forming the prosthetic device and includes a centrally disposed tapered channel 97 of the desired device configuration. A plurality of holes 98 are provided in the surface of channel 97 with each of the holes 98 in communication through body 96 with a vacuum line 99. In form ing the prosthetic device, the wires are first positioned in the mold cavity with the spherical and portions of the wires placed in holes 98 and maintained in position by means of the vacuum applied to line 99. The mold is then assembled and the cavity defined by channel 97 is filled with Silastic material.

The vacuum chuck provided by holes 98 ensures proper positioning of the ball contacts on the surface of the prosthetic device to provide the desired frequency stimulation by the device when inserted into the cochlea.

Figure 9a is a perspective view of an alternative embodiment 100 of prosthetic device which is molded to conform to the shape of the cochlea.

Insertion of the device in the cochlea is illustrated in the section view in Figure 9b in which a straight rod 102 such as steel wire is inserted into the molded body 100. The rod 102 is slowly extracted therefrom as the device is inserted into the cochlea whereby the body 100 reassumes its molded configuration.

Other different possible configurations of external and implanted subsystems are: 1.) A four or more channel electrode inside the scala tympanie in connection with a four channel implant and an external single channel soundprocessor. In this case either one electrode channel can be chosen for stimulation or a variable number of electrode contacts could be interconnected. In the latter case different thresholds for different electrodes contacts can be taken into account and compensated for by means of electronic circuitry.

2.) A single channel electrode fixed to the round window membrane or situated somewhere near the round window membrane combined with a larger ground electrode also positioned externally to the cochlea in connection with a single channel implant and an external single - channel sound - processor/transmitter.

The system described in 1.) has been tested in selected totally deaf volunteers and an open speech discrimination of 60-70% for unknown words or sentences could be obtained through stimulation onlywithout additional lipreading. This means that this prosthecis can already be regarded as a useful aid for the totally deaf, whereas the system described in 2.) is designed for the hard of hearing and for hearing impaired children.

For both the single channel and the multi-channel external portable stimulators there is the possibility of not transforming the sound waveform into pulse trains but using an analog waveform instead. In this case dynamic range compression 103 are compensation for dependence of loudness on frequency 104 are very important features of the external soundprocessor because the dynamic range between stimulus intensities necessary to cause threshold sensations and too loud sensations is much smaller in the case of electrical stimulations than for normal hearing and there may occur in some cases a strong influence of stimulation frequency on threshold intensities and on loudness.

This can be overcome by dynamic range compression using nonlinearities. This nonlinearity employed for the dynamic range compression may be of logarithmic form, obey a power law, a piece wise linear function, or some other suitable shape. It may be implemented by suitable connected differential amplifiers or operational amplifiers using diode networks or diode connected transistors.

In orderto reduce the introduction of unwanted frequencies by this dynamic range compression, the nonlinearity may be driven by a frequency shifted signal. The even order distortion products may then be eliminated by a narrow band rf-bandpass filter before downmixing the signal to the audio range.

An other possibility to reduce distortion products is to employ several nonlinearities within octave wide bands.

It is also possible to use a gain controlled amplifier posessing sufficiently small attack and release time constant of 2-10 ms and 100-200 ms respectively.

The characteristic of this gain controlled amplifier may be given the desired shape by inserting proper nonlinearities into the control signal path.

The dynamic range compression circuit may also precede the frequency adjustment circuitry. In this case only very slight, but accurate frequency shaping is necessary.

Fig. 10 iso the block circuit diagram of a sound processor including the transmitter.

A multichannel stimulator consists of several, essentially identical channels with separate rftransmission circuit. Each channel possesses a partcular frequency band. These bands are selected by the band pass filter. In the case pf a single channel stimulator only one of these channels is used. The band pass filter 103 may then be deleted. The dynamic range of the acoustic signal picked up by the microphone amounts to more than 80 dB. This large dynamic range has to be transformed into the range of stimulation intensities of approx. 10 dB.

This is achieved by dynamic range compression circuitry 107 and/or an input dependent gain controlled amplifier 105.

An advantage of the control circuit over the dynamic range compression using nonlinearelements is the small amount of additional nonlinear distortion; a drawback is that for a sudden onset of a very loud signal annoying peaks will appear at the output. Therefore dynamic range compression and gain control should be used at the same time.

Circuit 106 used for the adjustment of frequency response to the patient's frequency dependence of isoloudness characteristics contains frequency dependent components like RC- or LC-structures. It is also possible to insertthis circuit afterthe dynamic range compression circuit 107. In that case only verly slight, but nevertheless very accurate frequency shaping is necessary. The amplitude modulated transmitter 108 with its tank circuit 109 transmits the signal to the implanted tuned receiver circuit 110 and to the demoduiator 111. The demodulated signal is then coupled to the electrode 112.

Fig. 11 shows the circuit employed for the dynamic range compression. It is based on an integrated circuit 112 (TL441), which essentially contains four differential amplifiers whose outputs are paralleled and whose inputs are driven via voltage dividers of varying attenuation. The differential voltage between the points 122 and 123 depends logarithmically on the input voltage. The input voltage at 121 is coupled to the input 124 of the circuit 112 via the capacitor 113 which prevents any dc-voltage form reaching the input. The resistor 120 is used to establish dc-ground potential at the input of the circuit. The differential voltage between 122 and 123 istransformed to a single anded output in a conventional manner using an operational amplifier 112 together with resistors 115, 116, 117 and 118 connected as a differential amplifier.The trim-potentiomenter 119 is used to adjust the offset voltage.

Fig. 12 shows the circuit diagram of the amplitude modulated transmitter. It consists of the oscillator 120 generating carrier frequency of 12 MHz and the power amplifier. The tankcirnuit 126 and the implanted receiver circuit 127, which is tuned to the same frequency together form a band pass filter.

Such a bandpass filter, is current driven, shows a maximum of the voltage induced in the secondary ar critical coupling. Hence in the vicinity of the stationary point the induced voltage will only slightly depend on any unavoidable displacement of the primary, which is the transmitter tank circuit. Choosing the Qualities O of the tuned circuits properly one obtains a "critical Distance" of 10 to 12 mm. E.g. with a transmitter coil of a diameter of 23 mm the stimulation signal decreases by less than 5 /O for lateral displacement of 10 mm.

In order to obtain the necessary high Q of the tranmittertank circuit, emitter- or base-modulation is to be preferred over collectldr-modulation, since the latter leads to a saturation of the output.transis- tor 128, which should be avoided.

The audio-signal is coupled to the base of transistor 128 via resistor 133, coupling coil 134, and the resistor 130. The resistor 130 is selected so to obtain adequate rf-output power without changing the number of turns of the coupling.coil, which is a more tedious procedure. The capacitors 131 and 132 provide an rf-bypass at the modulation signal input and at the power supply, respectively. The Schottkydiode prevents unwanted parasitic oscillations in the output transistor during inadvertent saturation conditions.

The tank circuit of the transmitter is mounted to an ear-hook made of acrylic glass which is used to position the circuit directly over the implanted receiver circuit. The transmitter is miniaturized so that it also can find place on this ear-hook. This has the advantage of minimizing any rf-radiation from the device since any rf-carrying elements are shorter than 2 cm.

With the method for electrostimulation of the auditory system described above, it is possible to enhance hearing with the very hard of hearing and enable the deaf to obtain hearing functions.

The division of the audio signal into frequency bands and the selective stimulation of particular places along the cochlea improves the quality and the intelligibility of the sound impressions. The implanted receiver circuit contains passive components only. Therefore no externally supplied dcpower is necessary. Solely the stimulation signals have to transmitted across the skin.

The production of the prostheses including the multichannel electrode is comparatively simple. The production method described above ensures the exact positioning of the electrode contacts to allow the desired pitch sensation to be elicited. In the preferred embodiment pulse width modulation is used.

However, other modulation schemes as amplitude or frequency modulation may be used. Instead of digital stimulation it may be advantageous to use analogue stimulation signal after proper electronic processing described above. The frequency range of these stimulation signals may be in the audio frequency range.

The method of auditory stimulation utilizing the multi-frequency system in accordance with the present invention provides improved hearing in the deaf and hard of hearing. The use of frequency band signals enhances the perceived sound and the selective stimulation of the cochlea enhances the auditory response. Since the receivercomprises passive devices, no power supply other than the transmitted signal is required. The prosthetic device is readily manufactured with exact electrode positioning to achieve desired frequency response when stimulating the cochlea. While pulse modulation is employed in the preferred embodiment, other modulations such as amplitude or frequency can be employed. Analog signals can be employed as well as pulsed or digital signals in practicing the invention.

While in the described embodiment the audio frequency bands are transformed to corresponding signals having frequencies of 40-400 Hz, the corresponding signals can have the same frequencies as the audio bands or the frequencies may be unrelated.

Thus, while the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended

Claims

claims. CLAIMS

1. A multi-frequency system for electrical stimulation characterized by: transmission means for transmitting a plurality of carrier signals each of which is modulated by a signal representing a band of frequencies, multi-channel receiver means for receiving said transmitted signals with each channel responsive to one of said signals representing a band of frequencies, a multi-electrode prosthetic device, and means connecting a signal from each of said channels to at least one electrode of said prosthetic device whereby said prosthetic device provides electrical stimulation.

2. A multi-frequency system as defined by claini 1 characterized in that said system provides auditory stimulation and said band of frequencies are in the audio band and said prosthetic device is or is designed to be implanted in the cochlea.

3. A multi-frequency system as defined by claim 1 characterized in that said multi-channel receiver means comprises independent channels for processing said transmitted signals.

4. A multi-frequency system as defined by claim 2 characterized in that each independent channel comprises a tuned receiving coil and a signal detector.

5. A multi-frefquency system as defined by claim 2 characterized in that said multi-electrode prosthetic device comprises an elongated molded biocompatible body, a plurality of wires within said body, each wire terminating in a contact with the contact positioned at the surface of said body.

6. A multi-frequency system as defined by claim 5 characterized in that each wire is wrinkled prior to moding to facilitate flexing of said body upon insertion into the cochlea.

7. A multi-frequency system as defined by claim 5 characterized in that said contacts are selectively positioned to stimulate the cochlea in accordance with the frequency response of the cochlea.

8. A multi-frequency system as defined by claim 2 characterized in that said transmission means includes a plurality of channels corresponding to the plurality of bands of frequencies in the audio range, each channel including a band pass filter for selecting a band of audio frequency signals, a pulse generator, means connected to said filter and responsive to frequencies of signals from said band pass filter for controlling the frequency of said pulse generator, means responsive to amplitude of signals passed by said band pass filter for controlling pulses of said pulse generator, and a transmitter for modulating a carrier signal by the signal from said pulse generator.

9. A multi-frequency system as defined by claim 8 characterized in that said pulse generator is a monostable multivibrator and said means responsive to amplitude of signals controls pulse width.

10. Amulti-frequencysystem as defined by claim 8 characterized in that said plurality of channels correspond to the frequencies of 0,25-0,5 KHz, 0,5-1,0KHz, 1,0-2,0KHz, and 2,0-4,0 KHz.

11. A multi-frequency system as defined by claim 8 characterized in that said multichannel receiver means includes four channels with each channel including a coil, said transmitter means including two coils with each coil receiving two modulated carrier signals, a first two to said receiver coils being magnetically coupled to one of said transmitter coils, andtheothertwoofsaid receiver coils being magnetically coupled to the other of said transmitter coils.

12. A multi-frequency system as defined by claim 11 characterized in that said first two receiver coils are overlapping and the othertwo receiver coils are overlapping to minimize mutual magnetic coupling of said coils.

13. A multi-frequency system as defined by claim 1 characterized in that said transmission means further at least one coil for transmitting said plurality of modulated signals, said receiver means includes a plurality of coils corresponding in numbers to the number of channels in said receiver, said transmitter coil being magnetically coupled with a plurality of receiver coils.

14. A multi-frequency system as defined by claim 2 characterized in that said transmission means includes a plurality of channels corresponding to the plurality of bands of frequencies in the audio range, each channel including a band pass filter for selecting a band of audio frequency signals, means for compression of dynamic range, means for compensation of loudness dependance on frequency and a transmitter for modulating a carrier signal by the processed analog sound-signal.

15. Am multi-frequency system as defined by claim 14 characterized in that said multichannel receiver means includes four channels with each channel including a coil, said transmitter means including two coils with each coil receiving two modulated carrier signals, a first two of said receiver coils being magnetically coupled to one of said transmitter coils, and the other two of said receiver coils being magnetically coupled to the other of said transmitter coils.

16. A multi-frequency system as defined by claim 15 characterized in that said first two receiver coils are overlapping and the other two receiver coils are overlapping to minimize mutual magnetic coupling of said coils.

17. A multi-frequency system as defined by claim 15 characterized in that said dynamic range compression is achieved through the use of a suitable nonlinearity.

18. A multi-frequency system as defined by claim 17 characterized in that said nonlinearity is a logarithmic amplifier.

19. A multi-frequency system as defined by claim 15 characterized in that said dynamic range compression is achieved through driving a logarithmic amplifierwiththe signal upshifted in frequency and afterwards downshifting the signal.

20. A multi-frequency system as defined by claim 15 characterized in that said dynamic range compression is achieved through the use of nonlinearities arranged in octave wide bands.

21. A multi-frequency system as defined by claim 15 characterized in that said dynamic range compression is achieved through the use of a gain controlled amplifier.

22. A system as defined by claim 14 characterized in that said transmitter contains a nonsaturating base-modulated power amplifier.

23. A system as defined by claim 14 characterized in that said transmitter contains a nonsaturating emitter-modulated power amplifier.

24. Asystem as defined by claim 14characterized in that said transmission means contains a bandpass filter consisting of the transmitter tank circuit and the tuned receiver circuit.

25. A system as defined by claim 24 characterized in that the said bandpass filter is critically coupled.

26. A system as defined by claim 15 characterized in that said transmitter is mounted to an ear hook containing the transmitter coil.

27. A system as defined by any of claims 14 to 26 characterized in that the audio signal is applied to a single one of said transmission channels thereby avoiding splitting up of said audio signals into a plurality of bands.

28. A multi-channel receiver for subcutaneous placement characterized by a plurality of independent channels, each channel including a coil tuned to receive a transmitted signal, a detector interconnected with the coil to detect a modulation signal, and means for connecting the detected modulation signal to electrode stimulation means.

29. A multi-channel receiver as defined by claim 28 characterized in that said coils are provided in spaced apart groups with the coils in each group overlapping to minimize mutual magnetic coupling effects of the coils.

30. A multi-channel receiver as defined by claim 28 and further characterized by a multi-electrode prosthetic device for cochlea implantation.

31. A multi-channel receiver as defined by claim 30 characterized in that said prosthetic device comprises an elongated molded biocompatible body, a plurality of wires within said body, each wire terminating in a contact with the contact positioned at the surface of said body.

32. A multi-channel receiver as defined by claim 31 characterized in that said contacts are selectively positioned on the surface of said body to stimulate the cochlea in accordance with the frequency response of the cochlea.

33. A multi-electrode prosthetic device for cochlea implantation characterized by an elontated molded body of biocompatible material, a plurality of wires within said body, each wire terminating in a contact with the contact positioned at the surface of said body.

34. A multi-electrode prosthetic device for cochlea implantation as defined by claim 33, characterized in that said contacts are selectively positioned in said body to stimulate the cochlea in accordance with the frequency response of the cochlea.

35. A multi-electrode prosthetic device for cochlea implantation as defined by claim 33 characterized in that said wires are wrinkled to provide stress relief and facilitate flexing of said device.

36. A method of fabricating a multi-electrode prosthetic device having a molded elongated body, a plurality of wires within said body, each wire terminating in a ball with the ball positioned at the surface of said body, characterized in that the method includes the steps of a) providing a plurality of wires with contacts formed on one end thereof, b) placing said wires in a mold with said contacts maintained in position by vacuum chuck means, and c) filling said mold with biocompatible material.

37. The method of claim 36 and further characterized by the step of wrinkling said wires priorto plac ing in said mold whereby flexibility of the body is facilitated.

38. The method of enhancing auditory stimulation characterized by the stps of transmitting a plurality of mudolated signals with each signal corresponding to a frequency band in the audio range, receiving said plurality of modulated signals, detecting the modulation signals of said modulated signals, and applying each detected signal to electrode means implanted in a cochlea.

39. The method as defined by claim 38 characterized in that said step of applying said detected signals includes connecting each detected signal to electrode means located in the cochlea at a position having a frequency response corresponding to the frequency band of the modulated signal.

40. The method as defined by claim 38 characterized in that each transmitted signal is modulated by pulses having a frequency determined by the frequencies of signals in a frequency band and having a pulse width corresponding to the amplitude of signals in a frequency band.

41. A multi-frequency system for electrical stimulation, the system being according to claim 1 and substantially according to any one of the embodiments thereof hereinbefore described with reference to the accompanying drawings.

42. A multi-channel receiver according to claim 28 and substantially according to any one of the embodiments thereof hereinbefore described with reference to the accompanying drawings.

43. A multi-electrode prosthetic device according to claim 33 and substantially according to any one of the embodiments thereof hereinbefore described with reference to the accompanying drawings.

44. A method of fabricating a multi-electrode prosthetic device, the method being according to claim 36 and substantially as hereinbefore described with reference to the accompanying drawings.

45. A method of enhancing auditory stimulation according to claim 38 and substantially according to any one of the methods thereof hereinbefore described with reference to the accompanying drawings.