Transcript Cochlear Implant 1

Hearing Impairment
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Hair cells are responsible for translating mechanical information
into neural information.
Thus, with damaged hair cells, the auditory system has no way of
transforming acoustic pressure waves (sound) to neural impulses
which in turn leads to hearing impairment.
Hair cells damage might cause by diseases (e.g meningitis,
Meniere’s).
If large number of hair cells or auditory neurons throughout the
cochlea are damaged, then the person with such a loss is
diagnosed as profoundly deaf.
There is research which shows that the most common cause of
deafness is the loss of hair cells rather than the loss of auditory
neurons.
So, the remaining neurons could be used by exciting it with
electrical stimulation.
This make way to the wide use of cochlear implants.
Cochlear Implant
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Cochlear implants are beneficial for people with profound
sensorineural hearing loss, who do not benefit from conventional
hearing aids.
Basic components of cochlear implant :
• Microphone
• Speech Processor
• Transmitter
• Receiver
• Electrode Array
External components : microphone, speech processor,
transmitter.
Implanted components : receiver (in a drilled indentation on the
temporal bone), electrode array (inserted in the cochlear)
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Principle operation of Cochlear Implant
 Sound is detected by the microphone and converted into an
electrical signal
 The signal is sent to the speech processor
 Input signal is analysed and certain features are extracted and
coded in the speech processor
 These codes containing the extracted speech information are
sent to the transmitter
 The coded signal is sent via the transmitting coil, through the
skin, to the receiver as an RF signal
 The custom IC in the receiver decodes the RF signal to
determine the electrode number, stimulation level and stimulation
rate.
 The signal is transformed into the appropriate electrical pulses.
 The pulses are sent to the electrode array in the cochlea, which
stimulates the nerves inside the cochlea.
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Cochlear Implant Characteristics
 Electrode design (e.g., number of electrodes, electrode
configuration)
 Type of stimulation - analog or pulsatile
 Transmission link - transcutaneous or percutaneous
 Signal processing - waveform representation or feature
extraction.
Electrode Design
 Issues associated with electrode design are: electrode
placement, number of electrodes and spacing of contacts,
orientation of electrodes with respect to the excitable tissue and
electrode configuration.
 Electrodes may be placed near the round window of the cochlea
(extracochlear), or in the scala tympani (intracochlear) or on the
surface of the cochlear nucleus.
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Most commonly, placed in the scala tympani because it brings
the electrodes in close proximity with auditory neurons which lie
along the length of the cochlea.
It is preferred because it preserves the "place" mechanism used
in a normal cochlear for coding frequencies.
Number of electrodes and spacing between the electrodes
affects the place resolution for coding frequencies.
Larger number of electrodes, the finer the place resolution for
coding frequencies .
Single-channel implant: Only a single pair of electrodes
stimulating a fixed site in, or near, the cochlea. Only transmits
temporal information, loudness cues and rate information.
Multiple-channel implant: Uses a number (4-24) of electrodes
that stimulate different regions of the cochlea. Transmits placepitchinformation as well as temporal information.
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Multi-Channel Implant
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Type of Stimulation
 Two types : analog and pulsatile.
 In analog stimulation, an electrical analog of the acoustic
waveform itself is presented to the electrode.
 In multi-channel implants, the acoustic waveform is bandpass
filtered, and the filtered waveforms are presented to all
electrodes simultaneously in analog form.
 Nervous system will sort out and/or make use of all the
information contained in the raw acoustic waveforms.
 Disadvantage of analog stimulation is the simultaneous
stimulation may cause channel interactions.
 In pulsatile stimulation, the information is delivered to the
electrodes using a set of narrow pulses.
 Advantage is that the delivered in a non-overlapping (i.e., nonsimultaneous) fashion, thereby minimizing channel interactions.
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Modes of stimulation : bipolar, common-ground, monopolar.
Bipolar: All electrodes inside the cochlea; Stimulation takes place
between two electrodes.
Common-ground: All electrodes inside the cochlea; Stimulation
takes place between one electrode and all other electrodes.
Monopolar: One extra-cochlear electrode; Stimulation between
an intracochlear and the extra-cochlear electrodes.
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Transmission Link
 Two ways of transmitting the signals: through a transcutaneous
connection and through a percutaneous connection.
 Transcutaneous system transmits the stimuli through a radio
frequency link.
 External transmitter encode the stimulus information for radiofrequency transmission from an external coil to an implanted coil.
 Internal receiver decodes the signal and delivers the stimuli to
the electrodes.
 Transmitter and the implanted receiver are held in place on the
scalp by a magnet.
 Disadvantage:
- Implanted electronics may fail, and require a surgery
replacement.
- Transcutaneous connector contains magnetic materials
which are incompatible with MRI scanners.
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Percutaneous system transmits the stimuli to the electrodes
directly through plug connections.
No implanted electronics except the electrodes.
Advantages of percutaneous are flexibility and signal
transparency.