BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for inducing hearing and, more particularly, to a method 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 my article 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 I describe experiments I 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 an 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 interaction between the two ears of a cat. The acoustic reflex, however, is 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 my article entitled "Electrical Stimulation of the Human Cochlea in Sensory Deafness" published in Archives 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 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. An electrical excitation signal corresponding to an externally generated audio signal is conducted to the conductors. The excitation signal creates a uniform, alternating 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 method for inducing hearing.
It is a further object of the present invention to provide a method 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 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 for inducing the sensation of hearing by positioning an electrode within the lower scala of the cochlea and conducting electrical excitation signals to such electode so as to directly stimulate the nerve endings of the auditory nerve distributed along the basilar membrane within the cochlea.
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 embodiment 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 showng 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 preferred 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;
FIG. 7 is a block diagram of a preferred embodiment of apparatus for exciting an intra-cochlear electrode;
FIG. 8 is a circuit diagram of a preferred embodiment of the receiving elements of the circuit of FIG. 7; and
FIG. 9 is a view showing the physical configuration of a preferred embodiment of electrode and receiver.
DESCRIPTION OF THE PREFERRED EMBODIMENT
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. Hair 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 cerebral 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 arond 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 from 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 cochlea 15 (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 (nto 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 transmitted through the conductive cochlea fluid to the nerve endings in basilar membrane 21, thus replacing the naturally generated auditory electrical 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 the preferred embodiment of the present invention, and as shown in FIGS. 4-6, conductors 33 and 34 are made from gold wire wound on a manderel. 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 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 afford conductive means along a substantial length of basilar membrane 21, thereby exciting a relatively large frequency spectrum. More specifically, 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.
Once intro-cochlear electrode 30 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. In addition, the cochlea is well shielded within the heavy bony structure of the skull. A direct electrical connection may be made with normal wire conductive means but this introduces the risk of infection. The problem then becomes one of coupling electrical signals to the electrode within the cochlea without the use of normal wire conductive means.
Referring now to FIG. 7, there is shown a preferred embodiment of apparatus for exciting an intra-cochlear electrode. In the embodiment of FIG. 7, 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/U.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 reciever 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. 8.
Referring now to FIG. 8, 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. 9, 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 cicuit 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 sutructure, 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 then 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 location 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. 9, 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 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 74 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 74 is induced in antenna 76 and applied to receiver 77.
With the elements so positioned, operation is as described previously with respect to FIGS. 7 and 8. 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 75 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. 7-9 has been tested by implantation in selected patients at Sequoia Hospital, Redwood City, Calif. 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 six 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 ten, 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 10 approximately 90-95 percent of the time.
In the tests which I have conducted on human patients I have 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. I have 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 intra-cochlear electrode of the present invention.
I have recently conducted comparative tests between brain reception and electrical or acoustical stimulation in cats and human subjects 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 tests 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 my 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 for inducing 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 electric 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 embodiment 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 the present method for generating an alternating electric field along the basilar membrane within the cochlea utilizes an electrode adapted to be positioned within the lower scala of the cochlea, it will be appreciated by those skilled in the art that 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.