Neuroprosthetics Neuroprosthetics

Week 7Week 7

Cochlea ImplantsCochlea Implants

Cochlea ImplantsCochlea Implants

Signals from microphone transmitted to Signals from microphone transmitted to implanted electrodes in the auditory implanted electrodes in the auditory nervenerve

DSP used to extract featuresDSP used to extract features No claim that hearing is restoredNo claim that hearing is restored User can usually recognise speech – User can usually recognise speech –

assisted with lip reading etcassisted with lip reading etc Prefer no visible signs of implantPrefer no visible signs of implant Power, size, SP problems Power, size, SP problems

HistoryHistory

1800s Alesandro de Volta inserted 1800s Alesandro de Volta inserted electrodes into ear canals – bubbling!electrodes into ear canals – bubbling!

1957 Djourno & Eyries – electrodes on 1957 Djourno & Eyries – electrodes on auditory nerves, different pulse rates – auditory nerves, different pulse rates – papa + allopapa + allo

Implants now used for virtually all types of Implants now used for virtually all types of hearing losshearing loss

Now Bone conduction devices, Middle Ear Now Bone conduction devices, Middle Ear Implantable Hearing Devices (MEIHDs), Implantable Hearing Devices (MEIHDs), CIs, Auditory brainstem implants. CIs, Auditory brainstem implants.

Auditory MechanismsAuditory Mechanisms

Outer ear: pinna + outer ear canal: Outer ear: pinna + outer ear canal: gathers sound.gathers sound.

Connected to middle ear by tympanic Connected to middle ear by tympanic membranemembrane

Middle ear converts air pressure Middle ear converts air pressure variations into fluid pressure variations into fluid pressure variations.variations.

Inner ear is the cochlea – Inner ear is the cochlea – displacement of fluid by a travelling displacement of fluid by a travelling wave is converted to nerve action wave is converted to nerve action potentials by 30,000 hair cells.potentials by 30,000 hair cells.

Cochlea FunctionCochlea Function Cochlea outer hair cells contract in response Cochlea outer hair cells contract in response

to the travelling wave – act as controllable to the travelling wave – act as controllable amplifiers for the inner hair cellsamplifiers for the inner hair cells

Inner ear encodes frequencies in response to Inner ear encodes frequencies in response to fluid movements – dynamic range 120dBfluid movements – dynamic range 120dB

Lower and higher signal frequencies affect Lower and higher signal frequencies affect different regions (tonotopic organisation) different regions (tonotopic organisation) frequency selectivityfrequency selectivity

So cochlea acts as a spectrum analyser So cochlea acts as a spectrum analyser

Partial Hearing LossPartial Hearing Loss

For moderate loss in hair cells – For moderate loss in hair cells – treated by hearing aid or MEIHDs treated by hearing aid or MEIHDs

These methods amplify sounds for the These methods amplify sounds for the remaining hair cellsremaining hair cells

For profound loss in hair cells – For profound loss in hair cells – population is so low that no population is so low that no amplification helpsamplification helps

Direct stimulation of ganglion cells Direct stimulation of ganglion cells bypasses the hair cell transducers bypasses the hair cell transducers

Types of Hearing LossTypes of Hearing Loss

10% of world population have a hearing 10% of world population have a hearing loss – 2.5% Hearing Aid is OKloss – 2.5% Hearing Aid is OK

Conductive hearing loss – pathway to inner Conductive hearing loss – pathway to inner ear is impaired, inner ear fineear is impaired, inner ear fine

Sensorineural hearing loss – conduction to Sensorineural hearing loss – conduction to inner ear is fine, but transduction to nerve inner ear is fine, but transduction to nerve action potentials by hair cells is impaired.action potentials by hair cells is impaired.

Implants usually deal with one form or the Implants usually deal with one form or the other – small group with mixed hearing other – small group with mixed hearing loss (both) loss (both)

Reasons for Hearing LossReasons for Hearing Loss

Chronic infections of the middle earChronic infections of the middle ear Bacterial or viral invasion of the inner ear – causing Bacterial or viral invasion of the inner ear – causing

loss of hair cellsloss of hair cells Some antibiotics kill hair cellsSome antibiotics kill hair cells Genetic links – e.g. the gene connexin-26 creates a Genetic links – e.g. the gene connexin-26 creates a

protein necessary for potassium pathway in inner protein necessary for potassium pathway in inner ear - absence results in hair cells not being ear - absence results in hair cells not being generatedgenerated

Hair cell damage due to excessive noise, chemical Hair cell damage due to excessive noise, chemical pollutants, smoking, trauma, agingpollutants, smoking, trauma, aging

So implant type to be used requires an So implant type to be used requires an understanding of the reasons behind the hearing understanding of the reasons behind the hearing lossloss

Types of ImplantsTypes of Implants

Four types – depend on type of hearing lossFour types – depend on type of hearing loss Conductive – bone conduction IHD, vibratory Conductive – bone conduction IHD, vibratory

conduction skull to inner earconduction skull to inner ear Remaining 3 types treat sensorineural Remaining 3 types treat sensorineural

hearing loss:hearing loss: MEIHDs – treat moderate to severe – MEIHDs – treat moderate to severe –

amplification of sound (piezoelectric crystals)amplification of sound (piezoelectric crystals) CI – profound deaf – only spiral ganglion cells CI – profound deaf – only spiral ganglion cells

that project to auditory nerve remain – CI that project to auditory nerve remain – CI stimulates thesestimulates these

Direct electrical stimulation of the brainstem Direct electrical stimulation of the brainstem cochlea nucleus following tumor removalcochlea nucleus following tumor removal

Bone Conduction DevicesBone Conduction Devices

Bone anchored hearing apparatus Bone anchored hearing apparatus widely used since 1977widely used since 1977

Percutaneous system implanted in Percutaneous system implanted in 10,000 patients10,000 patients

Output speaker of a hearing aid is Output speaker of a hearing aid is coupled to a titanium post which is coupled to a titanium post which is attached to boneattached to bone

Vibrations in the skull are conveyed to Vibrations in the skull are conveyed to the inner ear – bypassing middle earthe inner ear – bypassing middle ear

Middle Ear DevicesMiddle Ear Devices

Several different techniques commercially Several different techniques commercially available – example here:available – example here:

SOUNDTECs Direct System is partly SOUNDTECs Direct System is partly implantable, electromagneticimplantable, electromagnetic

Implant is only a permanent magnet in a Implant is only a permanent magnet in a titanium canistertitanium canister

External digital sound processor – battery, External digital sound processor – battery, microphone, electromagnetic coil – fits behind microphone, electromagnetic coil – fits behind ear ear

Magnetic field from external coil interacts with Magnetic field from external coil interacts with permanent magnet across tympanic membrane permanent magnet across tympanic membrane

MEIHD LimitationsMEIHD Limitations

Cannot regain original frequency Cannot regain original frequency sensitivitysensitivity

Internal noise generated by external Internal noise generated by external amplificationamplification

Prolonged overdrive of remaining hair Prolonged overdrive of remaining hair cells may make hearing loss worsecells may make hearing loss worse

For long term hearing restoration For long term hearing restoration probably must look to cochlea implantsprobably must look to cochlea implants

Cochlea ImplantsCochlea Implants

Place Principle – the basilar membrane of Place Principle – the basilar membrane of the cochlea separates different frequencies the cochlea separates different frequencies from soundfrom sound

Temporal Principle – Time pattern in sounds Temporal Principle – Time pattern in sounds conveys information about the sound conveys information about the sound spectrumspectrum

Localised, controlled, charge-balanced, Localised, controlled, charge-balanced, biphasic waveforms used to depolarise biphasic waveforms used to depolarise pools of neurons along the cochleapools of neurons along the cochlea

Different electrodes stimulated dependant Different electrodes stimulated dependant on signal frequency – restores filteringon signal frequency – restores filtering

Stimulation coded for loudness and pitch Stimulation coded for loudness and pitch informationinformation

CI DevicesCI Devices

Features include a microphone, external sound Features include a microphone, external sound processor and power supply, transmitting processor and power supply, transmitting circuitry, receiver/stimulator, electrode arraycircuitry, receiver/stimulator, electrode array

Multichannel systems employ up to 31 Multichannel systems employ up to 31 electrodes – typical is only 6/7electrodes – typical is only 6/7

Attracting pair of magnets on surface + under Attracting pair of magnets on surface + under skin align antennae and hold external devicesskin align antennae and hold external devices

Encoded signal + Power transmitted Encoded signal + Power transmitted transcutaneously using RF antennae transcutaneously using RF antennae

Demodulator assigns information to the arrayDemodulator assigns information to the array

Signal ProcessingSignal Processing

Varies enormously between devicesVaries enormously between devices Preserve waveforms, speech Preserve waveforms, speech

envelope info or spectral featuresenvelope info or spectral features Original single electrode developed Original single electrode developed

in 1970sin 1970s Main speech frequency info 300-Main speech frequency info 300-

4,000Hz4,000Hz

Cochlea ElectrodesCochlea Electrodes

Placement is w.r.to ganglion cellsPlacement is w.r.to ganglion cells Usually placed in scala tympani (intracochlea)Usually placed in scala tympani (intracochlea) Placement aim is for coding fequenciesPlacement aim is for coding fequencies Multichannel electrode arrayMultichannel electrode array Silicone rubber carrier – shaped to fit in the Silicone rubber carrier – shaped to fit in the

scala typaniscala typani Platinum electrodes – the further in is the Platinum electrodes – the further in is the

insertion, so the lower the frequency response – insertion, so the lower the frequency response – minimize damage so 30 mm insertion at mostminimize damage so 30 mm insertion at most

Principles of opPrinciples of op

30,000 branches of auditory nerve30,000 branches of auditory nerve Can be divided into frequency groupsCan be divided into frequency groups Examples:Examples: The COMBI-40+ uses 12 pairs of The COMBI-40+ uses 12 pairs of

electrodes dispersed over 26.4 mmelectrodes dispersed over 26.4 mm The Nucleus uses 22 electrodes The Nucleus uses 22 electrodes

spaced 0.75mm apartspaced 0.75mm apart

Cochlea StimulationCochlea Stimulation

Two types – analog and pulsatileTwo types – analog and pulsatile Analog: acoustic waveform is replicated, Analog: acoustic waveform is replicated,

filtered and passed to all electrodesfiltered and passed to all electrodes Neural plasticity of brain sorts it outNeural plasticity of brain sorts it out Problems with channel interactionProblems with channel interaction Pulsatile: electrodes deliver narrow set of Pulsatile: electrodes deliver narrow set of

biphasic, charge balanced pulsesbiphasic, charge balanced pulses Pulse amplitude related to filtered waveformsPulse amplitude related to filtered waveforms Stimulation rate affects recognition (over Stimulation rate affects recognition (over

80%)80%) Clarion – 250,000 pulses/sec Clarion – 250,000 pulses/sec

Auditory Brainstem ImplantsAuditory Brainstem Implants

When auditory nerve has been When auditory nerve has been damageddamaged

Modified CI device used for direct Modified CI device used for direct stimulation of the cochlea nucleusstimulation of the cochlea nucleus

Multielectrodes placed on the surface Multielectrodes placed on the surface of the cochlea nucleusof the cochlea nucleus

Human trials have been approved but Human trials have been approved but only just getting under way only just getting under way

Final WordsFinal Words

Most common hearing loss due to loss of Most common hearing loss due to loss of inner ear hair cellsinner ear hair cells

Middle ear direct drive and cochlea Middle ear direct drive and cochlea implants are increasing in use.implants are increasing in use.

Further miniaturization, total implantation Further miniaturization, total implantation and greater lifetimes expectedand greater lifetimes expected

Possible genetic means of restoring hair Possible genetic means of restoring hair cells – if not then bioengineering cells – if not then bioengineering approachapproach

Next WeekNext Week

Visual NeuroprosthesesVisual Neuroprostheses