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Neuroprosthetics
Week 7
Cochlea Implants
Cochlea Implants
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Signals from microphone transmitted to
implanted electrodes in the auditory nerve
DSP used to extract features
No claim that hearing is restored
User can usually recognise speech –
assisted with lip reading etc
Prefer no visible signs of implant
Power, size, SP problems
History
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1800s Alesandro de Volta inserted electrodes
into ear canals – bubbling!
1957 Djourno & Eyries – electrodes on auditory
nerves, different pulse rates – papa + allo
Implants now used for virtually all types of
hearing loss
Now Bone conduction devices, Middle Ear
Implantable Hearing Devices (MEIHDs), CIs,
Auditory brainstem implants.
Auditory Mechanisms
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Outer ear: pinna + outer ear canal:
gathers sound.
Connected to middle ear by tympanic
membrane
Middle ear converts air pressure variations
into fluid pressure variations.
Inner ear is the cochlea – displacement of
fluid by a travelling wave is converted to
nerve action potentials by 30,000 hair
cells.
Cochlea Function
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Cochlea outer hair cells contract in response
to the travelling wave – act as controllable
amplifiers for the inner hair cells
Inner ear encodes frequencies in response to
fluid movements – dynamic range 120dB
Lower and higher signal frequencies affect
different regions (tonotopic organisation)
frequency selectivity
So cochlea acts as a spectrum analyser
Partial Hearing Loss
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For moderate loss in hair cells – treated by
hearing aid or MEIHDs
These methods amplify sounds for the
remaining hair cells
For profound loss in hair cells – population
is so low that no amplification helps
Direct stimulation of ganglion cells
bypasses the hair cell transducers
Types of Hearing Loss
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10% of world population have a hearing loss –
2.5% Hearing Aid is OK
Conductive hearing loss – pathway to inner ear
is impaired, inner ear fine
Sensorineural hearing loss – conduction to inner
ear is fine, but transduction to nerve action
potentials by hair cells is impaired.
Implants usually deal with one form or the other
– small group with mixed hearing loss (both)
Reasons for Hearing Loss
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Chronic infections of the middle ear
Bacterial or viral invasion of the inner ear – causing
loss of hair cells
Some antibiotics kill hair cells
Genetic links – e.g. the gene connexin-26 creates a
protein necessary for potassium pathway in inner ear
- absence results in hair cells not being generated
Hair cell damage due to excessive noise, chemical
pollutants, smoking, trauma, aging
So implant type to be used requires an understanding
of the reasons behind the hearing loss
Types of Implants
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Four types – depend on type of hearing loss
Conductive – bone conduction IHD, vibratory
conduction skull to inner ear
Remaining 3 types treat sensorineural hearing loss:
MEIHDs – treat moderate to severe – amplification
of sound (piezoelectric crystals)
CI – profound deaf – only spiral ganglion cells that
project to auditory nerve remain – CI stimulates
these
Direct electrical stimulation of the brainstem
cochlea nucleus following tumor removal
Bone Conduction Devices
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Bone anchored hearing apparatus widely
used since 1977
Percutaneous system implanted in 10,000
patients
Output speaker of a hearing aid is coupled
to a titanium post which is attached to bone
Vibrations in the skull are conveyed to the
inner ear – bypassing middle ear
Middle Ear Devices
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Several different techniques commercially
available – example here:
SOUNDTECs Direct System is partly implantable,
electromagnetic
Implant is only a permanent magnet in a titanium
canister
External digital sound processor – battery,
microphone, electromagnetic coil – fits behind ear
Magnetic field from external coil interacts with
permanent magnet across tympanic membrane
MEIHD Limitations
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Cannot regain original frequency sensitivity
Internal noise generated by external
amplification
Prolonged overdrive of remaining hair cells
may make hearing loss worse
For long term hearing restoration probably
must look to cochlea implants
Cochlea Implants
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Place Principle – the basilar membrane of the
cochlea separates different frequencies from
sound
Temporal Principle – Time pattern in sounds
conveys information about the sound spectrum
Localised, controlled, charge-balanced, biphasic
waveforms used to depolarise pools of neurons
along the cochlea
Different electrodes stimulated dependant on
signal frequency – restores filtering
Stimulation coded for loudness and pitch
information
CI Devices
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Features include a microphone, external sound
processor and power supply, transmitting
circuitry, receiver/stimulator, electrode array
Multichannel systems employ up to 31
electrodes – typical is only 6/7
Attracting pair of magnets on surface + under
skin align antennae and hold external devices
Encoded signal + Power transmitted
transcutaneously using RF antennae
Demodulator assigns information to the array
Signal Processing
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Varies enormously between devices
Preserve waveforms, speech envelope info
or spectral features
Original single electrode developed in
1970s
Main speech frequency info 300-4,000Hz
Cochlea Electrodes
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Placement is w.r.to ganglion cells
Usually placed in scala tympani (intracochlea)
Placement aim is for coding fequencies
Multichannel electrode array
Silicone rubber carrier – shaped to fit in the scala
typani
Platinum electrodes – the further in is the insertion,
so the lower the frequency response – minimize
damage so 30 mm insertion at most
Principles of op
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30,000 branches of auditory nerve
Can be divided into frequency groups
Examples:
The COMBI-40+ uses 12 pairs of
electrodes dispersed over 26.4 mm
The Nucleus uses 22 electrodes spaced
0.75mm apart
Cochlea Stimulation
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Two types – analog and pulsatile
Analog: acoustic waveform is replicated, filtered
and passed to all electrodes
Neural plasticity of brain sorts it out
Problems with channel interaction
Pulsatile: electrodes deliver narrow set of
biphasic, charge balanced pulses
Pulse amplitude related to filtered waveforms
Stimulation rate affects recognition (over 80%)
Clarion – 250,000 pulses/sec
Auditory Brainstem Implants
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When auditory nerve has been damaged
Modified CI device used for direct
stimulation of the cochlea nucleus
Multielectrodes placed on the surface of
the cochlea nucleus
Human trials have been approved but only
just getting under way
Final Words
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Most common hearing loss due to loss of
inner ear hair cells
Middle ear direct drive and cochlea
implants are increasing in use.
Further miniaturization, total implantation
and greater lifetimes expected
Possible genetic means of restoring hair
cells – if not then bioengineering approach
Next Week
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Visual Neuroprostheses