HARVARD MAHONEY NEUROSCIENCE INSTITUTE
HARVARD MEDICAL SCHOOL
220 Longwood Ave., Boston, MA 02115
_____________________________________________________
Correspondence/Circulation
1001 G Street, NW, #1025, Washington D.C. 20001
Fall 1994 Volume 3, Number 4
Cochlear Implants
Restoring Hearing to the Deaf
by Donald K. Eddington, Ph.D. and Michael L. Pierschalla
Until age 20, Michael, co-author of this article, enjoyed normal
hearing and a world filled with sound. He tells the story this way:
"From a very early age I loved all music and I loved to sing.
I spent the better part of my adolescence with my head firmly wedged
between a pair of stereo earphones - taking them off only to bathe
- and I bought a noisy guitar, convinced that it would soon bring
me a million dollars and a solid gold Cadillac.
"That world changed abruptly for me in 1975. With no warning,
with no significant malady, and for no apparent reason, I suddenly
lost my hearing over the span of three days. Many months later an
audiologist was able to detect a shadow of residual hearing that,
together with a hearing aid, provided some benefit - until one morning
I woke to find that hearing had vanished overnight.
"I realized then that I was facing a long future without my
hearing, that the best I could hope for was to hear again in my dreams
- and I made a vow never to forget the music, the sound of rain falling
on the sidewalk, my parents' voices. And then I went on and faced
life in a different way, looking for another identity.
"When we talk about the loss of one of our senses or abilities,
we talk almost exclusively in the language of accommodations, adaptation
and acceptance. Faced with this sort of challenge, our concept of
healing and recovery often takes on a new meaning - one that is focused
on a spiritual and psychological, rather than a physical, recovery.
If we can no longer expect to join nerves back together, then we try
instead to reconfigure the soul and the self."
Today, those who lose their hearing still have many adjustments to
make. Some choose to enter the vibrant deaf culture and form an expanded
circle of friends where fluent communication, based mainly on American
Sign Language (ASL) is a refreshing change from the more difficult
and stressful interactions with the hearing world.
Others choose a relatively new option, the cochlear implant, that
seeks to restore some hearing by bypassing a segment of the ear's
malfunctioning machinery and replacing it with an electrical substitute.
The cochlear implant provides many deaf people with enough hearing
to participate much more fluently in the hearing world. Many also
find that the implant makes it much easier to maintain and develop
the relationships they had established using spoken language.
About 6,000 individuals have had cochlear implants, since the late
1980's. However, a fully functional, artificial replacement of a major
neuronal function remains one of the great unconquered frontiers of
brain research. The cochlear implant is the first, and still the only,
neural prosthesis that is aiding a significant portion of a disabled
population.
Understanding how cochlear implants work is best accomplished by
first considering some fundamental aspects of normal hearing and the
problems that cause deafness.
Most people with profound deafness have lost the ability to translate
the acoustic energy of sound into the electrical signals carried to
the brain by the 30,000 fibers of the auditory nerve. In normal hearing,
sound waves travel along the external ear canal and cause the tympanic
membrane (ear drum) to vibrate. The three small bones of the middle
ear (malleus, incus and stapes) conduct these vibrations to the snail-shaped
cochlea of the inner ear.
The cochlea is divided along its length by the mechanically-tuned
basilar membrane. This membrane distributes vibrational energy longitudinally
by frequency: The lowest frequencies cause maximum membrane motion
near the cochlea's apex; the highest frequencies maximize motion near
the base. Four parallel rows of hair cells (named for their tufts
of hair-like cilia) extend along the length of the basilar membrane
and, when vibrated, elicit electrical activity on the auditory nerve
fibers.
Thus, a high-pitch sound causes the basilar membrane to vibrate mainly
near the cochlear base, and the basal nerve fibers are excited by
their hair cells. Similarly, hair cells near the cochlear apex vibrate
and excite their nerve fibers during the presentation of low-pitch
tones. More complex sounds like speech produce very complex but predictable
patterns of activity across the array of auditory nerve fibers. It
is these patterns of electrical activity that the brain interprets
as sound.
Destruction of cochlear hair cells and the related degeneration of
auditory nerve fibers results in sensorineural hearing loss. This
type of damage likely accounts for most of the 250,000 people in the
United States with profound deafness.
Elargement of an implanted cochlea shows how electrodes are positioned
to activate the auditory nerve fibers. Adapted by Leigh Coriale Design
and Illustration from "Functional Replacement of the Ear,"
by Gerald E. Leob. Copyright 1985 by Scientific Aberican, Inc., all
rights reserved.
Hair-cell damage that leads to deafness results from a number of causes,
including: acoustic trauma, ototoxic drugs, bacterial and viral infections,
autoimmune disease (probably the cause of Michael's hearing loss)
and genetic disorders.
Once severely damaged, hair cells neither recover nor regenerate.
For hearing-impaired individuals with sufficient residual hearing,
classical hearing aids that simply amplify sound can be very effective,
but they offer limited utility to many of the profoundly deaf.
Cochlear implants bypass the external and middle ears by using electrical
stimulation of electrodes implanted in the cochlea to reintroduce
the signals carried by auditory nerve fibers to the brain. A microphone
in a behind-the-ear hearing aid case is connected to a package of
electronics, called a sound processor, about as big as a Walkman,
that is worn on a belt or carried in a pocket.
An implant (shown in black) is positioned to directly stimulate the
cochlea and bypass the normal pathways of the ear canal and three
bones of the middle ear. Adapted by Leigh Coriale Designa and Illustration
from an image by Smith and Nephew Richards, Inc., with permision.
The sound processor translates the microphone signal into a set
of four to eight electrical stimuli. Directed to auditory nerve fibers
using an array of electrodes implanted in the deaf patient's cochlea,
these stimuli elicit patterns of nerve activity that the brain interprets
as sound.
The goal of this technology is to elicit patterns of nerve activity
that mimic those of a normal ear for a wide range of sounds. Ideally,
such a system would enable people deafened later in life to spontaneously
recognize all types of sound (including speech), and also provide
the input required for many children deafened at a young age to acquire
speech.
While this goal has not been completely realized, today's devices
enable about 10 percent of those implanted to communicate without
lip reading and the vast majority to communicate fluently when the
sound is combined with lip reading. Our group of audiologists, physicians
and scientists (from the Massachusetts Eye and Ear Infirmary, Harvard
Medical School and the Massachusetts Institute of Technology), funded
by the National Institutes of Health, is investigating the fundamental
mechanisms responsible for the hearing gained with cochlear implants
- and using this understanding to improve the range and clarity of
the sound the user experiences. One focus of current research is to
reduce interference between stimuli. We have all experienced hearing
a background conversation from a different telephone line while conversing
with the person we dialed. This kind of "crosstalk" also
occurs in implants and causes interference between the signals on
different electrodes. In addition, the effect of a stimulus can be
influenced by the stimuli that precede it. Techniques that provide
better spatial and temporal separation between stimuli have resulted
in 20 percent improvements in speech reception in the laboratory.
These and other improvements should move users toward the goal of
normal hearing as they are implemented in commercial devices.
Because cochlear implants do not restore normal hearing, a decision
whether to continue as a deaf person in the hearing world, learn ASL
and move into the deaf culture, or to pursue the implant option is
neither obvious nor casual. As Michael relates, "I had almost
never seen ASL before I became deaf, and didn't encounter it until
two years later. I thought it was a very beautiful, expressive language
- but so are French and Japanese and Finnish, and they are all equally
foreign to me! "As a deafened adult, my world was still suffused
with spoken English and I did not feel that I had the option of avoiding
it or finding a substitute. I became adept at speechreading but that
provides only 50 to 60 percent of the information needed to communicate
- and you still can't speechread the sound of a speeding car horn.
"My friends now remind me that in conversation I tended to lean
forward in my chair, tense with concentration, and that I had a habit
of talking a lot and asking little. One of them once remarked, 'You
know, it's much easier to get to know you than it is to get you to
know me.' When I look back on the partial restoration of my hearing
nine years ago, one of my most poignant memories is of simply relaxing
back in my chair and asking that same friend, 'So what have you been
up to this week?'
"To be given a chance to hear anything at all again, to talk
and listen more easily with family and friends, is an experience so
unique that I find it hard to put into words."
Dr. Eddington is Director, Cochlear Implant Research Laboratory,
Massachusetts Eye and Ear Infirmary; Associate Professor, Harvard
Medical School; and Principal Research Scientist, Massachusetts Institute
of Technology (MIT). Mr. Pierschalla is an artist and teacher who
collaborates as a subject with researchers at Harvard, MIT and Duke
Universities.
