| A Summary of Professor Graeme Clark’s Cochlear Implant Discoveries After commencing research on electrical stimulation
of the auditory pathways in 1967, Graeme Clark systematically initiated
and led the fundamental research resulting in the multiple-channel
cochlear implant. It is the first major advance in restoring speech
perception in tens of thousands of severely-to-profoundly people
worldwide, and has given spoken language to children born deaf or
deafened early in life. It is thus the first clinically effective
and safe interface between electronic technology and human consciousness.
This is a short summary of the discoveries made by Professor Clark
and his team that led to its safe and effective clinical application.
Discovery
#1: The limitations of mimicking the physiology of temporal and
place coding of sound frequencies with electrical stimulation of
the hearing nerves (1967 to 1975). The
research showed that single-channel electrical stimulation of the
cochlear (auditory) nerve could only mimic the coding of low frequencies,
and this would be inadequate for speech understanding. The physiological
studies demonstrated that the cells in the auditory brainstem could
not follow stimulus rates above 200-500 pulses/s. It was then discovered
how electrical currents could be partially restricted to separate
groups of nerve fibres along the cochlea, thus enabling the transmission
of mid-to-high speech frequencies to the brain on a place-coding
basis (thus exploiting the frequency ordering of the cochlea, and
auditory brain centres). This physiological and modelling research
indicated that multiple-channel rather than single-channel stimulation
was vital for transmitting the range of frequencies required for
understanding speech. Discovery #2: How to safely and effectively
use multiple-channel electrical stimulation of the hearing nerves
to transmit coded frequencies and intensities to the central nervous
system through an array of electrodes inserted into the cochlea
(1972 to 1985). This
required discovering: 1) how to insert the array into the correct
location in the cochlea to minimize injury to the tissues and auditory
nerves in particular; 2) the right mechanical properties for the
array to pass safely far enough around the basal turn of the cochlea
to lie opposite the nerves normally transmitting speech frequencies;
3) materials that were biocompatible and non-toxic to the cochlea
or auditory nerve; 4) electrode pads that were smooth to facilitate
insertion and reinsertion without trauma, with a surface area large
enough to minimize the current and charge density for safe electrical
stimulation, and having the electrode geometry for localizing current
to separate nerve fibres; and 5) the right design and associated
implant procedures to prevent the entry of infections from the middle
to the inner ear, and therefore avoiding the risk of meningitis.
Development
#1: The fully-implantable receiver-stimulator (1974-1977) 10-15
channels of speech frequency information, in accord with the brain’s
capacity to process critical bands, were transmitted to the Development #2: The pre-operative selection procedures,
asepsis routine for the operating theatre, and surgical techniques
(1975-1979) The
preoperative selection procedures included electrical stimulation
of the auditory nerve with an electrode placed in the middle ear.
The quality of the sounds heard determined whether the auditory
nerve was intact. A protocol was developed to minimize any risk
of seeding bacteria at the time of surgery. Surgical procedures
and instruments were developed, including the micro-claw to aid
the insertion of the electrode array. Discovery #3: The perceptual qualities
of sound frequencies and intensities when coded through multiple-channel
electrical stimulation of the auditory central nervous system in
severely-to-profoundly deaf adults who originally had hearing before
going deaf (1978 to1979). The
research demonstrated that pitch discrimination was only possible
for low rates of electrical stimulation, and that timbre (the quality
of the sound, that enables a distinction to be made between musical
instruments playing the same note) varied for place of stimulation
from the low to high frequency regions of the cochlea. Pitch and
timbre could both be identified for combined rate and place of stimulation,
but were also perceived as a blended sound. Furthermore, the sensation
on each electrode was described as vowel-like, and corresponded
to the vowel a normal hearing person would experience, if a similar
frequency region in the cochlea was excited by a speech single formant
frequency (a formant is a vocal resonant frequency of importance
for intelligibility). Loudness was related to current level, rate,
and other parameters. Discovery #4: Connected speech
was understood when its second formant frequency was coded as place
of stimulation and voicing as rate of stimulation across electrodes
in severely-to-profoundly deaf adults who originally had hearing
before going deaf (1978 to 1979). The
ground-breaking discovery in 1978, was that key mid-to-high frequencies
(e.g. second formants), of great importance for understanding speech,
presented non-simultaneously as electrical stimuli on a place frequency
coding basis, and the lower voicing frequencies as rate of stimulation,
enabled profoundly deaf adults with prior hearing to understand
conversational speech. This multiple-channel speech processing strategy
and implant was the first to give profoundly deaf people the ability
to understand connected speech (open-set speech understanding) both
with assistance from lipreading, and with
electrical stimulation alone. In 1985 when developed industrially
by Cochlear Pty Limited it became the first multiple-channel cochlear
implant to be approved as safe and effective by the US Food and
Drug Administration or any world health regulatory body for providing
open-set speech understanding to profoundly deaf adults who had
hearing before going deaf, with lipreading assistance and electrical stimulation alone. Discovery #5: Speech understanding occurred
with the second formant/voicing strategy through the integration
of complex speech signals along and across temporal and spatial
processing systems in the brain, and this was the foundation for
refinements in speech processing (1978 to
1985). The
discovery that the central auditory pathways integrated complex
speech signals along and across temporal and spatial processing
systems was made by analysing the information transmitted for the
recognition of speech features, studying the perception of complex
patterns of electrical stimulation, and evaluating the perception
of acoustic models of electrical stimulation in normal hearing listeners.
Understanding how the brain integrated auditory information not
only explained the success of the second formant/voicing speech
processing strategy, but was the foundation for refinements in speech
processing through the addition of more formant and spectral frequencies
and their place coding while still preserving the temporal aspects
of voicing across the spatial central nervous processing systems. Discovery #6: Improved speech understanding through
the transmission of additional speech information along the temporal
and spatial auditory central nervous processing systems, and in
particular the coding of formant and other frequency bands on a
place basis (1981 to 1989). The
discovery that the central auditory pathways integrated frequency
information along and across temporal and spatial systems formed
the basis for further studies that showed that additional formant
and other frequency bands of importance for intelligibility presented
non-simultaneously as place of stimulation, along with voicing as
rate of stimulation improved speech understanding. Discovery
#7: Improved hearing in noise and sound localization, were achieved
with bilateral cochlear implants or bimodal hearing (an implant
in one ear and hearing aid in the other) (1989-2002). Studies
with bilateral implants and bimodal hearing showed that intensity
differences between each ear could be readily detected and lead
to good sound localization, but this was not the case for differences
in the time of arrival of sound or phase information. Furthermore,
with bilateral implants patients could fuse the sounds from each
ear into the one image, especially when equivalent frequency sites
in each ear were excited. This was also seen for bimodal hearing.
Bilateral implants and bimodal hearing also made it possible for
patients to hear speech with competing noise in the opposite ear
(the head shadow effect). However, improved understanding of speech
in noise presented to both ears and due to central brain mechanisms
(squelch effect), were not very effective. Discovery
#8: The most important general factors for good speech perception
in people who had hearing before going deaf (1985-1992). Good
speech perception was achieved with factors that firstly maximized
the “bottom-up” transmission of sound information (i.e.
from cochlea to the auditory cortex), and secondly maximized the
“top-down” processing of this information (i.e. from
a higher or cortical level). The factors for good bottom-up”
transmission, were those that preserved the auditory brain pathways
from degenerating, and thus allowed the place and temporal coding
of sound. These were, in particular, a short duration of deafness,
and diseases that minimized damage of the cochlea. The factors for
good “top-down” processing were those that allowed time
for the person to learn to use a degraded signal in particular a
progressive hearing loss. Discovery #9: Children born deaf or deafened
early in life achieved with multiple-channel electrical stimulation
of the auditory central nervous system similar speech perception
scores to those of people who had hearing before going deaf, and
this enabled them to achieve near normal spoken language (1985 to
1990). In
1985 multiple-channel implantation and electrical stimulation showed
children born-deaf could understand speech as well as implanted
profoundly deaf adults with prior hearing, especially if they were
operated on at a young age. They were also able to develop near
normal language. In 1990 the multiple-channel formant speech processing
strategies and cochlear implant were the first to be approved by
the US Food and Drug Administration or any world regulatory body
as safe and effective for children from two years and above. It
was thus the first major advance in helping deaf children to communicate,
since sign language of the deaf was discovered at the Paris Deaf
School 200 years ago. Discovery #10:
The most important specific factor for the development of speech
in implanted children was the acquisition of the place coding of
frequency, and the general factors were those that assisted the
development of neural connectivity and “top-down” language processing
(1985-1990). The
research indicated that untreated deafness led to difficulties in
processing the place and temporal frequency cues of speech. These
perceptual skills correlated with speech perception, speech production
and language skills. The need to have the right neural connections
for processing place and temporal information was supported by the
finding that speech perception was better if there was prior exposure
to sound during the “plastic” stage of brain development.
In addition “top-down” neural processing through language
training also improved speech perception. Discovery #11:
The multiple-channel implant for infants and young children had
no greater risk of inner ear infection following otitis media than
normal if a fascial graft were placed around the array at its entry
point to facilitate the development of an electrode sheath, and
thus the risk of meningitis was minimized. Head growth had no adverse
effect on the implant if there was redundancy and fixation of the
lead wire at the floor of the mastoid antrum (1987-1992). The
research showed that Streptococcus pneumoniae middle ear infection
could be prevented from entering the inner ear by grafting fascia
around the electrode entry point to encourage the development of
a sheath to surround a single component electrode array. This provided
the best protection against the development of meningitis following
middle ear infection. It was also necessary to prevent middle ear
infection during the healing phase for one month postoperatively.
It was also shown that head growth would not extract the electrode
from the cochlea if a 20-25mm redundant loop was created in the
lead wire and the array was fixed to the floor of the mastoid antrum
which did not move relative to the entry to the inner ear. Furthermore
drilling the receiver-stimulator package bed did not distort head
growth; and electrical stimulation had no deleterious effect on
the maturing nervous system. Discovery #12: Implanted children could achieve the best speech perception
and spoken language, if diagnosed and implanted early, given early
intervention in developing listening skills, and then an auditory-verbal
or auditory-oral education. The
main aim in implanting deaf children is not speech perception per
se, but to develop receptive and expressive language so they can
communicate effectively in a world of sound. However, without adequate
speech perception, the development of language cannot be achieved
and the two show a strong correlation. Furthermore, as the implanted
children with an average hearing level of 106 dB had a comparable
speech perception result to hearing aid users with an average threshold
of 78 dB HL they have been habilitated and educated similarly. Early
diagnosis can be achieved with a screening program for all births
and then objective testing of hearing thresholds with for example
the steady state method for recording auditory evoked brain potentials
first reported by Rickards & Clark (1984) and Stapells et al
(1984). The pre-school early intervention program puts an emphasis
on developing receptive and expressive language through play; story
telling, listening skills, social skills, role models, music, and
parent guidance. The auditory-verbal method of education puts an
emphasis on hearing, and as with an auditory-oral education there
is no use of signed English or Sign Language of the Deaf. When
severely-to-profoundly deaf children are managed with early diagnosis,
early intervention, and auditory/oral or auditory/verbal education
approximately 50% have been shown in
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