The Auditory system and Cochlear Implant

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The Auditory system and Cochlear Implant
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The nature of sound
The travelling wave that is sound is like ripples
in a pond that travel from a source.
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The structure of the auditory system
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The Middle Ear
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Role of the ossicles

• To enhance the sound pressure before passing it
  on to the inner ear.
• How 1. Concentration of force
• 2. Leverage

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The Attenuation Reflex

• The attenuation reflex
• Contraction of muscles greatly diminishes
• sound conductance
• (Sound attenuation is much greater at low
  freq. than high freq.)

• 1. Adaptation to continuous sound at high
  intensities
• (inhibit saturation)
• 2. Damage protection but
• 3. Filter a high pitch sound.

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The Inner ear (Cochlea)
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Cross section of the cochlea
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Cross section of the cochlea (simplification)

• Perilymph
• (scala vestibuli / tympani)
• Endolymph
• (scala media) high K and low Na

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Physiology of the cochlea
Wider (x 5) Floppier (x 100)
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The Response of the basilar membrane
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Hair cells viewed through the scanning electron
microscope
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The Organ of Corti
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Movement of the basilar membrane
Scalar vestibuli ? Scalar tympani
Case of hyperpolarization
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Hair cell receptor potential
Saturation of receptor potential by a cilial
movement of 20 nm
Close relationship between sound pressure and
hair cell receptor potential.
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Transduction by hair cell

• Weak K current in normal condition
• Elastic filament linked
• Depolarization by K channel opening
• K ion activates voltage-gated Ca2 ch.

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The Innervation of Hair cells

• 15,000-20,000 outer hair cells, and 3,500 inner
  hair cells
• 35,000-50,000 ganglion cells
• 95 of ganglion neurons communicate with inner
  hair cells
• 110 ratio of inner hair cells and ganglion cells

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Amplification by outer hair cells

• Motor proteins
• (Not ATP-driven)
• 100-fold smaller signal
• without outer hair cells
• (e.g. Furosemide)
• Kanamycin kills outer
• hair cells and leads to
• deafness.
• Noisy ears
• very sensitive outer hair
• cells (tinnitus)

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Cochlear Implant needed
The loss of hair cell
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The Responses of an Auditory nerve fiber
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Cochlear Implant
Mimic the function of a normal cochlea
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The electrical stimulation should convey
information about the amplitude and the frequency
of the acoustic signal
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Implant Characteristic
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Electrode
Single vs. multiple channels (usually 4-8) The
Number of electrode (multi electrode usually 22)
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Electrode design (Esp. Electrode Configuration)
Monopolar electrode
Bipolar electrode
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Type of Stimulation

• How information is presented to the electrodes.

• Analog coding (continuous)
• Digital coding (pulsatile)

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Transmission link
Radio-frequency link connection
Percutaneous (direct) connection
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Signal Processing (speech processing strategy)

• Feature extraction
• e.g. MULTIPEAK (MPEAK)
• Waveform representation
• e.g. SPEAK (Spectral PEAK)
• CIS (Continuous Interleaved Sampling)
• CA (Compressed Analog)

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MPEAK
Fundamental Freq.
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SPEAK

• Center Freq. range from 250 10kHz (Nucleus CI
  20BPF)
• The Number of maxima selected varies from 5 to 10
  depending on the spectral composition of the
  input signal
• The selected electrodes are stimulated at a rate
  that varies between 180 300Hz

MPEAK vs. SPEAK

• Large improvements in performance were found
  with tests in noise

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The architecture of the spectra 22 processor
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CIS (Continuous Interleaved Sampling)

• CIS approach which addressed the channel
  interaction issue by using
• nonsimulataneous, interleaved pulses

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e.g. 6-3-5-2-4-1
The CIS strategy uses high-rate pulsatile
stimuli to capture the fine temporal details of
speech
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Compression (Nonlinearity Map)
Logarithm map

• Using
• Logarithm map
• Power-law transform

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Other strategy

• ACE(Advanced Combined Encoder)
• ACE strategy filters speech into a set number of
  channels and then selects the highest envelope
  signals for each cycle of stimulation

• SAS(Simultaneous Analog Strategy)
• Closely mimic the normal ear
• All incoming sound is compressed and filtered
• into 8-channels. Then simulataneous and
• continuously presented to the tonotopic
  electrode.

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Compressed analog(CA) approach in Ineraid device
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Summary
The SAS strategy has met with limited success,
whereas the SPEAK and CIS strategies have been
relative success
But..
It appears that no one system is effective for
all recipients Recent advances have made it
possible for one cochlear implant to offer
several speech processing strategies in the same
implant
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Make an addition to Non-linearity
Electromotile contraction
Spontaneous activity
Gate in the nodes of Ranvier