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Neuroprosthetics

Neuroprosthetics. Week 7 Cochlea Implants. Cochlea Implants. 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

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Neuroprosthetics

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  1. Neuroprosthetics Week 7 Cochlea Implants

  2. Cochlea Implants • 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

  3. History • 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.

  4. Auditory Mechanisms • 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.

  5. Cochlea Function • 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

  6. Partial Hearing Loss • 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

  7. Types of Hearing Loss • 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)

  8. Reasons for Hearing Loss • 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

  9. Types of Implants • 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

  10. Bone Conduction Devices • 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

  11. Middle Ear Devices • 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

  12. MEIHD Limitations • 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

  13. Cochlea Implants • 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

  14. CI Devices • 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

  15. Signal Processing • 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

  16. Cochlea Electrodes • 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

  17. Principles of op • 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

  18. Cochlea Stimulation • 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

  19. Auditory Brainstem Implants • 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

  20. Final Words • 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

  21. Next Week • Visual Neuroprostheses

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