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 • Prefer no visible signs of implant • Power, size, SP problems

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.

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.

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

Next Week • Visual Neuroprostheses

Neuroprosthetics