The Bionic Ear
How the cochlear implant (bionic ear) functions
This
shows the cross-section of the outer, middle and inner ears. In the
insert sound is transmitted outside to the cochlea, or inner ear. The
main section demonstrates a cross-section of the cochlea which has three
spiral compartments and there is a membrane lying across the cochlea
which vibrates to sound and acts as a filter so that high frequencies
stimulate the bottom end and low frequencies the top end, or apical
region.
This
diagram is a cross-section of the sense organ of hearing resting on the
brain, called the Organ of Corti. It has inner and outer hair cells
which vibrate to sound vibrations and the hairs on the cells act like
switches and allow electrical current to flow through the cells and into
the nerve fibres in response to the sound vibrations.
This
diagram shows the Organ of Corti in deafness and all the hair cells are
lost so no sound vibrations can be transmitted through them to the
brain. The idea underlying the cochlear implant, or Bionic Ear, was to
then artificially stimulate those hearing nerves with patterns that
would be similar to those that are created by sound with standard
hearing.
This
slide illustrates the basic waves in which the brain codes the sound
into pulses to be interpreted. On the left it is demonstrating temporal
coding. Notice on the top the high frequencies sound responses, or
spikes, in the nerves to each sound wave according to the temporal code
we hear and understand the pitch, according to the timing of the brain
cells responses.
On the right, demonstrates place coding, the inner ear filters the sound
to different frequency regions and these are connected spatially to all
centres in the brain so that there is a frequency scale preserved
throughout. According to the place theory we perceive a certain pitch
according to the site of the brain that is stimulated.
This
slide demonstrates that in order to produce the range of pitch needed
for speech understanding a multi-channel stimulation, or cochlear
implant, was required which was the significant innovation established
at the University of Melbourne research team. Notice sound is
transmitted through the skin by radio-waves to a receiver implant, which
decodes the signals and stimulates the different frequency regions of
the brain on a place coding basis, and also transmits the lower
frequencies for voicing.
This
image is a mould of the human cochlea showing how tiny it is. It has
two and three quarter spirals around a central axis and the top loops
for balance. The inner ear has three cavities which are about 1.5mm in
diameter. There are up to 20,000 hair cells and 20,000 nerve fibres
taking information to the brain. This was the challenge faced with a
cochlear implant, how and what nerve to stimulate? The first
evaluations on putting an electrode bundle into that spiral was that it
wouldn’t go very far and certainly not reach the speech frequency
region. The answer came shown in figure 8.
When
Professor Clark was examining a Turban Shell on the beach with grass
blades he found that if the grass blades were flexible at the tip and
stiffer at the base they would go far enough around the spiral. This
mechanical principle was applied to that used for electrodes for
cochlear implants.
This
slide demonstrates the circuitry needed for the first prototype
receiver-stimulator which had 10 silicone chips marked with the circuit
design as in that large circuit layout.
This
slide shows the prototype receiver-stimulator used in the first patient
on the first of August 1978 and 1979. It has two coils on the outside
for receiving electro-conductive signals for power and also data.
This
image shows the surgery being undertaken at the Royal Victorian Eye and
Ear Hospital in a flow of sterile air with Professor Clark in the
insert.
This
is a photograph of the actual first operation on Rod Saunders by
Professor Clark and Dr. Brian Pyman on the 1st of August 1978.
This
is a diagram of the electrode; free fitting bundle of 20 wires passed
around the spiral of the cochlea to lie opposite the speech
frequencies.
This
is a photograph of the first patient Rod Saunders, three weeks after his
surgery showing the incision.
This
is an image of the computer used to develop a way of processing speech
so it would get through a bottle neck to the brain and be heard as real
speech signals. It was small by comparison to modern computers and had
16k of RAM.
An
illustration of a raw speech wave formed from part of the word
‘otolaryngology’ to illustrate how complex the signal is and the
challenge facing the team to produce a code for transmitting to brain.
This
slide shows a diagram of the cochlea and the first finding where the
patient explained that stimulating different electrodes were heard not
only as sharp and dull sounds but they were like vowels and the vowels
varied according to the site of stimulation. It was noted that the
frequency of site of stimulation corresponded to the frequency of a
special signal in speech sounds called formants.
This
shows an organ pipe arrangement to illustrate what a formant is. It’s a
resonance in the vocal tract due to changes in dimensions with different
speech sounds and conveys a lot of intelligibility. So the aim of the
first processors was to select out the formants.
Shows
the first strategy, the second formant frequency which is very important
for intelligibility was coded for place of stimulation and perceived by
the brain as timbre. Sound pressure was coded as current level and
perceived as loudness and the voicing frequency was coded and stimulus
pulse rate and perceived as pitch, and it was transmitted across each
site of electrical stimulation.
This
illustrates that there are many features in speech signals that needed
to be transmitted and coded. This is called a spectrum, spectrogram for
the word ‘bat’. Notice that the ‘b’ sound has a short burst of energy
with different frequencies. The vowel ‘a’ has at least two strong bands
of stimulation called the formants and the rate of stimulation relates
to the voicing and the ‘t’ sound is a burst of high frequency energy.
Photograph
showing Rod Saunders speaking with his wife using the first speech
processor developed by the University of Melbourne’s Department of
Otolaryngology.
This
slide shows the first commercially produced device by Nucleus later the
company Cochlear Pty Ltd. It has a more reliable implant with a single
coil and a smaller speech processor that’s still a headband to transmit
signals through to the implant.
This
graph shows the word correct scores for words and sentences with
improvements in speech processing and it can be seen how now in four
sentences the scores approximate 80% correct, which means the person can
have a fully meaningful conversation over the telephone and not need to
lip read.
This
image demonstrates the change in the system for operating on children in
1985. This implant has a magnet in the centre of the coils for
receiving and transmitting information and that enabled children to
apply it easily.
This
is an image of the first two young children operated on in 1985 and
1986.
This
image shows a demonstration of how the latest the Nucleus Freedom
cochlear implant works