Prosthetic Device For The Deaf

Abstract

There is disclosed a method and apparatus for inducing the sensation of intelligible hearing by direct electrical excitation of the auditory nerve endings distributed along the basilar membrane within the cochlea. An electrode is positioned within the lower scala of the cochlea by insertion through the round window. The electrode consists of a resilient base member shaped to conform to the inner surface of the lower scala, such base member extending along the basilar membrane. The base member retains a pair of conductors which extend parallel to the length of the basilar membrane. Means are also provided for transmitting an excitation signal to a receiver implanted with and connected to the conductors. Several configurations for the electrode are disclosed as well as several techniques for the excitation thereof.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending application Ser. No. 75,142, filed Sept. 24, 1970 now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a prosthetic device for the deaf and, more particularly, to a method and means for inducing the sensation of intelligible hearing, by direct electrical excitation of the auditory nerve endings distributed along the basilar membrane within the cochlea, in people suffering from sensory deafness. This condition is untreatable by acoustic amplification or bone conduction.

2. Description of the Prior Art

The fundamentals of the hearing process, whereby the vibrations of the surrounding air called "sound" are sensed by the auditory system and transmitted to the brain, are well defined. For present purposes, such hearing process may be briefly described as follows: The auditory system may be divided into its three component parts, namely the external, the middle and the internal ear. The external ear is outermost and includes the auricle attached to the side of the head and the external auditory meatus. Sound vibrations in the air are focused by the auricle and conveyed to the opening of the external ear canal which transmits such vibrations to the tympanic membrane which seals the inner end of the auditory meatus and forms the dividing line between the external and middle ears.

The middle ear is positioned within a space in the temporal bone of the skull and serves to transmit the vibratory movements of the tympanic membrane to the internal ear. The middle ear includes a series of bones called the "auditory ossicles" which include the malleus, or hammer; the incus, or anvil; and the stapes or stirrup. The hammer is directly attached to the tympanic membrane whereas the stirrup is attached to a membrane positioned in a minute opening, called the "oval window" in the bony area containing the internal ear. The auditory ossicles are interconnected so that the vibratory movements of the tympanic membrane are transmitted to the oval window, and sound is thus transmitted from the external to the inner ear.

The portion of the inner ear specifically concerned with hearing consists of the cochlea, a long, narrow duct within the temporal bone, which is wound spirally around its axis for approximately two and one-half turns. The cochlea is divided by a pair of membranes extending longitudinally therethrough into an upper, a middle and a lower scala. The oval window presents an opening into the upper scala, an additional minute opening, called the "round window", providing an opening into the lower scala, the round window being closed by a membrane. The cochlea is filled with a fluid, the perilymph, which is free to circulate through the upper and lower scalas which are interconnected at the apex of the cochlea.

The membrane between the middle scala and the lower scala, called the "basilar membrane", extends the entire length of the cochlea duct. The auditory pathways from the cochlea terminate in the cerebral cortex of the brain. The auditory nerve endings, distributed along the basilar membrane, are in direct functional connection with the hair cells contained in the middle scala.

The vibratory movements of the tympanic membrane which are transmitted by the auditory ossicles to the oval window are distributed through the cochlea fluid throughout the cochlea. This vibratory input manifests itself in an alternating electrical field within the structure of the cochlea (which electrical field appears and has been detected at the round window). This electrical field (generated by the hair cells) is sensed by the nerve endings in the basilar membrane and transmitted via the auditory nerve to the cerebral cortex of the brain which interprets such electrical signals as sound.

The loss of hearing, or a decrease in hearing sensitivity, may result from damage or abnormalities in the external, the middle or the internal ear. Where the hearing problem is a loss of sensitivity, the problem is usually solved by the use of a conventional hearing aid which simply amplifies the sound before transmission to the tympanic membrane. On the other hand, where hearing sensitivity is reduced to a point where additional amplification or bone conduction is useless, such conventional hearing aids are incapable of generating the sensation of hearing.

Where total loss of hearing is due to malfunctions in the external or middle ear, such as a stiffening of the tympanic membrane or an improper functioning of the auditory ossicles, hearing can usually be restored through surgical procedures whereby either the tympanic membrane or one or more of the auditory ossicles are replaced by man-made or human substitutes. However, the total loss of hearing as a result of difficulty in the external or middle ear represent a minority of actual cases. The majority of instances of total loss of hearing results from either sensory or neural deafness. In the former case, deafness results from a reduction in the sensitivity of the cochlea in the internal ear which may be caused, for example, from a loss of hair cells, a chemical change in the perilymph, etc. In the latter case, deafness results from damage to the auditory nerve itself, either through disease or physical rupture. In either case, where total deafness results, such that a conventional amplifying hearing aid is useless, no technology presently exists for successfully restoring the sensation of hearing.

Regarding the prior art which is considered to be relevant to the instant invention, reference is made to an article by Robin P. Michelson entitled "The Crossed Cochlea Effect", published in the Transactions of the American Laryngological, Rhinological and Otoligical Society, pp. 626 to 644, 1968. In this publication Michelson describes experiments conducted to determine the extent of auditory reflexes in cats in order to obtain a better understanding of certain auditory functions and their interactions. In the experiments in audio signal (sound pressure signal) was applied to one ear of each cat and monitored by a cochlear microphonic electrode in the same ear. The signal was modified by an electrical stimulation in the contralateral ear. The purpose was to determine the levels of electrical stimulation in the contralateral ear which might suppress the cochlear microphonic signal in the acoustically stimulated ear. These levels were found to be approximately 250 microvolts to 2 millivolts. The center frequency of the tuning curve indicated a minimum stimulation or threshold level for acoustic reflex in the range of 250 to 500 microvolts. These stimulation levels are of the same order as the cochlear microphonic, i.e., the naturally generated electrical signal within the cochlea. Through these experiments it was demonstrated that there is an acoustic reflex ineraction between the two ears of a cat. The acoustic reflex, however, it not the same as the sensation of hearing. The experiments, therefore, did not demonstrate that the cats were actually hearing an audio signal. In fact, as will appear from the description hereinafter, the electrical and sound pressure stimuli used in these experiments was well below the minimum perception threshold required for the cats to hear.

The present invention involves the use of electrical stimulation of the auditory organ to produce hearing in the deaf. Reference is made to a second article by Robin P. Michelson entitled "Electrical Stimulation of the Human Cochlea in Sensory Deafness," published in Archieves of Otolaryngology, March 1971, Vol. 93, pp. 317-323, which describes the efforts of other scientists prior to the present invention to produce hearing by electrical stimulation of the auditory organ, as well as results achieved with the use of the present invention. This article refers to an implanted electrode system developed by James H. Doyle, which system is described in detail in U.S. Pat. No. 3,449,768. Doyle utilizes what he calls a "neural potential generator" which produces 1 KHz clock pulses modulated in amplitude and width to create a complex modulation scheme which is intended to duplicate the firing rates and potentials of the neurons along the basilar membrane. A complex electrode is utilized consisting of a multiplicity of wires driven from a subcutaneous transformer in a unipolar manner from ground plane to the individual electrode wires. The electrode is so dimensioned that once it is inserted in the lower scala it is free to move therein. Thus, the pulses produced by the neural potential generator are distributed in a random fashion along the basilar membrane without regard for the place frequency relationship. The place frequency relationship, first discovered by Von Bekesy, simply states that particular portions along the basilar membrane are related to specific frequencies. The area of the basilar membrane closest to the round window is associated with the high frequencies. The opposite end of the basilar membrane near the helicatrima is associated with the low frequencies. Doyle states that his patients heard the carrier frequency produced by the neural potential generator. The Doyle system has not been successful in inducing the sensation of intelligible hearing.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is disclosed a method for inducing the sensation of intelligible hearing by direct electrical stimulation of the auditory nerve endings of the auditory nerve. Since the present technique completely by-passes the external and middle ears and the hair cells of the inner ear, it is possible to induce the sensation of intelligible hearing in the absence of these structures. Thus, the present invention may be effectively used to induce the sensation of hearing in people suffering from deafness caused by abnormalities in any of these areas. However, the primary use will be in cases of sensory deafness which has, heretofore, been untreatable.

Briefly, the sensation of hearing is induced by positioning an electrode within the lower scala of the cochlea, such electrode being surgically inserted through the round window. The electrode consists of a resilient base member shaped to conform to the inner surface of the lower scala, such base member extending along the basilar membrane. The base member retains a pair of conductors which extend parallel to the length of the basilar membrane. Means are also provided for transmitting an excitation signal to a receiver implanted with and connected to the conductors. The excitation signal creates an electrical field between the conductors. This field is transmitted through the conductive cochlea fluid to the nerve endings in the basilar membrane, thus replacing the naturally generated auditory electric field.

It is therefore an object of the present invention to provide a prosthetic device for the deaf.

It is a further object of the present invention to provide a method and apparatus for inducing the sensation of hearing in individuals suffering from total or near total sensory deafness.

It is a still further object of the present invention to provide a method and apparatus for inducing the sensation of hearing by electrical stimulation of the nerve endings of the auditory nerve.

It is another object of the present invention to provide a method and apparatus for inducing the sensation of hearing by positioning an electrode within the lower scala of the cochlea and coupling electrical excitation signals to such electrode so as to directly stimulate the nerve endings of the auditory nerve distributed along the basilar membrane within the cochlea.

It is still another object of the present invention to provide means for exciting an intra-cochlear electrode.

Another object of the present invention is the provision of an intra-cochlear electrode which may be used to stimulate the nerve endings of the auditory nerve.

Still other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the preferred embodiments constructed in accordance therewith, taken in conjunction with the accompanying drawings wherein like numerals designate like parts in the several figures and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the fundamental elements of the hearing process;

FIG. 2 is an enlarged, front elevation view, partly in section, of the human cochlea;

FIG. 3 is an enlarged, cross-sectional view taken along the line 3--3 in FIG. 2;

FIG. 4 is an enlarged, front elevation view of a first embodiment of intra-cochlear electrode;

FIG. 5 is a cross-sectional view taken along the line 5--5 in FIG. 4;

FIG. 6 is a cross-sectional view of the lower scala of the cochlea, similar to FIG. 3, showing the intra-cochlear electrode of FIGS. 4 and 5 in place;

FIGS. 7, 8 and 10 are front elevation views, partly in section, of alternate forms of intra-cochlear electrodes;

FIGS. 9 and 11 are cross-sectional views taken along the lines 9--9 and 11--11, respectively, in FIGS. 8 and 10, respectively;

FIG. 12 is a block diagram of a preferred embodiment of apparatus for exciting an intra-cochlear electrode;

FIG. 13 is a circuit diagram of a preferred embodiment of the receiving elements of the circuit of FIG. 12;

FIG. 14 is a view showing the physical configuration of a preferred embodiment of electrode and receiver; and

FIG. 15 is a pictorial representation of an alternate embodiment of means for exciting an intra-cochlear electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and, more particularly, to FIG. 1 thereof, there is shown, in block diagram form, the fundamental elements of the hearing process. Sound vibrations caused by an external sound pressure generator 10 set the air in motion producing spherical pressure waves 11. Pressure waves 11 are caught by the external ear 12 and transmitted to the tympanic membrane 13 which is displaced in response to such waves. The vibratory movements of the tympanic membrane 13 are transmitted via the auditory ossicles 14 to the oval window of the cochlea 15. Air cells within cochlea 15 function as a transducer to generate an alternating electrical field within cochlea 15. This electrical field is sensed by the nerve endings distributed through the basilar membrane and transmitted via the auditory nerve 16 to the celebral cortex 17 of the brain, which interprets such electrical signals as sound.

Referring now to FIGS. 2 and 3, the cochlea 15 is a long, narrow duct within the temporal bone which is wound spirally around its axis for approximately 21/2 turns. The cochlea is divided by a pair of membranes 21 and 22, extending longitudinally therethrough, into an upper scala 23, a middle scala 24 and a lower scala 25. The oval window 26, which is sealed by a membrane in contact with the stirrup of the auditory ossicles, presents an opening into upper scala 23. The round window 27, which is also closed by a membrane, provides an opening into lower scala 25. Cochlea 15 is filled with a fluid, the perilymph, which is free to circulate through upper scala 23 and lower scala 25 which are interconnected at the apex 28 of cochlea 15.

Membrane 21 between middle scala 24 and lower scala 25, called the basilar membrane, extends the entire length of cochlea 15. The auditory nerve 16 of the brain terminates in cochlea 15 and the nerve endings are distributed along basilar membrane 21. Minute hair cells 29 extend from basilar membrane 21 into middle scala 24.

The vibratory movements of tympanic membrane 13, which are transmitted by auditory ossicles 14 to oval window 26, are distributed through the cochlea fluid throughout cochlea 15. This vibratory input manifests itself in an alternating electrical field within the structure of the cochlea (which electrical field appears and has been detected at round window 27). This electrical field (generated by hair cells 29) is sensed by the nerve endings of the auditory nerve in basilar membrane 21 and transmitted via the auditory nerve 16 to the cerebral cortex 17 of the brain which interprets such electrical signals as sound.

Where total loss of hearing results from sensory deafness, i.e. a reduction in the sensitivity of the cochlea, no method or apparatus presently exists for restoring the sensation of hearing. However, in accordance with the present invention, it has been discovered that the sensation of hearing in people suffering from sensory deafness can be induced by direct electrical excitation of the auditory nerve endings distributed along basilar membrane 21 within cochlea 15 by using a bipolar electrode in the lower scala without the necessity of a complex encoding system such as disclosed in Doyle.

Referring now to FIGS. 4-6, direct electrical excitation of the auditory nerve endings within cochlea 15 is achieved by use of an intra-cochlear electrode, generally designated 30. The body 31 of electrode 30 is molded of a medically acceptable resilient material, such as silicone or other rubber or plastic material, which is of such a shape as to fit through round window 27 and into lower scala 25 of cochlea 15. Body 31 of electrode 30 includes a notch 32 which is designed to fit the round window margin and retain body 31 within lower scala 25 of cochlea 15. Electrode 30 further comprises a pair of gold or other suitable inert conductors 33 and 34 which are imbedded in and retained by base member 31. External leads 35 and 36 are connected to the ends of conductors 33 and 34, respectively, whereby leads 35 and 36 supply electrical signals to contacts 33 and 34, respectively. Body 31 may also include a stiffening member (not shown) imbedded therein, such as a strand of wire, to obtain the desired degree of resiliency.

As described more fully hereinafter, and as shown in FIG. 6, electrode 30 is inserted through round window 27 of cochlea 15 into lower scala 25 where it extends along the basilar membrane for approximately three-fourths of a turn thereof. According to the embodiment of FIGS. 4-6, conductors 33 and 34 are positioned side-by-side, adjacent basilar membrane 21, each of conductors 33 and 34 extending parallel to the length of membrane 21. In addition, the shape of base member 31 is such so as to provide a space between the surface 37 thereof opposite conductors 33 and 34 and the wall of lower scala 25 to permit circulation of the cochlea fluid through lower scala 25 as well as a fluid escape path during insertion.

With an electrical excitation signal applied to conductors 33 and 34 via leads 35 and 36, respectively, a uniform alternating electrical field is generated therebetween. Because of the position of the conductors 33 and 34 in the lower scala, the electrical field generated therebetween is applied so as to allow place frequency selection to take place unlike the Doyle system. This field is tramsmitted through the conductive cochlea fluid to the nerve endings in basilar membrane 21, thus replacing the naturally generated auditory electric field. The electric field generated by conductors 33 and 34 is sensed by the nerve endings distributed along the basilar membrane 21 and conducted via the auditory nerve 16 to the cerebral cortex 17 of the brain which interprets such electrical signals as sound.

According to a preferred embodiment of the present invention, and as shown in FIGS. 4-6, conductors 33 and 34 are made from 2 mil gold wire wound on a 5 mil mandrel for approximately 30 turns. Conductors 33 and 34 so formed are inserted into notches in base member 31 so as to slightly extend beyond the outer periphery of base member 31. The conductors are then imbedded within base member 31 by filling such notches with additional resilient material. Three to five strands of 2 mil gold wire may serve as leads 35 and 36 for conducting an electrical excitation signal to conductors 33 and 34. In addition, as explained previously, conductors 33 and 34 extend parallel to the length of basilar membrane 21 for approximately three-fourths of a turn. This affords conductive means along a substantial length of basilar membrane 21, thereby exciting a relatively large frequency spectrum. More specificially, it has been found through experimentation that the nerve endings within basilar membrane 21 are frequency selective. The nerve endings adjacent round window 27 respond to frequencies at the high end of the audio spectrum and decrease in frequency sensitivity as the apex 28 of cochlea 15 is approached. Accordingly, by extending conductors 33 and 34 for a substantial length along basilar membrane 21, a relatively large frequency spectrum may be excited.

Referring now to FIG. 7, there is shown an alternate embodiment for an intra-cochlear electrode, generally designated 40. The external configuration of electrode 40 is identical to the external configuration of electrode 30 and includes a base member 41 composed of medically acceptable resilient material. Imbedded within base member 41 are a pair of inert conductors 42 and 43 which are positioned on opposite sides of base member 41, rather than being adjacent, on the same side as in the case of electrode 30. However, in spite of this different placement of conductors 42 and 43, the operation of electrode 40 is identical to the operation of electrode 30. In addition, external lead wires 44 and 45 are connected by weld, pressure form or solder to one end of conductors 42 and 43, respectively. A notch 46 is also provided for retaining electrode 40 within cochlea 15 by pressure fit with the round window margin.

Other configurations of intra-cochlear electrodes suitable to other manufacturing methods are shown in FIGS. 8-11. In FIGS. 8 and 9, an intra-cochlear electrode, generally designated 50, includes a base member 51 and multiple point contacts 52 and 53 plus gold flashing 54 and 55 over the length of base member 51 on opposite sides thereof. Vacuum deposition or other attachment means may be used for attaching the gold flashing. Lead wires 56 and 57 are connected to the multiple point contacts 52 and 53, respectively, to conduct electrical excitation signals to gold flashing 54 and 55, respectively.

In FIGS. 10 and 11, an intra-cochlear electrode, generally designated 60, includes a base member 61 and large, flexible conductors 62 and 63 imbedded directly within base member 61 plus flashed or vacuum deposited gold or platinum surfaces 64 and 65 positioned on the surface of base member 61 on opposite sides thereof. As before, a pair of lead wires 66 and 67 conduct electrical excitation signals to conductors 62 and 63, respectively.

Once intra-cochlear electrode 30, 40, 50 or 60 is positioned within lower scala 25 of cochlea 15, as will be explained more fully hereinafter, there must then be provided a means for coupling electrical signals to the conductors thereof. The problem with inducing electrical signals within the cochlea is, of course, the fact that no orifices are available for ready access to the tympanic cavity. The cochlea is well shielded within the heavy bony structure of the skull so that no direct entrance is possible without risk of infection. The problem then become one of coupling electrical signals to the electrode within the cochlea without the use of normal wire conductive means.

Referring now to FIG. 12, there is shown a preferred embodiment of apparatus for exciting an intra-cochlear electrode. In this embodiment of FIG. 12, the vibrations of the surrounding air are sensed by a microphone 70 which converts the mechanical vibrations to an electrical signal in the audio spectrum which is applied to a preamplifier 71. The output of preamplifier 71 is applied via a tone control network 72, to be described more fully hereinafter, to a modulator 73. Modulator 73 is operative to modulate the output of a combination oscillator V.H.F. transmitter 74. The output of oscillator/transmitter 74 is applied to an antenna 75 which, in its preferred form, is an inductive coil. The transmitting network, consisting of elements 70-75, may be mounted externally of the body to sense the sound waves and convert such sound waves into a modulated R.F. signal. This modulated R.F. signal is sensed by a receiving antenna 76, which may also be an inductive coil, and applied to a receiver 77. The output of receiver 77 is demodulated by a demodulator 78 to restore the original audio excitation signal appearing at the output of tone control network 72. Finally, the output of demodulator 78 is applied to an intra-cochlear electrode 79.

The transmitting network consisting of elements 70 through 75 may have any suitable configuration since elements 70-75 are positioned externally of the body, as will be explained more fully hereinafter, and size and complexity are not problems. On the other hand, since components 76-78 will be positioned internally of the body with electrode 79, they should be as simple as possible. A preferred configuration for elements 76-78 is shown in FIG. 13.

Referring now to FIG. 13, receiver 77 may comprise a tuned circuit consisting of inductive coil 76 and a capacitor 81 tuned to the frequency of oscillator/transmitter 74. In the case where modulator 73 is an amplitude modulator, demodulator 78 may simply comprise a diode connected to one side of capacitor 81. The output of diode 82 may be shunted by a capacitor 83 and conducted via resistors 84 and 85 and a lead 87 to one conductor of electrode 79. An additional diode 86 shunts the junction between resistors 84 and 85, the other side of capacitor 81 being connected via a lead 88 to the other conductor of electrode 79. In such circuit, capacitor 83 and resistor 84 act as a filter and current limiter, respectively. Diode 86 is a noise limiting diode such that noise peaks occurring due to electromagnetic discharges can be limited by forward conduction of diode 86. Resistor 85 also acts as a current limiting resistor in feeding the audio signal to electrode 79.

Referring now to FIG. 14, the physical configuration of a preferred embodiment of the present invention is shown. Intra-cochlear electrode 79 is made integral with a continuous length 90 of medically acceptable resilient material in which leads 87 and 88 are imbedded. One end of leads 87 and 88 are connected to the conductors within electrode 79 whereas the other ends of leads 87 and 88 are connected to a small integrated circuit chip or substrate 91 carrying the inductors, capacitors, resistors and diodes. Included with chip 91 is receiving coil 76. Chip 91 is also imbedded within the resilient material. This entire structure, generally designated 92, would then be implanted "in vivo". A typical surgical procedure is as follows: The patient is placed on an operating table with the appropriate ear in a horizontal position exposed in a sterile field. The auricle is folded forwardly and clamped in position and an incision made posterior to the ear. Entry to the middle ear is gained by elevating the skin along the auditory meatus which permits a direct by-pass of the tympanic membrane. With the skin along the auditory meatus and the tympanic membrane elevated, visual contact may be made with the middle ear and the oval and round windows at the entrance to the cochlea. A bony promontory protruding above the round window is then removed to permit free access to the round window. The round window membrane is then removed and a portion of the upper margin excavated for easy access to the lower scala. Electrode 79 is then inserted through the round window such that the electrode conductors lay in close proximity to the basilar membrane between the lower scala and the scala media. Electrode 79 is inserted into the lower scala until the notch therein slips into the round window margin. A channel is then excavated in the bony structure along the auditory canal for ocation of material 90 containing leads 87 and 88, material 90 then being sutured into position. The elevated skin along the auditory canal is then carefully returned to its original position and the canal packed to assure proper adhesion.

The skin posterior to the incision is elevated and a small portion of the muscular structure attached to the skull removed to receive chip 91 which is then sutured in place. A pair of test leads 93 and 94, as shown in FIG. 14, are connected directly to leads 87 and 88 and extend outwardly from material 90 adjacent chip 91. Test leads 93 and 94 are brought out through the incision. The transmitting network is then activated and a signal transmitted to receiver 77. The voltage across the intra-cochlear electrode is monitored on an oscilloscope via test leads 93 and 94 to insure operability. After the electrode is tested and found operative, test leads 93 and 94 are clipped and the incision sutured. The external ear is then returned to its normal position and the surgical procedure is completed.

With the surgical procedure completed, receiver 77 and antenna 76 in chip 91 are positioned immediately posterior to the ear close to and under the skin. A unit may then be mounted behind the ear, such unit including microphone 70, oscillator/transmitter 74 and transmitting antenna 75. The output of microphone 70 may be conducted through electrical leads to a pocket-carried unit containing preamplifier 71, tone control network 72, modulator 73 and a suitable power supply (not shown). The output of modulator 73 is then coupled back to the ear-mounted unit to oscillator/transmitter 74 and transmitting antenna 75. In this manner, transmitting and receiving antennas 75 and 76 will be positioned in close proximity to each other, only a thin layer of skin separating the two elements. As a result, the current passing through antenna 75 is induced in antenna 76 and applied to receiver 77.

With the elements so positioned, operation is as described previously with respect to FIGS. 12 and 13. In summary, the vibrations of the surrounding air are sensed by microphone 70 and converted to an amplified, shaped, modulated R.F. signal by components 71-74. The modulated signal is transmitted by antenna 74 to antenna 76 where receiver 77 and demodulator 78 reproduce the original audio excitation signal and apply it via leads 87 and 88 to electrode 79. With such electrical excitation signal applied to electrode 79, an electric field is generated between the conductors thereof. The electrical field so generated varies in amplitude proportioned to the pressure vibrations of the surrounding air, i.e., the audio signal to be heard. In other words, such field is an analog of the audio signal to be heard. The field generated between the conductors of electrode 79 is transmitted through the conductive cochlea fluid to the nerve endings in the basilar membrane, thus replacing the naturally generated auditory electric field. The electric field so generated is sensed by the nerve endings distributed along the basilar membrane and conducted via the auditory nerve to the cerebral cortex of the brain which interprets such electrical signals as sound.

Tone control network 72 is provided to shape the frequency spectrum of the signal applied to electrode 79, if desired. More specifically, initial tests with the present system have shown that it is not easy and, as a matter of fact, quite difficult, for a patient who has never heard to properly interpret the electrical stimulus now being applied to the auditory nerve endings. For this reason, it has been necessary to initially shape the frequency spectrum applied to a particular patient to correspond to stimuli his brain is capable of interpreting. As the patient gains experience in interpreting the signals applied to his cochlea, the frequency spectrum of the applied signal is slowly increased. Accordingly, tone control 72 is inserted between preamplifier 71 and modulator 73 to provide the desired shaping of the applied excitation signal.

The above-described system, including the intra-cochlear electrode shown in FIG. 4 and the electronics described with respect to FIGS. 12-14 have been tested by implantation in selected patients at Sequoia Hospital, Redwood City, California. The surgery has been performed by Dr. Robin P. Michelson. In one such implant procedure, the surgical approach was identical to that described hereinbefore. The patient was tested the following day by transmitting signals to the receiver and then in turn to the electrode. The patient exhibited the ability to distinguish tones over the frequency range 125 Hz to 4,000 Hz. His frequency discrimination at one octave steps from 250 Hz to 4,000 Hz was excellent. He exhibited amplitude discrimination of pure tones with a calculated change of less than 2 db. The patient further exhibited a dynamic range, i.e. threshold to maximum listening level, of approximately 10 db. He was given a Spondee Test and was able to recognize 6 of 35 words. The patient's previous score on this test with a hearing aid was zero. In further tests on this patient, transmitting to the patient a random series of the numbers one through 10, the patient is presently capable of correctly distinguishing the numbers approximately 65 percent of the time. In a second patient, where a similar implant procedure has been performed, such patient is capable of distinguishing a random series of the numbers one through ten approximately 90-95 percent of the time.

Several other configurations of excitation means are possible. With respect to the embodiment of FIG. 12, modulator 73 need not be an amplitude modulator but may be a frequency modulator, a pulse-width modulator or any other conventional form of modulator. In addition, it is theoretically possible to completely eliminate the use of a modulator. More specifically, the output of tone control network 72 may be applied directly to transmitting coil 75. Receiving coil 76 would then be connected directly to leads 87 and 88 of electrode 79, although filtering and noise and current limiting components may be employed. In other words, it is theoretically not necessary to modulate the audio signal on an R.F. carrier and then demodulate the signal, but rather the audio signal may be directly applied to receiving antenna 76 and electrode 79. The potential difficulty with this approach is possible patient sensitivity to random audio frequency noise in the atmosphere so that the modulation approach is preferred.

Referring now to FIG. 15, there is shown a still further embodiment of a means for exciting an intra-cochlear electrode where the auditory ossicles are intact. As explained previously, the tympanic membrane 101 couples sound vibrations through the hammer 102, the anvil 103 and the stirrup 104 to the membrane at the oval window of the cochlea. Accordingly, a small, permanent magnet 105 may be attached to the stirrup 104, as shown. An audio pick-up coil 106 may then be imbedded within the base of an intra-cochlear electrode 108 and connected via leads 107 to the conductors thereof. Coil 106 would be sutured adjacent or surrounding magnet 105.

In operation, motion of permanent magnet 105 in proximity to coil 106 induces an d.m.f. within coil 106. This e.m.f. is then carried by leads 107 to the electrode and presented to the basilar membrane between the scala media and the lower scala within the cochlea. The conversion of this e.m.f. to sound would then be as described previously.

In tests which have been conducted on human patients it has been found that the threshold of auditory perception under electrical stimulation is a function of frequency. The voltage measured across the conductors of the intra-cochlear electrode which is required to stimulate auditory perception increases as frequency increases. At 1 KHz, three patients exhibited a threshold of auditory perception of approximately 0.5 volt while at 100 Hz, this threshold was observed to be approximately 0.1 volt. At higher frequencies, in the range of 2 to 5 KHz, stimulation levels as high as 1 volt were required. It has been found that the minimum electrical stimulation required for auditory perception in human patients is about 0.05 volt. Thus, in order to induce hearing in a human subject, it is necessary that at least 0.05 volt be impressed across the conductors of the intracochlear electrode of the present invention.

Comparative tests between brain reception and electrical or acoustical stimulation in cats and human subjects have been recently conducted to determine the stimulation level required to achieve equivalent hearing results. The brain reception of electrical and acoustical stimulation in cats was determined by recording the electrical response of the inferior colliculus, one of the higher hearing centers in the brain, since obviously a cat cannot relate to the investigator its level of auditory perception. The test demonstrated that cats appear to show identical responses from electrical and acoustical stimulation when stimulated at the same level as human patients. Since the minimum auditory perception threshold in humans is approximately 0.05 volt, it can be deduced that a comparable minimum perception level occurs in cats. Because the maximum electrical stimulation utilized in the previous tests described in the aforementioned Michelson article entitled "The Crossed Cochlea Effect" was 250 microvolts, it is apparent from the comparative tests discussed hereinbefore that the cats used in the previous tests could not hear at the levels of stimulation utilized.

Therefore, and in accordance with the present invention, there is disclosed a method and apparatus for including the sensation of hearing by direct electrical stimulation of the auditory nerve endings of the auditory nerve. Since the present technique completely by-passes the external and middle ears and most of the internal ear other than the basilar membrane, it may be effectively used to induce the sensation of hearing in people suffering from deafness caused by abnormalities in any of these areas. However, the primary use will be in the case of sensory deafness which has, heretofore, been untreatable.

In accordance with the present invention, the sensation of hearing is induced by positioning an intra-cochlear electrode within the lower scala of the cochlea, such electrode being surgically inserted through the round window. The electrode includes a pair of conductors which extend parallel to the length of the basilar membrane. Means are disclosed for transmitting an excitation signal to a receiver implanted with and connected to the conductors. Such excitation signal creates a uniform, alternating electrical field between the conductors, which electrical field is transmitted through the conductive cochlea fluid to the nerve endings in the basilar membrane, thus replacing the naturally generated auditory electric field. The electrical field so generated is sensed by the nerve endings distributed through basilar membrane 21 and conducted via the auditory nerve to the cerebral cortex of the brain which interprets such electrical signals as sound.

While the invention has been described with respect to the preferred physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications and improvements may be made without departing from the scope and spirit of the invention. For example, although the preferred embodiment of apparatus for generating an alternating electrical field along the basilar membrane within the cochlea comprises an electrode adapted to be positioned within the lower scala of the cochlea, it will be appreciated by those skilled in the art tht it is theoretically possible, although not presently practical, to position an electrode within the upper scala or the middle scala of the cochlea. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.

Claims

I claim:

1. Apparatus for inducing sensations of hearing in a human subject comprising

an elongate curved electrode adapted to be positioned within the lower scala of a cochlea of the subject, with the electrode extending lengthwise along a portion of the lower scala,

receiving means coupled to said electrode for receiving an electrical excitation signal being an analog of an externally generated audio signal and for applying said excitation signal uniformly to said electrode, and

transmitting means responsive to said externally generated audio signal for transmitting said electrical excitation signal to said receiving means.

2. The apparatus of claim 1 wherein said electrode includes

an elongate resilient base member shaped to substantially conform to part of the inner surface of the lower scala, and

a pair of conductors secured to said base member for receiving said electrical excitation signal from said receiving means, said conductors extending lengthwise of the base member whereby each of the conductors is positioned proximate auditory nerve endings distributed along the same longitudinal section of the cochlea.

3. The apparatus of claim 2 wherein said base member has a cross-sectional area less than the cross-sectional area of the portion of the lower scala within which it is positioned whereby flow of cochlea fluid past the base member is relatively unimpeded.

4. Apparatus according to claim 2 wherein said transmitting means comprises:

transducer means responsive to said externally generated audio signal for converting said audio signal into said electrical excitation signal; and

transmitting antenna means adapted to be positioned adjacent the external ear of the subject for radiating said excitation signal;

and wherein said receiving means for receiving and applying said excitation signal to said conductors comprises;

receiving antenna means adapted to be positioned under the skin adjacent said external ear; and

a pair of electrical leads connected between said receiving antenna means and said conductors.

5. Apparatus according to claim 4 wherein said receiving antenna means is connected via said electrical leads to said conductors, said receiving antenna means and said leads being imbedded within a resilient material, said receiving antenna means being adapted to be implanted between the skin and the temporal bone of the skull, posterior to the ear of the subject, and wherein said transducer means and said transmitting antenna means are positioned exterior to the skin, said transmitting antenna means being adapted to be positioned posterior to said ear adjacent to said receiving antenna means.

6. Apparatus according to claim 4 wherein said transmitting means further comprises:

means for generating a carrier signal having a frequency outside of the audio spectrum, said carrier signal being coupled to said transmitting antenna means; and

means for modulating said carrier signal with said electrical excitation signal; and wherein said means for receiving and applying said excitation signal to said conductors further includes:

tuned circuit means having a resonant frequency corresponding to the frequency of said carrier signal; and

means coupled between said tuned circuit means and said pair of electrical leads for demodulating said carrier signal and for applying said excitation signal to said electrical leads.

7. Apparatus according to claim 6 wherein said modulator means is an amplitude modulator and wherein said means for demodulating comprises:

a diode coupled between said tuned circuit means and said pair of electrical leads.

8. Apparatus according to claim 6 wherein said tuned circuit means, said demodulator means and said receiving antenna means are formed on a microelectronic substrate.

9. Apparatus according to claim 2 wherein said transmitting means comprises:

permanent magnet means permanently attached to the stapes of the auditory ossicles of the subject; and wherein said receiving means includes,

an inductive pick-up coil imbedded within said resilient base member and connected to said conductors, said inductive pick-up coil being positioned adjacent said permanent magnet.

10. Apparatus according to claim 2 wherein said conductors are flexible and are imbedded within said base member in side-by-side relationship proximate to the basilar membrane of the cochlea.

11. Apparatus according to claim 10 wherein a portion of each said conductor projects beyond the outer periphery of said base member.

12. Apparatus according to claim 10 wherein said conductors are positioned on opposite sides of said base member.

13. Apparatus according to claim 2 wherein each of said conductors includes

multiple point contacts imbedded within said base member, said point contacts being arranged along a line adapted to extend parallel to the length of the basilar membrane within the cochlea; and

a conductive coating on the surface of said base member, said point contacts being in electrical contact with said coating.

14. Apparatus according to claim 2 wherein said receiving means applies a potential of at least 0.05 volts across said conductors.

15. Apparatus for inducing sensations of hearing in a human subject comprising

elongate electrode means adapted to be positioned in a cochlea of the subject, lengthwise of a portion of the lower scala, responsive to an alternating electrical excitation signal for applying a uniform electrical field to stimulate auditory nerve endings located along a corresponding lengthwise portion of the basilar membrane to which said electrode means is proximate, and

generator means for inductively coupling to said electrode means an alternating electrical excitation signal which is an analog of an audio signal.

16. The apparatus of claim 15 wherein said electrode means includes an elongate curved resilient base member and a pair of elongate conductors secured to said base member lengthwise thereof whereby said conductors are positioned lengthwise of the cochlea.

17. The apparatus of claim 16 wherein said generator means includes first means adapted to be implanted under the skin of the subject for applying said excitation signal to said conductor; and

second means responsive to said audio signal for inductively coupling said excitation signal to said first means.

18. The apparatus of claim 16 wherein each of said conductors comprises an elongate member adapted to extend parallel to the length of the basilar membrane and wherein said generator means generates a uniform field along a substantial portion of said basilar membrane.

19. Apparatus according to claim 16 wherein said generating means is operative to generate a uniform field along a substantial portion of said basilar membrane.

20. A method for inducing the sensation of hearing in a human subject comprising:

positioning an electrode within the cochlea, said electrode including a base member and a pair of conductors retained upon said base member said electrode being positioned with said conductors extending longitudinally along a portion of the chamber; and

inductively coupling an electrical excitation signal being an analog of an externally generated audio signal to said conductors.

21. The method of claim 20 wherein a potential of not less than about 0.05 volts is impressed across said conductor.

22. The method of claim 20 wherein the step of inductively coupling an electrical excitation signal to said conductors comprises:

implanting a receiving antenna under the skin adjacent the external ear;

connecting said receiving antenna to said conductors;

positioning a transducer externally of the skin, said transducer converting said externally generated audio signal into said electrical excitation signal;

coupling the output of said transducer to a transmitting antenna; and

positioning said transmitting antenna externally of the skin adjacent the external ear adjacent said receiving antenna.

23. The method of claim 22 wherein the step of coupling the output of said transducer to a transmitting antenna comprises:

generating a carrier signal having a frequency outside of the audio spectrum;

coupling said carrier signal to said transmitting antenna;

modulating said carrier signal with said electrical excitation signal; and wherein said step of connecting said receiving antenna to said conductors comprises:

connecting said receiving antenna to a tuned circuit having a resonant frequency corresponding to the frequency of said carrier signal;

demodulating said carrier signal; and

coupling said demodulated carrier signal to said electrical leads.

24. The method of claim 20 wherein the step of inductively coupling an electrical excitation signal to said conductors comprises:

attaching a permanent magnet to the stapes of the auditory ossicles;

embedding an inductive pick-up coil within said base member;

connecting said inductive pick-up coil to said conductors; and

positioning said inductive pick-up coil adjacent said permanent magnet.