Neuroprosthetics Week 7 Cochlea Implants

Signals from microphone transmitted to implanted electrodes in the auditory nerve Signals from microphone transmitted to implanted electrodes in the auditory nerve DSP used to extract features DSP used to extract features No claim that hearing is restored No claim that hearing is restored User can usually recognise speech – assisted with lip reading etc User can usually recognise speech – assisted with lip reading etc Prefer no visible signs of implant Prefer no visible signs of implant Power, size, SP problems Power, size, SP problems

History 1800s Alesandro de Volta inserted electrodes into ear canals – bubbling! 1800s Alesandro de Volta inserted electrodes into ear canals – bubbling! 1957 Djourno & Eyries – electrodes on auditory nerves, different pulse rates – papa + allo 1957 Djourno & Eyries – electrodes on auditory nerves, different pulse rates – papa + allo Implants now used for virtually all types of hearing loss Implants now used for virtually all types of hearing loss Now Bone conduction devices, Middle Ear Implantable Hearing Devices (MEIHDs), CIs, Auditory brainstem implants. Now Bone conduction devices, Middle Ear Implantable Hearing Devices (MEIHDs), CIs, Auditory brainstem implants.

Auditory Mechanisms Outer ear: pinna + outer ear canal: gathers sound. Outer ear: pinna + outer ear canal: gathers sound. Connected to middle ear by tympanic membrane Connected to middle ear by tympanic membrane Middle ear converts air pressure variations into fluid pressure variations. 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. 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 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 Inner ear encodes frequencies in response to fluid movements – dynamic range 120dB Lower and higher signal frequencies affect different regions (tonotopic organisation) frequency selectivity Lower and higher signal frequencies affect different regions (tonotopic organisation) frequency selectivity So cochlea acts as a spectrum analyser So cochlea acts as a spectrum analyser

Partial Hearing Loss For moderate loss in hair cells – treated by hearing aid or MEIHDs For moderate loss in hair cells – treated by hearing aid or MEIHDs These methods amplify sounds for the remaining hair cells These methods amplify sounds for the remaining hair cells For profound loss in hair cells – population is so low that no amplification helps For profound loss in hair cells – population is so low that no amplification helps Direct stimulation of ganglion cells bypasses the hair cell transducers 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 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 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. 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) 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 Chronic infections of the middle ear Bacterial or viral invasion of the inner ear – causing loss of hair cells Bacterial or viral invasion of the inner ear – causing loss of hair cells Some antibiotics kill 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 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 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 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 Four types – depend on type of hearing loss Conductive – bone conduction IHD, vibratory conduction skull to inner ear Conductive – bone conduction IHD, vibratory conduction skull to inner ear Remaining 3 types treat sensorineural hearing loss: Remaining 3 types treat sensorineural hearing loss: MEIHDs – treat moderate to severe – amplification of sound (piezoelectric crystals) 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 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 Direct electrical stimulation of the brainstem cochlea nucleus following tumor removal

Bone Conduction Devices Bone anchored hearing apparatus widely used since 1977 Bone anchored hearing apparatus widely used since 1977 Percutaneous system implanted in 10,000 patients Percutaneous system implanted in 10,000 patients Output speaker of a hearing aid is coupled to a titanium post which is attached to bone 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 Vibrations in the skull are conveyed to the inner ear – bypassing middle ear

Middle Ear Devices Several different techniques commercially available – example here: Several different techniques commercially available – example here: SOUNDTECs Direct System is partly implantable, electromagnetic SOUNDTECs Direct System is partly implantable, electromagnetic Implant is only a permanent magnet in a titanium canister Implant is only a permanent magnet in a titanium canister External digital sound processor – battery, microphone, electromagnetic coil – fits behind ear External digital sound processor – battery, microphone, electromagnetic coil – fits behind ear Magnetic field from external coil interacts with permanent magnet across tympanic membrane Magnetic field from external coil interacts with permanent magnet across tympanic membrane

MEIHD Limitations Cannot regain original frequency sensitivity Cannot regain original frequency sensitivity Internal noise generated by external amplification Internal noise generated by external amplification Prolonged overdrive of remaining hair cells may make hearing loss worse Prolonged overdrive of remaining hair cells may make hearing loss worse For long term hearing restoration probably must look to cochlea implants 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 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 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 Localised, controlled, charge-balanced, biphasic waveforms used to depolarise pools of neurons along the cochlea Different electrodes stimulated dependant on signal frequency – restores filtering Different electrodes stimulated dependant on signal frequency – restores filtering Stimulation coded for loudness and pitch information 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 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 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 Attracting pair of magnets on surface + under skin align antennae and hold external devices Encoded signal + Power transmitted transcutaneously using RF antennae Encoded signal + Power transmitted transcutaneously using RF antennae Demodulator assigns information to the array Demodulator assigns information to the array

Signal Processing Varies enormously between devices Varies enormously between devices Preserve waveforms, speech envelope info or spectral features Preserve waveforms, speech envelope info or spectral features Original single electrode developed in 1970s Original single electrode developed in 1970s Main speech frequency info 300-4,000Hz Main speech frequency info 300-4,000Hz

Cochlea Electrodes Placement is w.r.to ganglion cells Placement is w.r.to ganglion cells Usually placed in scala tympani (intracochlea) Usually placed in scala tympani (intracochlea) Placement aim is for coding fequencies Placement aim is for coding fequencies Multichannel electrode array Multichannel electrode array Silicone rubber carrier – shaped to fit in the scala typani 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 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 30,000 branches of auditory nerve Can be divided into frequency groups Can be divided into frequency groups Examples: Examples: The COMBI-40+ uses 12 pairs of electrodes dispersed over 26.4 mm The COMBI-40+ uses 12 pairs of electrodes dispersed over 26.4 mm The Nucleus uses 22 electrodes spaced 0.75mm apart The Nucleus uses 22 electrodes spaced 0.75mm apart

Cochlea Stimulation Two types – analog and pulsatile Two types – analog and pulsatile Analog: acoustic waveform is replicated, filtered and passed to all electrodes Analog: acoustic waveform is replicated, filtered and passed to all electrodes Neural plasticity of brain sorts it out Neural plasticity of brain sorts it out Problems with channel interaction Problems with channel interaction Pulsatile: electrodes deliver narrow set of biphasic, charge balanced pulses Pulsatile: electrodes deliver narrow set of biphasic, charge balanced pulses Pulse amplitude related to filtered waveforms Pulse amplitude related to filtered waveforms Stimulation rate affects recognition (over 80%) Stimulation rate affects recognition (over 80%) Clarion – 250,000 pulses/sec Clarion – 250,000 pulses/sec

Auditory Brainstem Implants When auditory nerve has been damaged When auditory nerve has been damaged Modified CI device used for direct stimulation of the cochlea nucleus Modified CI device used for direct stimulation of the cochlea nucleus Multielectrodes placed on the surface of the cochlea nucleus Multielectrodes placed on the surface of the cochlea nucleus Human trials have been approved but only just getting under way 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 Most common hearing loss due to loss of inner ear hair cells Middle ear direct drive and cochlea implants are increasing in use. Middle ear direct drive and cochlea implants are increasing in use. Further miniaturization, total implantation and greater lifetimes expected Further miniaturization, total implantation and greater lifetimes expected Possible genetic means of restoring hair cells – if not then bioengineering approach Possible genetic means of restoring hair cells – if not then bioengineering approach

Next Week Visual Neuroprostheses Visual Neuroprostheses