Auditory Implants
Deafness
Severe
Profound
71-95dB HL
95dB + HL
20-30Hz per second is minimum hearing frequency
20,000Hz is maximum hearing frequency
Sensory hearing loss requires CI
Loud speech may be understood at 1 foot from ear
distinguish vowels but not consonants
Does not rely on audition as primary modality for communication
80dB is threshold level for CI
CI
Surgically implanted auditory prosthesis
Electrode array that sits inside the cochlea
Provides some degree of hearing to those with severe/profound HS
For people who do get enough benefit from hearing aids
might not be able to hear high freq. with HAs - require CI
less than 50% of words from a sentence - could benefit from HAs
can get access to sounds up to 30dB
electrical stimulation of hearing
Components
Transmitter coil
Sound processor
Microphones
Electrode Array
Receiver coil and stimulator
How CI Work
Hair cells in the cochlea are damaged or absent in most forms of permanent deafness
Neurons that link the cochlea to the auditory system (spiral ganglion cells) remain and can be stimulated
CI produces electrical stimulation that produces a neural response in the SGC
Intact auditory nerve transmits the resulting signal to the brain
internal device
receives signals from outside and transfers to an electrical current to the cochlea
Electrode array (12-22 electrodes) inserted in the scala tympani
converting sound
Acousting signal picked up my microhone
Speech processor converts signal to suitable stimulation
Transmitting coil transmits signal as radio frequency across skin
Internal receiver decodes electrical signal and transmits to EA
Activated electrodes stimulate cochlea/auditory nerve interface
Electrical impulses conveyed by auditory nerve to brain which are interpreted as sound
Frequency picked up depending on position in the cochlea
low frequency further in the cochlea
each electrode sends electrical pulses corresponding to particular frequency range
CI design aspects
External hardware
Signal processing
Electrical stimulation
Electrode design
Adverse event prevention and longevity
Future/biological interventions (drug delivery)
CI gives discrete signals that are different to those naturally given by your ear
biocompatible but body does react to the electrodes
signal and electrical output
pics up sound waves
Converts to digital signal to N frequency bands
Filter bank
selects all bands or those with highest amplitudes
Signal Processing
Analogue to digital conversion
amplification and compression
multi-mic noise suppression (beam forming)
enhances signals in a particular direction
Filter bank
band-pass signals for use in individual electrodes
decide the total signal bandwidth
Narrow bands to mimic the healthy auditory system
Electrical Stimulation
Pitch shift/mismatch
Limitations in neural synchronisation
Channel interaction (spread of excitation)
How deeply the electrode is placed into the cochlea
Length of EA
frequencies coded by the electrode
Size of the cochlea
Neural response
Temporal response/synchronised firing
refractory responses
Neural synchronisation variations between patients
current spread
overlap in stimulation between adjacent channels
variability between patients
Challenges
Speech perception in background noise
Frequency selectivity/channel interaction
Pitch
Cochlear health/residual hearing preservation
Spatial hearing
Music perception
Adverse events and device longevity
understand here sound comes from
foreign body inserted
small background noise can disrupt hearing of speech
Small differences in frequencies fall within the same electrode channel
fixed rate of electrical pulses
output of 2 mics to prioritise speech and reduce background noise
Hybrid or EAS
CIHA or bimodal
Neural response telemetry (NRT) is a method of capturing the action potential of the distal portion of the auditory nerve in cochlear implant (CI) users