Hearing Impaired

1
Hearing Impaired

• Physiology of Cochlear Implants

Jay B. Dean, Ph.D.
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Overview

• Auditory Pathway
• Inner Ear
• Cochlea
• Basilar Membrane
• Frequency Discrimination
• Signal Transduction sound waves to electrical
  impulses
• Deafness
• Cochlear Implants
• Cochlear Implants the view from the brain
  (J.C. Middle brooks, J.A. Bierer R.L. Snyder,
  2005)

3
What is a Disability?

• Neuromuscular impairment
• Sensory deprivation (vision, hearing or speech)
• Cognitive dysfunction (learning, higher
  processing)

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Sensory Hearing
http//www.brainconnection.com/med/medart/l/anat/9
90705.jpg
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Outer, middle inner ear
http//www.american-hearing.org/images/ear.jpg
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Sound Waves
http//www.privateline.com/TelephoneHistory/soundw
aves.html
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Sound Waves Air Pressure Waves
http//clerccenter.gallaudet.edu/infotogo/535/ear.
gif
Examples of a steady-state sound (pure tones)
above and dynamic sound (speech) below
http//www.ucihs.uci.edu/hesp/publications/houseea
rinstitutemain.htm
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Middle Ear
http//depts.washington.edu/otoweb/patients/pts_sp
ecialties/pts_hear-n-bal/images/middle_ear.jpg
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Sensitivity of the Ear
The minimum sound pressure level perceptible to
the ear at a particular frequency is called the
threshold of hearing at that frequency. This is
different for each individual, even between
people with 'normal' hearing capacities. It is
also age related, with a progressive loss in
sensitivity at the high frequencies occurring
with increasing age.
A young, healthy ear can respond over a frequency
range of 20 Hz to 20,000 Hz.
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Auditory Cortex
http//www.colorado.edu/epob/epob3730rlynch/image/
figure8-16.jpg
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Inner Ear

• 3 cavities in the the temporal bone, containing
• vestibule - next to the oval window
• 3 semi-circular canals - which are the body's
  sensor for balance and orientation
• cochlea - a bony spiral organ, about 35 mm
  long, shaped like a snail shell of 2 1/2 turns.
• This is where mechanical vibrations transmitted
  from the middle ear are transformed into nerve
  impulses to be perceived by the brain as sound.
• Each division contains an incompressible fluid
  called perilymph.

The inner ear is the most important part of the
ear for hearing and contains the structures which
are damaged by excessive noise.
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Cochlea

• The cochlea is itself divided lengthwise into
  three chambers
• scala vestibuli - which has the oval window at
  its base
• scala tympani - which ends in the round window
  (a simple membrane which acts as a pressure
  release) and
• scala media - which contains the true hearing
  sensory structure - the organ of Corti.

http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Cochlea
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Endolymph vs. Perilymph
The dividing membranes are called the basilar
membrane and Reissner's membrane. The organ of
Corti, which contains the sensory hearing cells,
is supported on the basilar membrane in the scala
media which is filled with endolymph fluid. The
scala vestibuli and scala tympani are connected
at the apex of the cochlea by an opening called
the helicotrema, and are filled with perilymph
fluid. The scala media is at a slightly higher
electrical potential than the other two chambers
(80 mV). This potential difference is important
for the correct functioning of the cochlea.
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Basilar Membrane
The organ of Corti contains the sensory hair
cells which are embedded in supporting cells
attached to the basilar membrane. There are two
types of hair cells - inner and outer. The
inner hair cells, of which there are about
10,000, form a single row along the inside spiral
of the cochlea. The outer hair cells, of which
there are about 20,000, are in three parallel
rows towards the outside of the spiral.
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/images/l101_12.jpg
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Frequency Discrimination
The ear is able to detect different frequencies
in sound due to the characteristics of the
basilar membrane. The basilar membrane has the
mechanical properties of elasticity
(springiness), damping (friction) and mass
(inertia), the first two of which change along
its length. At the basal end (near the oval
window), the membrane is narrow and rigid, while
at the apex, it is wider and floppier. The
elasticity interacts with the inertia of the
fluids in the cochlea to support a wave-like
motion traveling from the basal end to the apex.
apex
basal
basal
apex
Position of the Peak Vibration of the Basilar
Membrane for Sounds of Different Frequency
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Frequency Discrimination (continued)
Basilar Membrane
At each point along the membrane the ratio of
stiffness to mass varies, and this determines an
upper frequency above which a wave will not
travel. At the basal end, where the stiffness
is high, this cut off frequency is also high, and
most frequencies in the auditory range will
travel as a wave. Towards the apex, the
stiffness decreases, and so the cut-off frequency
is less and high frequency waves will not travel
in this region.
apex
basal
basal
apex
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Frequency Discrimination (continued)
Basilar Membrane
apex
basal
Hence the basilar membrane acts to sort the
incoming sound waves into different frequency
components - high near the oval window and low
near the apex. This frequency discrimination is
essential to good hearing.
basal
apex
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Stereocilia
Each hair cell has a cluster of hair-like
structures, called stereocilia, on its upper
surface. The stereocilia are arranged in "w" or
"v" formations.
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Stereocilia, Tectorial Membrane
Above the hair cells is the tectorial membrane,
which is attached to the lining of the cochlea
wall and may be attached to the outer hair cell
stereocilia. When a traveling wave displaces
the basilar membrane, a shearing movement of the
stereocilia occurs. These function much like a
microphone - small back and forth movements of
the cilia change the flow of electric current
through the hair cells.
http//www.safetyline.wa.gov.au/institute/level2/c
ourse18/lecture101/l101_08.asp
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Inner/Outer Hair Cells
The inner hair cells are the primary sensory
cells. They directly connect to individual nerve
fibres of the auditory nerve. The sound-induced
voltage changes within the inner hair cells lead
to electrical activity in the nerve, which is
sent to the brain. The outer hair cells appear
to serve an additional, mechanical purpose, only
recently discovered. Experiments have shown that
they are likely to lengthen and shorten in
sympathy with the electrical signals passed by
their hairs.
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Signal transduction
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Auditory Nerve
AUDITORY NERVE Nerve fibres carry the impulses
from the hair cells. They pass through the spiral
ganglia, to join together to become the auditory
nerve. This connects to the cochlea nuclei in
the brain stem and hence to the higher auditory
centres in the temporal lobe of the brain. Here
the messages, received and analysed by the ear,
are interpreted.
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Causes of Deafness

• EAR CANAL - Most ear canal problems are due to
  either excessive wax, or foreign bodies in the
  ear, most of these problems are easily treatable.
• 2. EAR DRUM - This can either be perforated or
  broken, perforated ear drums can be assisted by a
  hearing-aid, but badly damaged ear-drums can lead
  to complete loss of hearing.
• 3. EAR BONES - There can be a bone growth which
  may cause the bones to weld together
  (Otosclerosis) and not work as they should. A
  hearing aid may help but more often surgery is
  needed.
• 4. COCHLEAR - Age related hearing loss
  (Presbyacusis) is due to the tiny hair cells
  dying. Sometimes these may be damaged due to
  illness, this is helped by use of a hearing aid.
• 5. NERVE DEAFNESS - Mainly due to illness (e.g.
  mumps) the nerves are damaged thus the sound
  signal is not sent to the brain as it should
  normally. can be helped with use of a hearing
  aid.
• 6. GLUE EAR - This is where the ear draws fluid
  from Eustachian Tube which is connected to the
  nose, this is easily helped by minor surgery.
  http//www.norfolkdeaf.org
  .uk/types.htm

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Deafness
The cochlea converts the sound waves into
electrical signals. These signals are then passed
to the brain. Around 80 of deafness occurs due
to damage to the cochlea cells. Deafness is in
fact one of the most common of all disabilities,
and still very little is known about it.
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What is a cochlear implant? A cochlear implant
is an implanted electronic hearing device,
designed to produce useful hearing sensations to
a person with severe to profound nerve deafness
by electrically stimulating nerves inside the
inner ear.

http//www.wasa-shhh.org/cochlear_implants.htm
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• What is a cochlear implant?
• These implants usually consist of 2 main
  components
• The externally worn microphone (5), speech
  processor (3) and transmitter system (4).
• The implanted receiver (2) and electrode system
  (1), which contains the electronic circuits that
  receive signals from the external system and send
  electrical currents to the inner ear.

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Cochlear Implant
1. Sounds in the environment are picked up by the
small directional microphone.2. A thin cable
(cord) sends the sound from the microphone to the
Spectra 22 speech processor.3. The speech
processor amplifies, filters and digitizes sound
into coded signals.4. These coded signals are
sent from the speech processor to the
transmitting coil via the cables.5. The
transmitting coil sends the signals across the
skin to the implanted receiver/stimulator via an
FM radio signal.6. The receiver/stimulator
delivers the correct amount of electrical
stimulation to the appropriate electrodes on the
array.7. The electrodes along the array
stimulate the remaining auditory nerve fibers in
the cochlea.8. The resulting electrical sound
information is sent through the auditory system
to the brain for interpretation .
http//depts.washington.edu/otoweb/patients/pts_s
pecialties/pts_hear-n-bal/pts_hear-n-bal_cochlear-
implant.htm
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Implanted Electrode
http//www.bcm.edu/oto/jsolab/cochlear_implants/co
chlear_implant.htm
The implanted part is an electronic device that
is put under the skin behind the ear.
An electrode connected to the device is inserted
into the inner ear.
The electrode is simply a bundle of tiny wires
that have open contacts spread out along the
length of the cochlea. Thus, the electrical
signals can be sent to different areas of the
cochlea and represent different frequency sounds.
30
http//www.hearingloss-wa.org/implant20image.JPG
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• Who uses cochlear implants?
• severely to profoundly deaf adults and children
  who get little or no benefit from hearing aids.
• even individuals with severe or profound "nerve
  deafness" may be able to benefit from cochlear
  implants.

32

• What determines the success of cochlear implants?
• How long the patient has been deaf
• patients who have been deaf for a short time do
  better than those who have been deaf a long time
• How old they were when they became
  deaf--whether they were deaf before they could
  speak
• How old they were when they got the cochlear
  implant--younger patients, as a group, do better
  than older patients who have been deaf for a long
  time
• How long they have used the implant
• How quickly they learn

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• What determines the success of cochlear implants
  (continued)?
• How good and dedicated their learning support
  structure is
• The health and structure of their
  cochlea--number of nerve (spiral ganglion) cells
  that they have
• Implanting variables, such as the depth and
  type of implanted electrode and signal processing
  technique
• Intelligence and communicativeness of patient

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How does a cochlear implant work? A cochlear
implant receives sound from the outside
environment, processes it, and sends small
electric currents near the auditory nerve.
These electric currents activate the nerve,
which then sends a signal to the brain. The brain
learns to recognize this signal and the person
experiences this as "hearing". The cochlear
implant somewhat simulates natural hearing, where
sound creates an electric current that stimulates
the auditory nerve. However, the result is not
the same as normal hearing.
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Reference no. 16
36
Cochlear implants the view from the brain, 1

• Cochlear implants (therapeutic option)
• Patients who lack cochlear hair cells
• And who have surviving auditory nerve fibers
• C.I. used for gt 2 decades
• gt60,000 devices implanted
• However, few studies of how CNS responds to
  stimulation of the auditory nerve by
  intracochlear electrodes

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Implanted Electrode
http//www.bcm.edu/oto/jsolab/cochlear_implants/co
chlear_implant.htm
The implanted part is an electronic device that
is put under the skin behind the ear.
An electrode connected to the device is inserted
into the inner ear.
The electrode is simply a bundle of tiny wires
that have open contacts spread out along the
length of the cochlea. Thus, the electrical
signals can be sent to different areas of the
cochlea and represent different frequency sounds.
38
Reference no. 15
Multi-site neural recording technology
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Multi-site neural recording technology
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Cochlear implants the view from the brain, 2

• Normal ear spectral analysis of sound
  (frequencies) is accomplished by the mechanical
  frequency sensitivity of the cochlea
• Map of sound frequency tonotopic organization
• Frequency analysis inner hair cells
• Synaptic activation auditory nerve fibers

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Cochlear implants the view from the brain, 3

• Cochlear prosthesis
• Cochlear signal transduction ? microphone
• Spectral analysis of fluid waves (cochlea) ? band
  pass filters within the speech processor

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Cochlear implants the view from the brain, 4

• lt 24 intra-cochlear electrodes _at_ different
  positions along the basilar membrane

Basilar Membrane
basal
apex
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Cochlear implants the view from the brain, 5

• Amplitude-modulated electrical pulse train
  (recorded by the electrode) that stimulates the
  auditory nerve
• Normally, this information
• is relayed tonotopically to
• the ICC A.C.

ICC, low freq. (dorsolaterally) high
freq. (ventromedially) Spatial tuning
curve Tonotopic axis of the ICC A.C.
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Spatial Tuning Curves derived by recording
successively along the tonotopic axis of the ICC
or AC
X-axis stimulus intensity (dB) Y-axis ICC
depth (mm)
45
Central representation of spectral information,
pp. 488-490

• Multi-site recording probes simultaneous
  recordings of action potentials activity along
  the tonotopic axis of the ICC or AC.
• Thus ? spatial tuning curves for cochlear
  implants vs. visually placed recording electrodes
• Result ?

46
Multi-site neural recording technology
Cochlear implant
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Central representation of spectral information,
pp. 488-490

• Result
• Cochlear implant most focused or restricted
  activation (i.e. V shaped spatial tuning curve)
  was similar to that activated by 1/3-octave-wide
  noise burst in normal hearing
• Careful placement of bipolar electrode pairs
• Radial dimension of cochlear spiral ? spatial
  tuning curve produced by pure tones.
• Importanceoptimizing design of clinical
  electrode arrays !

48
Central representation of spectral information,
pp. 488-490

• Auditory cortex (Figure 1)
• Patterns of excitation vary with cochlear
  electrode configuration and place of excitation
• MP, BP or TP electrodes
• MP ? TP electrodes infer increasingly restricted
  cochlear electrical fields i.e. more focused
  higher spatial resolution activation of auditory
  cortical neurons

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R

• MP
• BP
• TP

C.I.
50

• Firing rate responses (color coded) evoked by
  various cochlear-implant stimuli in the auditory
  cortex (guinea pig).
• Vertical axis cortical place
• top caudo-medial
• bottom rostro-lateral
• Horizontal axis time relative to stimulus onset
• Blue ? Yellow ? Red
• Low FR ? high FR
• (where FR firing rate in action
  potentials/second)
• ? FR weighted centroid of activity

Monopolar electrode
Bipolar electrode
Tripolar electrode
Basal cochlea High frequencies
Apical cochlea Low frequencies
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Basilar Membrane
basal
apex
52

• Firing rate responses (color coded) evoked by
  various cochlear-implant stimuli in the auditory
  cortex (guinea pig).
• Vertical axis cortical place
• top caudo-medial
• bottom rostro-lateral
• Horizontal axis time relative to stimulus onset
• Blue ? Yellow ? Red
• Low FR ? high FR
• (where FR firing rate in action
  potentials/second)
• ? FR weighted centroid of activity

Cochlear stimulation
Basal coochlea High frequencies
Apical cochlea Low frequencies
Monopolar electrode
Bipolar electrode
Tripolar electrode
53
Central representation of spectral information,
pp. 488-490

• Figure 1, cortical images of neural activity
  shift
• Activation of caudal-to-rostral cortical loci
  (top-to-bottom) which represents high-to-low
  frequencies in the auditory cortex
• Notice the restricted area of activation in the
  AC using TP electrodes at the cochlea
• Spectral information is transmitted best by
  restricted electrode configurations (i.e., BP
  TP).

54
Central representation of temporal information,
pp. 490-491

• Temporal information (timing, direction, tempo,
  rhythm, etc.)
• Amplitude envelope of sound waves
• C.I. transmit amplitude envelopes by amplitude
  modulation of constant-rate electrical pulse
  trains. How effective is this?
• Normal hearing neurons in the auditory pathway
  fire action potentials in synchrony (phase lock)
  to amplitude-modulated stimuli

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Central representation of temporal information,
pp. 490-491

• In the C.I.
• Initial evidence indicates that neurons in the
  auditory pathway also phase lock to stimuli
  applied to cochlear devices
• Auditory cortical neurons can phase lock across
  the range of modulation frequencies relevant for
  speech perception in cochlear implants

56
Central auditory plasticity, p. 491

• Neural plasticity in the auditory system is
  caused by
• Deafness
• Chronic electrical stimulation of the cochlea
• Deafness
• Disrupts tonotopic organization
• Disruption of tonotopy increases with time
• Chronic cochlear electrical stimulation
• Distorts ICC tonotopy (MP)
• However, MP stimulation preserves tonotopy

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Central auditory plasticity, p. 491

• Neural plasticity in the auditory system is
  caused by
• Deafness
• Chronic electrical stimulation of the cochlea
• Deafness
• Disrupts tonotopic organization
• Disruption of tonotopy increases with time
• Chronic cochlear electrical stimulation
• Distorts ICC tonotopy (MP)
• However, TP stimulation preserves tonotopy