The Miracle of Hearing: The Biomedical Engineering of Cochlear Implants
Lauren Falahee
Keywords: cochlear implant, hearing loss, auditory nerve, signal processing, speech
recognition
The cochlear implant is a biomedical hearing device that allows individuals who suffer
from severe hearing loss to interpret environmental sounds and understand speech. The
cochlear implant has been successful in many individuals, giving them ability to live life
with sound. The implant uses a microphone to detect sounds from the environment and
send them to a speech processor, where they are converted into signals. The signals are
transmitted to an electrode within the cochlea of the ear, where they electrically stimulate
the auditory nerve. The auditory nerve sends the stimulus to the brain, where it is
interpreted as sound. Although the cochlear implant allows its users to hear a
substantial amount, there are some limitations. Individuals with the implant have
difficulty pinpointing voices within a loud environment or crowd, and occasionally have
trouble with speech recognition. Some solutions to this include using bilateral implants,
which take advantage of natural hearing effects, and using electrodes that more closely
mimic the natural way of “hearing.” With further advances in the cochlear implant,
hearing impaired individuals may one day be able to obtain normal hearing abilities.
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Introduction
[Play clip of Clair de Lune (or another classical masterpiece). Imagine not being able to
hear this masterpiece, or the other beautiful noises of life such as the singing of birds, the
babbling of brooks, or the sound of a loved one’s voice.]
For one little girl, Cecilia, the fear of never being able to hear anything was once a
reality. Cecilia was diagnosed with severe hearing loss at the age of 15 months [1]. She
initially used hearing aids, but they only provided slight improvement and did not aid in
her speech and auditory development [1]. Her doctors determined that a cochlear implant
would be a much better option. After receiving her cochlear implant, Cecilia was able to
develop her speech and hear much more effectively. As a teenager, she was
mainstreamed in public school and received a GPA of 3.94 through spoken language
learning [1]. She also participated on her high school’s volleyball team, ecstatic to have
the ability to contribute to her school’s pride [1]. Cecilia considers herself to be a hearing
individual and the cochlear implant has enhanced her life [1]. Her full story can be
watched at the following link.
[http://aemf.org/who-we-help/cecilia/] [1].
[I think it would be helpful to show this clip because it represents Cecilia’s perspective
and gives readers the ability to listen to how she sounds when she speaks with the
implant.]
One of the greatest achievements of biomedical engineering is the development of the
cochlear implant. The cochlear implant is a device that is implanted within the ear canal
of a hearing impaired individual, allowing them to hear noises and interpret speech. The
main goal of engineering is to enhance the quality of life of others, and providing the gift
of hearing has met that goal. The cochlear implant has given deaf individuals the chance
to live life with sound, their own personal playlist. Their joy is the driving force behind
the engineering of cochlear implants. This article discusses the technology and
development of cochlear implants, how cochlear implants affect people’s lives, and
possible future development of the implant.
The Development of the Cochlear Implant
The main technology behind cochlear implants dates back to 1880, when a scientist
named Alessandro Voltas inserted electrically stimulated metal rods into his ear canal [2].
Voltas discovered that electrical stimulation of the ear canal produced auditory sensation
[2]. His findings were further tested in 1961, when electrical stimulation of the auditory
nerve was tested on two deaf individuals with successful results [2]. The individuals
reported changes in ‘loudness’ based on the level of stimulation, and changes in pitch
based on the rate of stimulation [2]. This experiment was repeated in 1964, when an
electrode was placed directly on the cochlea of deaf patients, eliciting auditory results [2].
It can be seen in Figure 1 that the cochlea is directly attached to the auditory nerve, which
extends into the brain. The key experimental finding in the 1960 experiments was that
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the specific stimulation of the cochlea led to auditory sensation. This discovery fueled a
push toward the development of cochlear implants.
Figure 1. An image showing the parts of the ear that are used in normal hearing. Note that the
cochlea is spiral shaped and attached to the auditory nerve, which leads to the brain [3].
Single Channel versus Multichannel Cochlear Implants
In 1972, single channel cochlear implants were made available to the public, followed by
multi-channel implants in 1984 [2]. The number of channels within a cochlear implant is
related to the number of stimulation sites within the cochlea [4]. The channels transfer
different frequencies to different stimulation sites [4]. Therefore, single channel implants
only have one stimulation site in the ear, and all frequencies are sent to this one site [4].
The single channel implant gives users speech-reading enhancement, but is limited in its
auditory ability due to only having one stimulation site [2].
The multi-channel cochlear implant consists of a receiver and an intracochlear electrode
array [2]. Multi-channel implants, as their name suggests, have multiple channels that
allow different frequencies to travel to different stimulation sites [4]. This allows more
signals to be sent to the brain, allowing for enhanced hearing by the user [2]. The multichannel system also functions more closely to the natural ear, allowing for a more
complete representation of sound than single channel implants [4]. Speech processing is
also much more effective with multi-channel implants, and it was found that speech
understanding decreased in users with implants that had less than four channels [4]. Both
of the implants work by stimulating the auditory nerve, allowing signals to be sent to the
brain and interpreted as sound.
How do cochlear implants work?
Cochlear implants receive noises and transmit them to the auditory nerve, bypassing
damaged parts of the ear [5]. The auditory nerve sends the signals to the brain where
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they are interpreted as sound [5]. It should be noted that normal hearing is not achieved
by the cochlear implant, but it does allow the user to hear certain sounds of the
environment and understand speech, which may be considered a better option then
hearing nothing at all [5]. Individuals who have lost their hearing and receive a cochlear
implant must relearn how to interpret sound and speech through the implant.
Breaking Down the Cochlear Implant
The cochlear implant consists of two sections, the external portion and the internal
portion [5]. The external portion is located behind the ear and consists of the transmitter,
the speech/sound processor, and the microphone, as seen in Figure 2 [5]. The circular
transmitter is attached to the head directly above the ear. A wire attaches the transmitter
and the crescent shaped device containing the processor and microphone, as seen in
Figure 2 [5]. The microphone “hears” noises from the surrounding environment, which
are arranged into signals by the speech processor [6]. These signals are sent to the
transmitter via the wire, and then transmitted to the receiver under the skin as FM radio
signals [6].
A
Figure 2. Image showing the external and internal components of a cochlear implant [7].
[I think an animated image should be here showing exactly where the signals/impulses
are travelling within the implant.]
The internal portion of the implant is surgically placed under the skin and in the cochlea
of the ear, labeled A in Figure 2. It consists of the receiver and electrode system, as seen
in Figure 2 [5]. The receiver converts the FM signals from the transmitter into electrical
impulses [6]. The electrical impulses are transferred from the receiver to the electrode
array in the cochlea, which sends the impulses to the auditory nerve [5]. The auditory
nerve transmits the signals to the brain where they are interpreted as sound [6].
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Putting the Implant in Place
Cochlear implant surgery is not as complicated as most people might believe. The entire
surgery takes between two and four hours and is done using general anesthesia [3]. The
surgery is minimally invasive, involving only a small incision and a 3-4 millimeter hole
drilled into the temporal bone [3]. The receiver is placed under the skin and skull, while
the electrode wires are threaded throughout the cochlear spirals, which can be seen in
Figure 1 [3]. Two to four weeks after the implantation, the implant is turned on and finetuned to match the user’s hearing needs [3]. Turning on the implant has the ability to
change the user’s life forever.
Changing People’s Lives
There is no doubt that the cochlear implant is changing the lives of deaf individuals of all
ages. Cochlear implants allow older individuals who have lost their hearing over time or
due to an accident to reconnect with sound [8]. It takes them about six months to learn
how speech and noises sound with the implant, but once they become familiar with it,
they are able to live a normal life without the disappointment of never being able to hear
again [8]. For young children, the implant has a larger impact because it helps them
interact socially with non-deaf children and learn to develop speech [8].
Young children can learn to hear and speak after receiving the implant, allowing them to
live and interact with the majority of society [8]. During the early years of childhood,
“auditory input and communication” are extremely important in the development of
language, behavior, and cognitive skills [9]. Cochlear implants help augment language
and communication skills in deaf children [9]. It is also believed that cochlear implants
help children become more attentive and develop their social skills because they are able
to communicate with their peers and family [9]. Besides the sheer ability to experience
sound, cochlear implants have a profound effect on the development of young children
and affect how they live the rest of their life.
Engineering has provided a means for deaf individuals to increase their quality of life and
provide them with the opportunity to hear, seen in success stories such as Cecilia’s. As of
December 2012, 219,000 people worldwide have received cochlear implants [10].
Although this is a small number, this biomedical device has made a difference in those
individuals’ lives. Today, about 40% of the children who are born deaf or with severe
hearing loss are given a cochlear implant [10]. The use of the cochlear implant is
growing and giving more individuals the opportunity to hear in a world they once could
not. Engineers are motivated by and celebrate this technology due to its beneficial effect
on the hearing impaired. However, there have been some negative reactions to cochlear
implants from the deaf community.
There was a strong reaction to cochlear implants from the deaf community, resulting in
protesting the use of cochlear implants in the 1980s and early 1990s [11]. Some
members of the deaf community emphatically reject cochlear implants because they
believe being deaf should not be considered a disability [11]. Some in the deaf
community wish to be considered a minority culture with their own language, sign
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language [11]. They believe that the increased amount of deaf individuals receiving
cochlear implants translates into the reduction of their community. They fear that their
visual language of signing, seen in Figure 3, is being overtaken by spoken language [11].
But, a deaf individual’s response to cochlear implants is entirely based on their personal
opinion. Although some deaf individuals have reacted negatively to cochlear implants,
many have reacted positively to this new technology, which is still being developed.
Figure 3. Two individuals having a conversation in sign language [12].
The Next Steps
Although the cochlear implant has positively impacted many people’s lives, there are still
some ways in which it can be further developed. The technology behind cochlear
implants is relatively old, and individuals have been receiving cochlear implants since the
1970s [2]. But, there are still many ways for the implant to be improved to someday give
individuals normal hearing abilities.
The current cochlear implant does not allow its users the full ability to hear normally, and
hearing can be challenging in noisy environments [13]. Bilateral implants may be the
solution for implant users to be able to localize noise and voices, such as hearing one
voice amongst a noisy crowd [13]. The bilateral implant takes advantage of two physical
effects of hearing, known as head shadowing and binaural summation. Head shadowing
distinguishes voices in a crowd due to the “shadowing” of one ear from loud noises by
the head and shoulders, as seen in Figure 4 [13]. Binaural summation is another physical
effect of hearing that uses the overlapped input from both ears to improve speech
reception and sound recognition. The bilateral implant helps give the user the benefits of
head shadowing and binaural summation in order to hear more effectively [13].
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Figure 4. An image depicting the physical effect of head shadowing. The right ear is not being
impacted by the intense noise on the left side of the head due to “shadowing” from the head and
shoulders [14].
Another way to improve speech reception in patients is to use combined electric and
acoustic stimulation in one cochlea [13]. Some hearing loss patients still possess residual
hearing abilities below a certain frequency [13]. Therefore, the implant should be
surgically placed so as not to disturb the natural ability of the ear to experience acoustic
stimulation [13]. Implants are also being developed using an electrode that mimics the
natural “hearing” process of the cochlea [13]. Other research involves improving the
actual design of the implant, such as decreasing the size of both the external and internal
devices [2]. With future engineering and improvements in technology, the cochlear
implant could be greatly improved and may one day give its user the ability to hear with
completely normal abilities.
Conclusion
The cochlear implant is a life-changing device that gives hearing-impaired individuals the
ability to experience sound. Advances in signal transmitting and electrode technology
have given deaf individuals the ability to interpret sound through electrical stimulation of
the auditory nerve. The technology behind cochlear implants is still being further
developed, but may one day give deaf individuals the full ability to hear. It cannot be
denied that many individuals, such as Cecilia, have benefited from cochlear implants. It
is a joy to watch her story and see her life transform with the implant, allowing her to be
mainstreamed in public school and participate in sports. With the implantation of more
cochlear implants, more individuals will be blessed with the miracle of hearing due to
this wonderful biomedical device.
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