Cochlear
implants as we know them now are the result of intensive research over the last
four decades. However, there is a
long history of attempts to provide hearing by the electrical stimulation of the
auditory system. The centuries old interest
in the biology application of electricity was the basis for the development of
cochlear implants.
THE EARLY YEARS: Research
during the late 18th and 19th centuries
Interest in the electrical methods of stimulating
hearing had its beginnings in the late 18th century when Alessandro
Volta discovered the electrolytic cell.
Volta was the first to stimulate the auditory system electrically, by
connecting a battery of 30 or 40 ‘couples’ (approximately 50V) to two metal
rods that were inserted into his ears. When
the circuits were completed, he received the sensation of ‘une recousse dans la tate’ (“a boom within the
head”), followed by a sound similar to that of boiling of thick soup. Volta’s
observation sparked sporadic attempts to investigate the phenomenon over the
next 50 years, but the sensation the patients described was always momentary and
lacked tonal quality.
Crude
applications of electrical stimulation were described through the 18th
and 19th century in Paris, Amsterdam, London, and Berlin. (Clark
and niparko). Since sound is an
alternating, disturbance in an elastic medium, it was soon realized that
stimulating the auditory system with a direct current could not reproduce a
satisfactory hearing sensation. The next step was taken by Duchenne of
Boulogne who, in 1855, stimulated the ear with an alternating current that
he produced by inserting a vibrator into a circuit containing a condenser and
induction coil. What resulted was a sound that resembled, ‘the beating of a
fly’s wings between a pane of glass and a curtain’. (Clark) This was better,
but still not satisfactory.
In
1868, Brenner published a more extensive investigation of these effects
that studied the effects of altering the polarity, rate and intensity of the
stimulus, and placement of the electrodes, on the hearing sensation produced
(cited by Simmons 1966). He found
that hearing was better with an electrical stimulus that created a negative
polarity in the ear, and that correct placement of the electrodes could reduce
the unpleasant side effects. Brenner
used bipolar stimulation, meaning that one electrode was placed in saline in the
external auditory meatus, and the other was placed on a more distant part of the
body. This electrode is now
referred to as the Brenner electrode (Clark et al).
INTEREST RENEWED: Breaking
ground during the early 1900’s
The
initial optimism surrounding the bioelectrical approaches to cure deafness was
followed by a period of skepticism as the applications appeared to be invasive
and required ongoing critical evaluation. However, in the 1930’s,
interest was renewed in the problem of reproducing hearing artificially.
This coincided with the introduction of the thermionic valve, which
allowed for the auditory system to be stimulated electrically with significantly
greater precision.
The
work of Wever and Bray (1930) demonstrated that the electrical response
recorded form the vicinity of the auditory nerve of a cat was similar in
frequency and amplitude to the sounds to which the ear had been exposed. Meanwhile, the Russian investigators Gersuni and Volokhov
in 1936 examined the effects of an alternating electrical stimulus on hearing.
They found that hearing could persist following the surgical removal of the
tympanic membrane and ossicles, and thus hypothesized that the cochlea was the
site of stimulation.
Another
set of researchers, Stevens and Jones (1939), thought that
electrical could be transduced into sound vibrations before it reached the inner
year. Hearing induced in this way has been called the electrophonic effect.
They were able to determine whether a linear or non-linear transducer was
involved by the presence and strength of the overtones, which were detected when
the subject heard beats. The studies by Stevens and Jones (1939), as well as
Jones et al (1940) indicated that when the cochlea was stimulated electrically,
there were three mechanisms, which produced hearing:
1.
The middle ear could act as a transducer, which obeys the ‘square
law’ and convert alternations in the strength of an electrical field into the
mechanical vibrations that produce sound.
2. Electrical energy could be converted into sound by a direct effect on the basilar membrane, which would then vibrate maximally at a point determined by the frequency and these vibrations would stimulate the hair cells
3. Direct stimulation of the auditory nerve produced a crude hearing sensation.
Their
conclusions were basically correct, although now other body tissues have been
shown to act as transducers (Clark).
A
wealth of research in the 1940’s and 1950’s into the mechanisms involved in
electrophonic hearing indicated that hearing is produced by transducing
electrical energy into sound vibrations and that residual cochlear function is
also required. It became apparent
that total perception deafness could not be corrected by inducing a widespread
electrical filed in the region of cochlea. Instead, a more localized stimulation
of the auditory nerve fibers is required.
In
1950, Lundberg performed one of the first recorded attempts to stimulate
the auditory nerve with a sinusoidal current during a neurosurgical operation.
His patient could only hear noise. However,
a more detail study followed in 1957 by Djourno and Eyries,
provided the first detailed description of the effects of directly stimulating
the auditory nerve in deafness. In their study, the stimulus appears to have
been well controlled. Djourno and
Eyries placed a wire on the auditory nerves that were exposed during an
operation for cholesteatoma. When
the current was applied to the wire, the patient described generally
high-frequency sounds that resembled a “roulette wheel of the casino” and a
“cricket”. The signal generator
provided up to 1,000-Hz and the patient gradually developed limited recognition
of common words and improved speech-reading capabilities. The patient was found able to discern differences in pitch at
increments of 100 pulses and was found able to distinguish words such as
“pap’, mamn”and “allo”.
In
1964, Doyle et al., reported inserting an array of electrodes into the
cochlea of a patient with total perceptive deafness. The electrodes were designed to limit the spread of the
electrical field and were stimulated in sequence with threshold square waves
that were superimposed with speech signals.
The four electrodes were not especially implanted to take advantage of
the spatial distribution of the auditory nerve fibers responding to different
frequencies, and the result obtained was only satisfactory. However, it was
significant that the patient was able to repeat phrases.
Yet
another researcher, Simmons (1966) provided a more extensive study in
which electrodes were placed through the promontory and vestibule directly into
the modiolar segment of the auditory nerves.
The nerve fibers representing different frequencies could be stimulated.
The patient was tested to assess the effect of alterations in the
frequency and intensity of the signal. The
subject demonstrated that in addition to being able to discern the length of
signal duration, some degree of tonality could be achieved.
The
clinical applications of electrical stimulation of the auditory nerve were
refined by House (1976) and Michelson (1971) through scala tympani
implantation of electrodes driven by implantable receiver-stimulators.
Dr. William House observed the percepts of patients when small electric
currents were introduced to the promontory during middle ear procedures under
local anesthesia. But technical
barriers proved frustrating. During the early sixties, House implanted several
devices in totally deaf volunteer patients. Although these were rejected due to
lack of biocompatibility of the insulating material, that they worked for a
short time provided optimism towards a solution for sensorineural deafness.
(House testimonial). House
teamed up with Jack Urban, a very innovative engineer, to
ultimately make cochlear implants a clinical reality. The new devices benefited
from the increasing capabilities for microcircuit fabrication derived form space
exploration and computer development.
RAPID PROGRESS: The commercial marketing of Cochlear Implants
In
1972, a speech processor was developed to interface with the House 3M
single-electrode implant and was the first to be commercially marketed.
More than 1,000 of these devices were implanted between 1972 to the mid
1980s. In 1980, the age criteria
for use of this device was lowered from 18 to 2 years. ).
During the 1980’s, several hundred children had been implanted with the
House 3M single channel device. The FDA formally approved the
marketing of the 3M/ House cochlear implant in November 1984. By
the late eighties, virtually all of the major concerns about the long-term
success and safety of cochlear implants were largely resolved.
During
this same period, work outside the United States was progressing, most notably
in Australia where Clark and colleagues were developing a multi-channel cochlear
implant that, in the last half of the eighties, was to become the single-most
used implant in the world under the name "Nucleus Multi-channel Cochlear
Implant".
Multiple channel devices were introduced in 1984, and enhanced the
spectral perception and speech recognition capabilities compared to the
single-channel device, as reported in large adult clinical trials.
THE
LAST DECADE TO PRESENT
Through
the 1990’s, clinical and basic science studies have resulted in progress in
implant technology and in clinical approaches to cochlear implants. Electrode
and speech processor now produce coding strategies that are associated with
successively higher performance levels. The
commercial success of the Nucleus device has triggered the acceptance of
implants as assistive devices. Over
the years, implant patients have become more numerous and risks have been
minimized. More people have accepted that implants are here to stay, and are
increasingly being recommended.