U.S. patent number 3,751,605 [Application Number 05/223,416] was granted by the patent office on 1973-08-07 for method for inducing hearing.
This patent grant is currently assigned to Beckman Instruments, Inc.. Invention is credited to Robin P. Michelson.
United States Patent |
3,751,605 |
Michelson |
August 7, 1973 |
METHOD FOR INDUCING HEARING
Abstract
There is disclosed 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. 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. An electrical excitation signal corresponding to an
externally generated audio signal is conducted to the conductors of
the electrode thereby generating a uniform, alternating electrical
field along the basilar membrane which replaces the naturally
generated auditory electrical field.
Inventors: |
Michelson; Robin P. (Redwood
City, CA) |
Assignee: |
Beckman Instruments, Inc.
(Fullerton, CA)
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Family
ID: |
22836398 |
Appl.
No.: |
05/223,416 |
Filed: |
February 4, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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75076 |
Sep 24, 1970 |
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Current U.S.
Class: |
607/57; 623/10;
607/137; 600/25 |
Current CPC
Class: |
A61N
1/36038 (20170801) |
Current International
Class: |
A61F
11/04 (20060101); A61F 11/00 (20060101); A61N
1/36 (20060101); H04r 025/00 () |
Field of
Search: |
;179/17R,17BC,17E
;128/1R ;3/1 ;181/23 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
The Crossed Cochlea Effect by Michelson, X-actions of American
Largngological, Rhinological & Othological Society, Inc.,
Presented 1/3/68, Publication Received Library of Medicine
1/8/69..
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: D'Amico; Thomas
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of my copending
application Ser. No. 75,076, filed Sept. 24, 1970 now abandoned.
Claims
I claim:
1. A method for inducing the sensation of hearing on human subjects
comprising:
generating an alternating electrical field along the basilar
membrane within the cochlea, said electrical field being an analog
of an audio signal to be heard.
2. The method of claim 1 wherein the step of generating an
alternating electrical field along the basilar membrane
comprises:
positioning an electrode within the cochlea, said electrode
including a base member and a pair of conductors retained by said
base member; and
conducting an electrical excitation signal which is an analog of
said audio signal to said conductors.
3. The method of claim 2 wherein a potential of not less than about
.05 volts is impressed across said conductors.
4. A method for inducing the sensation of hearing in human subjects
comprising:
generating a uniform, alternating electrical field along a
substantial portion of the basilar membrane within the cochlea,
said electrical field being an analog of to an audio signal to be
heard.
5. The method of claim 4 wherein the step of generating a uniform,
alternating electrical field along the basilar membrane
comprises:
positioning an electrode within the lower scala of the cochlea,
said electrode including a base member and a pair of elongated
conductors retained by said base member, said conductors adapted to
extend parallel to the length of the basilar membrane; and
conducting an electrical excitation signal corresponding to said
audio signal to said conductors.
6. The method of claim 5 wherein a potential of not less than about
0.05 volts is impressed across said conductors.
7. A method for inducing the sensation of hearing in human subjects
comprising:
positioning an electrode within the lower scala of the cochlea,
said electrode including a resilient base member and a pair of
conductors retained by said base member; and
conducting an electrical excitation signal which is an analog of an
externally generated audio signal to said conductors.
8. The method of claim 7 wherein each of said conductors comprises
an elongated member adapted to extend parallel to the length of the
basilar membrane, said conductors being positioned side-by-side in
said base member, immediately adjacent said basilar membrane.
9. The method of claim 7 wherein a potential of not less than 0.05
volts is impressed across said conductors.
Description
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.infin. 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.
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