by Dave Sindrey
I hope this information will give therapists\parents some bearings when
dealing with their audiologist and develop a more collaborative relationship between them.
Module 1 - Parts of the Cochlear Implant System
Module 2 - The Dynamic Range of Hearing
Module 3 - Mapping
Module 4 - -
Module 5 - -
Module 6 - Trouble Shooting the Nucleus 22 and the Nucleus 24
Throughout this work, the pronoun "she" is used for the audiologist and
"he" for the client.
Module 1 - Parts of the Cochlear Implant System
This module is about the pathway of hearing in listening with hearing at normal
thresholds, the fitting of hearing aids for those with a sensorineural loss, and then the
pathway of hearing for those with cochlear implants. It provides a framework for
what I will cover in the next couple of modules. Please read it even if you are
familiar with the content so that you will follow my explanation. I'll skim over
some topics that I will cover in more detail later.
The Pathway of Hearing
When discussing the ear it is usually divided into three parts: the outer ear (the pinna
<the ear you see> and the ear canal), the middle ear (the ear drum, the three tiny
bones of the middle ear <ossicles>, and the Eustachian tube <a tube that connects
the middle ear to your throat in order to equalize air pressure and to drain>, and the
inner ear (the system of balance <vestibular system>, the vestibular nerve, the
cochlea, and the auditory nerve).
As you consider how the ear is designed to naturally convert the sound collected at the
pinna to finally be processed for meaning by the brain, you will see how the cochlear
implant works in a similar way to deliver usable information to the auditory nerve. The
ear is a series of steps that converts the sound in air waves to a mechanical force,
sorted, filtered and emphasized with fine representation of the information first
received. It ends up with a natural electrical impulse created by the inner hair
cells of the cochlea.
The outer ear and the middle ear work to "conduct" sound to the inner ear. If
there is a problem in this area, the loss is described as "conductive".
Information about the source of a sound is carried by particles of air in a domino effect.
There is no sound in space. The sounds of speech set the air in motion quite
distinctly from one another. The air particles vibrate one another to carry the
information to be collected and emphasized by our outer ear (the pinna gives a little
boost to frequencies between 1000 Hz and 2000 Hz. This is a prime area for the
sounds of speech. Ear canals (and especially children's ear canals, being smaller)
give an added boost on top of that to the area around 2000 to 4000 Hz.
The eardrum acts as microphone and converts the information received concerning the
movement of air particles into something the brain can interpret. Three tiny bones
connected to the ear drum within the middle ear cavity move along with the eardrum's
movement. The way that these bones move conveys information to the oval window.
The bones (because moving from a big area - the eardrum - to a little area - the
oval window, plus a little lever action) add some force to the information. The last
little bone is fixed to this window to the cochlea. As the bone moves it sets up a
wave in the fluid within the cochlea. High pitched sound would cause fast vibrations
and low pitched sounds would cause slow vibrations in the fluid of the cochlea. There is
another opening from the cochlea to the middle ear, also covered with membrane, that
allows for the fluid within the cochlea to move better. This is the round
window and the point near which the surgeon makes a cochleostomy to inset the electrode
array.
When there is a problem with the cochlea or with the auditory nerve past it the loss is
described as "sensorineural" ("sensory" for the cochlea and
"neural" for the nerve).
The cochlea is actually a shell shaped cavity within the bone of the skull It's
about as big as your baby finger tip from the nail up. The oval window (the opening
in the cochlea connected to the ossicles (the three bones in the middle ear space) is
vibrated, which sends a wave through the cochlea. There are thousands of tiny hair cells
in the cochlea - all in rows. There are two different types of hair cells in the
cochlea, named for where they are in the row - inner hair cells and outer hair cells.
If you can imagine stretching out the two and a half turns of the cochlea, the hair
cells are organized along the length of the cochlea like keys on a piano. The part
nearest the oval window is called the "basal" end of the cochlea, and the part
farthest from the oval window is the "apical" end. Just like a piano
keyboard, different parts of the cochlea are designed to receive different
frequencies/pitches. The "basal" end vibrates better with higher pitches,
and as lower pitches are received, more "apical" parts of the cochlea are
vibrated. The hair cells are like seaweed floating in the fluid of the inner ear.
When the part of the cochlea they reside in vibrates, the hair cells bend and that
triggers a small natural electric impulse which is connected to the auditory nerve.
The inner hair cells send the information and the outer hair cell around it works as a
natural amplifier. They emphasize peaks of information received along the
cochlea. The outer hair cells are the first hair cells to be damaged when there is a
sensorineural loss. They help people to hear sounds below 60dB and allow for some fine
tune discrimination of frequencies. The outer hair cells emit a sound when they are
provoked and when they are resting. In a test called "evoked otoacoustic
emissions" a microphone in the outer ear looks for small but measurable sounds that
the outer hair cells generate when the cochlea is healthy. These emissions are
not found in children with conductive or cochlear based hearing losses worse than 30 dB.
So, if a child who responds as profoundly deaf has otoacoustic emissions, the
problem is probably past the cochlea.
A cochlear implant is designed to take the place of the inner hair cells of the cochlea.
A cochlear implant system collects sound, processes it and then carries a usable
signal to the nerve. The cochlear implant cannot bypass a problem with the auditory
nerve (brain stem implants are another matter).
Typically the problem with sensorineural losses is within the cochlea.
The "Mapping for Dummies" deals with the cochlear implants ability to organize
sound so that it can be presented in the most useful way for each listener. This
next section talks about basic parts of the system just so that we all have the same base
to start from as the modules continue.
The Pathway of Hearing Through a Cochlear Implant
The cochlear implant system has both internal components and external components.
The internal component is the receiver stimulator. It looks like a small plastic
mouse with a string of electrodes as its tail. In the plastic body, there is a
magnet, and an internal component that receives the information and stimulates the string
of electrodes. More on that in later modules.
The surgeon drills a path through the bone just behind the ear to get access to the middle
ear space. This entry spot is also where the receiver/stimulator is placed.
The surgeon prepares a small nickel sized bed for it to lie, and sutures it in place.
Again, this is the part of the internal component that receives the information and
stimulates the string of electrodes. The surgeon makes an opening from the middle
ear space into the cochlea through or near the round window. When the skin is
stitched over the stimulator/receiver with the electrodes in place, the child must usually
then wait (according to program and child's swelling) anywhere from ten days to 6 weeks
before the implant can be stimulated.
The external components consist of a microphone, a series of cords, a transmitting coil
and a speech processor.
The microphone picks up sounds and transmits the sounds along a long cord to the speech
processor. This is a microcomputer a little bigger than a case for an audiotape. The
processor sorts the incoming sound and sends it back up the cord to the head set and
across a small cord to the transmitting coil. This is a small round plastic circle
attached to the small cord. It houses a magnet that holds it it place over the skin
to the internal component under the skin. The information is transmitted across the
skin by Radio Frequency transmission. The internal component receives the
information and stimulates the selected electrodes according to the processor's parameters
set by the audiologist. These parameters are specific to each child. The way
that a processor's parameters are set to deliver sound to a child is called the
"map".
If you apply the analogy of a damaged keyboard to the cochlea, the electrodes are laid
down by the surgeon along the damaged keys (the hair cells). When the electrodes are
stimulated, electrical energy bypasses the damaged hair cells and stimulates the nerve
directly above it. The information is passes up the child's auditory system and
perceived as sound.
Back To Top
Module 2 - The Dynamic Range of Hearing
- for normal thresholds and for sensorineural loss with and without a cochlear implant
The Dynamic Range of Hearing
This is the most important concept in "mapping". If you understand the
concept of "dynamic range", you will be able to add on all other information in
relation to it. Here's an explanation of dynamic range as it applies to the usual
pathway of hearing:
In listeners with hearing at normal limits, the sound is carried from its source by
vibrating air particles, which vibrate adjacent air particles and so on. When the
sound waves enter the ear canal and then strikes the eardrum, it does so in a way that
reflects the way it was set in motion. High pitch sounds (high "frequency"
sounds) cause the air particles to vibrate quickly. Low pitch sounds (low
"frequency" sounds) cause the air particles to vibrate more slowly. How
many cycles these vibrations go through in a second is called their "frequency"
and is measured in Hertz (Hz). A high pitched sound could be 10000 Hz and a low
pitch sound might be measured at 200 Hz. Healthy young adults with
"normal" hearing will hear sounds from as low as 20 HZ to as high as 20 000 Hz.
A mouse can hear higher pitches. An elephant can hear lower pitches. An
elephant can hear the low rumblings of far off herds, a mouse can hear the scratching of
insects. A human's hearing seems designed for the sounds of speech.
The intensity of the sound, The force of the sound is measured in decibels (dB). This is
the force at which the air particles strike the eardrum. Healthy young adults with
"normal" hearing will hear sounds at 0 dB Hearing Level (HL). They
actually defined 0 dB HL as the softest sound that healthy young adults with normal
hearing can hear 50% of the time. When the scale reaches 120 dB, most people will
experience painful sensations and the sound may start to cause damage to remaining hair
cells in the cochlea. Frequency in Hertz (Hz) and decibels in Hearing Level (dB HL)
is the scale that we measure and compare our hearing to on an audiogram. The level 0
dB is different for each frequency. If the sound is measured in true sound pressure
level (SPL) the levels would be different for each frequency. The HL audiogram
levels them out so that we can chart a patient's hearing against what is considered normal
- hearing from 20 to 20 000 Hz from 0 dB HL to 120 HL at all frequencies. This isn't
quite true. The audiologist is primarily concerned with the range of hearing that
will transmit the sounds of speech. An audiologist will typically test only up to
8000 Hz. It is also too time consuming to test the level of hearing at each
frequency. The audiologist tests at 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 6000
Hz and sometimes 8000 Hz. She can then guess (interpolate) between the points.
If there is a great difference between two tested points on the audiogram, she may
test between them.
The Dynamic Range of hearing is the area of usable hearing, from the softest sound you can
hear (50% of presentations for audiograms) to the loudest you can tolerate - "Maximum
Comfort Level" (MCL). "Loudness Discomfort Level" is the point at which
sound becomes uncomfortable (LDL). An important aspect of the dynamic range is that
sound grows louder to the listener as you increase the decibels towards the upper limit of
the dynamic range. The important things we have to hear in order to tell one
speech sound from another is the peaks of sound at different frequencies in relation to
other sounds of speech, their loudness in relation to other sounds of speech, and how they
change over time as they are spoken.
A person requires a good "clean" dynamic range. I mean by this one that
allows them to hear all the frequencies that carry the sounds of speech. They have to hear
these sounds at different relative intensities. They also need for that system to
react, recover, and then rereact to the sound as quickly as possible to capture the
changes as they happen.
A person with a sensorineural hearing loss has a restricted dynamic range.They don't start
hearing until much higher decibel levels. After that point loudness grows quite
quickly, because for hearing-impaired (SN loss) and for those with normal hearing, the
Level of Discomfort and/or the level at which the remaining hair cells might be damaged is
the same. So a person with thresholds measured at normal limits might have a dynamic
range of 120dB across frequencies, while a person with a profound sensorineural loss might
have only a dynamic range of 30 dB at the low frequencies and 10 dB at the high
frequencies.
If you think of listening to speech on the radio, you can hear and understand what they
say best if the speech is not just at where you can just hear it 50% of the time (at
threshold). It's better if it is turned up to your Most Comfortable listening level
which is deeper into your dynamic range. It would be too uncomfortable to listen at
your Loudness Discomfort Level. You need it loud enough but not too loud.
An audiologist should aim to amplify the sounds of speech so that they are loud enough to
be deep within that particular users dynamic range. If overamplified, the speech
sounds are sent at or past the point where the Maximum Output Limiting of the Hearing aid
will allow. The audiologist sets the hearing aid so that it will not amplify sounds
past the point where sound will be uncomfortable and/or is likely to damage residual
hearing. When sounds hit this level or past it the hearing aid amplifies it less to keep
the sound within the user's dynamic range. It is better when speech is placed deep
within but not at the top of the dynamic range. When this compression occurs
distortion is usually added to the signal. The "Desired Sensation Level"
or DSL program (Seewald, et. al. 1991) looks at where how loud speech is at different
frequencies (they looked at the child's own speech, the speech of an adult male, an adult
female and another child). They averaged this and came up with values across frequencies -
and called it the Long Term Averaged Speech Spectrum (LTASS).
The DSL program considers the sounds of speech that it is trying to amplify for the child
to best hear it. Based on a child's residual hearing (dynamic range of hearing) this
program makes recommendations for how much amplification would be optimal to place speech
to be heard and understood. This is the only fitting program that considers speech and
amplification fitting this way. The two cochlear implant programs I have worked at
require a good fitting with DSL targets to qualify as an appropriate trial with
amplification. There have been cases where children have come to the centers fit
with other fitting procedures and speech for the higher frequencies (in terms of the
LTASS) do not even reach threshold. There is a dynamic range of usable hearing above
that point which is not utilized. Other cases occur frequently where the output limiting
of the hearing-aid is set so conservatively that sound can not be amplified much past or
even to the point of threshold. Children with hearing aids set this way will still
have reasonable aided audiograms as this only shows the lowest level heard through the
hearing aids 50% of the time. For some frequencies a few decibels above the
threshold may be held constant (can't get louder) because of conservative MPO fitting.
The DSL approach to fitting hearing aids is similar to the approach taken in mapping.
The audiologists first step is to define the child's dynamic range of hearing
across electrodes. She must find the lowest level of stimulation that the child will
respond to at each electrode - called threshold (T). This threshold is a little different
in definition than that used for audiograms. The audiologists looks for a consistent
response (not 50%). It is usually established by passing threshold twice in an
ascending method. I'll talk more about this later. The perceived loudness of sound
is related to the total charge delivered. As the audiologists increases the charge
she should approach Threshold.
Past threshold the listener should also have a perceived increase in loudness with
increase in the charge delivered. The audiologist must also try to estimate the
Loudness Discomfort Level (LDL). For each user these levels will be different.
The C level is the Comfort Level which is set by adult users. For children this is
much harder to estimate. When an audiologist finds the Loudness Discomfort Level
(LDL) for an electrode the C level is typically around the 70% point from T level to this
LDL. I'll talk more about this later too.
If you think of ladders climbing from each electrode. Think of ladders side by side.
The top of each ladder represent the loudest yet comfortable level for listening to
speech (C level). The bottom of each ladder represents the T level. The lowest
level of stimulation that is consistently heard. Each ladder should grow in loudness
from the lowest rung to the uppermost rung. The row of ladders must work together to
represent sounds across frequencies. It is important that the tops of the ladders
are all pretty much the same loudness. This is the C level where speech is sent. If
the uppermost levels are "sawtooth" with the listener perceiving some electrodes
as louder than others, it can throw off the intelligibility of the speech it presents.
The ladders have to line up in terms of loudness growth. This is called
balancing. The audiologist will try to fit the map so that the C levels are balanced
in terms of loudness across all electrodes. Ideally, the mid points in the dynamic
range should be balanced as well, so that the ladders all work together as a dynamic
range.
Back To Top
Module 3 - Mapping
I think the best way to get these concepts is to learn from a basic framework and then
to add to it exceptions and new developments as we go. I'll start by explaining the basics
of the mapping process through the Nucleus 22 device. As you learn about stimulation
mode and speech coding strategies etc. you will see how different internal devices are
designed for different purposes.
For the explanation that follows, imagine that the cochlea is stretched out straight. The
X's below represent the electrodes in the electrode array.
Imagine you are facing the child with a cochlear implant and the implant is in his right
ear.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
right ear [X X X X X X X X X X X X X X X X X X X X X X
"[" can represent the division between inner ear and middle ear.
The part of the cochlea nearest the opening from the middle ear is called the
"basal" (base) end and the part of the cochlea at its furthest point is the
"apical" end. The most "basal" electrode is then the one nearest
the round window and is termed electrode 1. The most "apical" electrode is
termed electrode 22. The cochlea is naturally organized to process high pitches at
the "basal" end and going down towards lower pitches as further points of the
cochlea (apical end) are stimulated.
The numbering of the Electrodes is for their positioning as you progress into the
cochlea. It can confuse you a little when you try to remember that high frequency
information is allocated to the low number electrodes.
Remember that the internal device looks like a computer mouse with the magnet and
receiver/stimulator housed in its body and the wires leading from it through the tail to
the stimulating electrodes at the tip of its tail. The tail can be inserted up to 25 mm
through the round window into the cochlea (about 1 1/2 turns of the 2 1/2 turns of the
cochlea - further than that the cochlea does not respond very well to electrical
stimulation). Each electrode is connected to the receiver/stimulator by its own insulated
platinum-iridium wire. From the receiver/stimulator the tail is is a flexible
carrier that houses these wires and takes them to the round window. Then there are ten
extra non-stimulating electrodes that add support. At the end of this plastic string
are the 22 stimulating electrodes. They taper from 0.6 mm in diameter to 0.4 mm at
its tip at the apical end. The electrodes are evenly spaced along the array.
The cochlea actually has three chambers. If you could stretch it out into a straight
tube and then looked down one end you would see three triangular spaces that run the
length of the cochlea. The electrode array is (almost always) placed in the lower
chamber called the scala tympani. The electrode array lie close to the basilar
membrane. This is a long sheet of tissue that separates the lower chamber from the
middle chamber. It is what vibrates with beautiful precision in the healthy ear to
the waves in the cochlea send off by movement of the middle ear ossicles (bones) which
were set off by the air waves striking the ear drum). Normally the fluid is pushed
at the oval window. this sets up the wave in the uppermost chamber called the scala
vestibuli (pnemonic - "vest" is a top). The peak of the wave indents
downward on the top of the middle chamber called the scala media
("middle"). In this middle space there are different structures that help
to refine the signal sent. The hair cells that we talked about in earlier modules
are on the floor of this chamber on the basilar membrane. The inner hair cells bend as the
basilar membrane is pushed downward into the scala tympani. The outer haircells
enhance the cochlear response by sharpening the peaks of basilar membrane movement.
The outer hair cells work as a natural amplifier. When these are gone the ear cannot
receive sounds softer than 60 dB. When the inner hair cells are bent a natural
charge is sent to the neural tissue it connects to. Here's an important point - The
faster these cells are stimulated and the more of these cells that combine together, the
greater the perception of loudness.
Think of the hair cells as piano keys with each (about 20,000) responsible for different
pitches. In most sensory neural deafness the problem is with these
"keys". The inside of the "piano" is working fine. The
surgeon lays down the electrode array so that an electrical pulse can be sent past the
damaged key to strike a cord in the piano below. These pulses are sent in concert
with the information received at the microphone. The sounds of speech are heard as
patterns of electrodes playing past a broken keyboard to the piano below.
The neural tissue is actually above not below the array. The electrode array is
placed in the scala tympani along the basilar membrane. As the current is sent from
the electrode in an attempt to stimulate a specific area of neural tissue, it has been
shown that the closer the electrode array is placed to the inside turn of the cochlea, the
cleaner the transmission.
With the increase of electrical energy there is an increased perception of loudness.
We are going to display increased amplitude of the electrical pulse as height over the
electrode array...
greater electrical pulse electrode 15 - E
lower electrical pulse electrode 9 - E
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
right ear [X X X X X X X X X X X X X X X X X X X X X X
child facing you
For each implant user the level at which an electrical pulse first elicits sound and
the point at which the level of an electrical pulse becomes uncomfortable will be
different.
For the processor to present information that reflects the complexity of speech, It
needs to know where the user can just hear sound (T-levels or threshold) and where the
sound is loud but comfortable (C-level or comfort level). This is the dynamic range.
Here is a basic concept necessary for understanding stimulation mode - When an electrode
is stimulated the current must also pass through another electrode. The electrode
stimulated is the active electrode and the electrode that the current passes through is
the indifferent electrode. Remember that as more neural tissue is stimulated, there is an
increased perception of loudness. The indifferent electrode passes less electrical
current but it will still stimulate some neural tissue and add to the percept of
loudness.
I think of the stimulated or active electrode as the tent pole and the indifferent
electrode as the tent peg. The tent tip slopes to the peg. The tent tip (the
active electrode) is the perceived point for the signal. The tent also recruits
neural tissue on its way down to the peg. The further away the peg, the more neural
tissue that is recruited. The auditory system still seems to be able to pick out the
tent tip from the slope of the tent. That is even though other points of the cochlea are
receiving stimulation, the system can pick out the greatest point of stimulation as the
message.
With E as the current sent to the active electrode (in this case electrode #5) and the
indifferent electrode with current marked as I, this is how Bipolar (BP) Stimulation looks
E
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
right ear [X X X X X X X X X X X X X X X X X X X X X X
child facing you
The audiologist must define a good dynamic range with growth of loudness and an upper
limit. With the BP mode levels often go very high without recruiting enough neural
tissue to get a sufficient level of loudness. That is why the default stimulation
mode is BP+1. This is where the active electrode skips one electrode and uses
the next as the indifferent. This wider stimulation mode will get an earlier percept
of loudness. BP+2 skips two electrodes, BP+3 skips three, BP+4 skips four, and BP+5 skips
5 electrodes. BP+5 is the widest bipolar stimulation mode. The reason audiologists
may hesitate at using the wider stimulation modes is that it has a cost of electrodes at
the apical end...
BP+5
E
I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
right ear [X X X X X X X X X X X X X X X X X X X X X X
child facing you
Note how electrode 17 doesn't have an electrode to use as it's indifferent. With 22
operating electrodes, the BP mode can make use of 21, the BP+1 can make use of 20, the
BP+2 can make use of 19, BP+3 can make use of 18, BP+4 can make use of 17, and BP+5 can
only make use of 16.
New software will allow audiologists to multiple mode program the electrodes in an attempt
to set levels as wide only on electrodes that require it. As I understand it, as an
example, electrodes 1 through 10 may be set in BP+5 while 11 through 18 could be set in
BP+3, 19 could be set in BP+2, and 20 in BP+1. If these apical electrodes could not
get an adequate percept of loudness, then it is better to use less electrodes with a more
definable dynamic range.
Common ground is different than bipolar mode of stimulation. For all modes of stimulation
there must be an active and an indifferent site of stimulation. Common Ground
stimulation uses all of the electrodes aside from the active electrode as the
indifferent. This mode groups all of the electrodes (aside from that stimulated) as
one. There was a worry in early years that with such a diverse spread of current,
there might be a contamination of information (hard to pick out the peak from the
rest). As it turned out, those who use common ground stimulation seem to do as well
as those programmed in BP modes. There are two main reasons with the Nucleus 22 to
start a child off in Common Ground stimulation - 1. finding problem electrodes 2.
predicting T-levels and C-levels. I'll explain this more in "Initial Mapping
Sessions" below.
An audiologist would not want to start in Common Ground if a child had a partial insertion
of electrodes. Remember that we are using [ as the division between middle ear and
inner ear.
Partial Insertion - Common Ground Stimulation
E
I I I I I I I I I I I I I I I I I I I I I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
right ear X X X X [X X X X X X X X X X X X X X X X X X
child facing you
In this example, as electrode 9 is stimulated in Common Ground, electrodes outside the
cochlea are stimulated as indifferent. This could result in non-auditory
stimulation. These sensations may be described as painful, warm, or you may see the
child's eye twitch involuntarily if the facial nerve is stimulated. Some feel a
tingling in their tongue.
The new nucleus 24 device has 24 electrodes. Two are designed to stay outside the
cochlea. They are to be used as the indifferent electrodes. With this wide
separation between active and indifferent electrodes, T-levels and C-levels can be reached
with much less electrical charge. The real advantage of this has to do with the
slowing down of the processor as it reaches higher stimulation levels. The
stimulation mode with the Nucleus 24 is called Monopolar. Four plate electrodes that
act as one are housed within the case of the receiver/stimulator (that's MP2) and the
other electrode is called a ball electrode (much bigger than the cochlear electrodes). The
impedance of these electrodes is much greater and there is very little chance that
nonauditory sensations will occur with stimulation.
Stimulus Level
Up to this point I have said that increasing the current amplitude is how the processor
gets an increase in perceived loudness. That was a little simplified. The
processor tries to get an increase in loudness using two parameters - current amplitude
and pulse width. In this way the processor can get a range of loudness in steps from
soft to loud. The range of steps it uses goes from 1 to 239.
Pulse width is the amount of time the processor holds the signal in place. Think of a
match being held in place. The same intensity match would feel hotter if held to
your hand for a longer time. Pulse amplitude is the amount of electrical current
that is sent through the electrode. These two parameters determine the intensity of
the signal. The range of steps is designed to economically provide a good range with
little power and constant in terms of speed.
From levels 1 to 65 the processor holds the pulse width (length of stimulation) at 19.2
microseconds. The growth in loudness up to 65 is obtained by increasing the current
intensity (from 20 to 1000 uA). From level 65 to 229 the current intensity is held
constant (at 1000 uA) while the pulse width is widened. The pulse width is then
widened to get an increase in loudness. It grows from 19.2 microseconds to 400
microseconds at stimulus level 229. After that pulse width is held constant again and up
to level 239 there is again an increase in current intensity (from 1000 to 1700 uA).
Here's the problem. High stimulation levels cost time and energy.
High stimulation levels will drain batteries and delay the processor in sending
information at high enough speeds.
The SPEAK strategy is now used with both Nucleus 22 and 24 devices. It analyzes the
incoming information from the microphone and determines where the peaks of information
are. With 20 active electrodes the processor will pick an average of 6 peaks of
information (called maxima) from the sound it samples at one point of time. As an
analogy - Each sample is sent to the electrodes as one frame in a running film. Its
a little different though. To send each frame of information, the selected electrodes have
to be stimulated one at a time in sequence. The auditory system doesn't do well when
two peaks are sent simultaneously. The peaks should be sent, though, quickly one
after another. The nerve reacts and then re-reacts quite quickly.
When a processor has to hold the signal at pulse widths that take a long time it slows the
processing system down while it cycles through each peak of information (each maxima)
within a frame. Once it gets through one frame and another sample is ready some
information is skipped. The running film is missing frames of information.
If the average level of stimulation is over 185 units at C-levels, the processor has to
slow down quite a bit to provide long enough pulse widths. At these levels the system
warns the audiologist that SPEAK cannot work effectively at these levels.
As you saw above, one way to get C-levels with less stimulation is to broaden the
stimulation mode. An audiologist will go from BP+1 to BP+2 (or wider) when the
levels are too high to run SPEAK optimally. Lower stimulation levels mean less drain
on the batteries as well. This is why different maps will result in different
battery life. This is also why broadening the stimulation mode may mean increased battery
life.
This is also why when an audiologist changes the stimulation mode, she will also have to
test and reset all T-levels and C-levels because they will change.
Monopolar stimulation has a different range. With the broader stimulation mode, the
processor can get a range from soft to loud with much lower stimulus levels. The
Nucleus 24 in monopolar mode uses only current intensity (with pulse width held at 25
microseconds) to increase loudness. The pulse width can be changed, but remains constant
while current level is increased. This avoids lengthy pulse width times and reduces
the power drain on the processor. The levels in the Current Level scale are
different than those in the Stimulus Level scale. The audiologists are still getting
used to these new numbers and what a "normal" range will be.
The new processor is faster too because it can "multi-task". It is
sampling while sending.
(The other advantage of the 24 is something it can do called "telemetry". The
internal array can be used to check the functioning of the electrodes, the functioning of
the electrodes in terms of its place in the cochlea, and the neural response to each
electrode by the client - more on that later).
So far I have described stimulation mode and stimulus level. Stimulation mode
describes how the active electrode works with an indifferent electrode when stimulated
(the tent tip to peg analogy). The stimulus level refers to how much stimulus is
presented to get a range of percept from soft sound to loud but comfortable.
To set the processor to send information within a range (the dynamic range) of usable
hearing, the audiologist must find the T-levels and C-levels for each electrode.
There is always a chance that during surgery one or more of the electrodes will be
damaged. In common ground stimulation the audiologist can better isolate a problem
electrode. In Bipolar modes the problem could be with the active or the indifferent
and even with adult users they are hard to isolate due to more subtle differences between
shorted and nonshorted T-levels and C-levels. In common ground the problem electrode will
disappear in the group as indifferent but will show itself strongly through elevated T and
C-levels when it is the active electrode for shorted electrodes and no response will be
elicited for those with open circuits.
Problems with the electrode can be described as either short circuit or open circuit.
A short circuit is where the insulation coating around a number of electrode wires is
damaged. The wires themselves are not broken but are somehow connecting to each
other. Channels affected will show higher threshold levels and no growth in loudness
as stimulation is increased. If a short circuit happens between an active electrode
and one of the nonactive stiffening ring electrodes, then nonauditory sensations could
occur outside of the cochlea through stimulation of the "non"active electrode.
If an intermittent short occurs (the two wires touch to provide the short and at times
move out of contact to resolve the short) the potential is there for providing overly loud
input signals. If the C-levels are set for when the short is present, the processor
will send speech to that level of stimulation. If the short is resolved (if the
wires move out of contact) then the processor will still send the same amount of
stimulation and this will now exceed the level comfortable for a child. An
intermittent problem is suspected when large changes in a C-level for a particular channel
is seen. The audiologist will then eliminate (deactivate) that electrode from the
map (no stimulation will be sent to that electrode except for as indifferent).
An open circuit is where the wire leading to an electrode is broken. In bipolar mode there
would be no response to stimulation when the affected electrode is used as active or
indifferent. In common ground the audiologist can identify the problem electrode
which will not produce a sound sensation when used as active.
For the Nucleus 22 common ground mode is used to set an initial map. For the Nucleus
24 monopolar stimulation is used.
In monopolar stimulation, the affected electrodes, if shorted, will sound normal, but will
result in elevated stimulation levels. The telemetry function of the internal device
can help to identify trouble electrodes. The audiologist can run low stimulation sweeps
through different stimulation modes. These are at low levels and are not heard by
the child. They take a short time to administer (less than 1 minute) and will give
the audiologist information about the functioning of the electrodes. This is done
before creating a map in monopolar mode. Audiologists should conduct this test
before each mapping session. Intermittent shorts may appear or worsen over time.
The audiologist selects Client/Implant Test on the computer screen and the sweep is done
in four modes -
1. CG (common ground) which tests the impedances of all intracochlear electrodes (less
than 700 ohm = short, greater than 20 kohm = open circuit). This test can't test the
extracochlear electrodes.
2. MP1 tests the impedances of R1 (the ball electrode) (if greater than 20 kohm = open
circuit or high impedance)
3. MP2 tests the impedances of R2 (plate electrode)(if greater than 20 kohm = open circuit
or high impedance)
These two tests can test intracochlear impedances for open circuit but can not detect
intra-short circuits.
4. MP1+2 measures the impedance of the MP1+2 stimulation mode. If only one
extra-cochlear electrode is open circuit or high impedance this test will not find it.
Together the above four tests work to show that everything is working.
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Module 4
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Module 5
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Module 6 - Trouble Shooting the Nucleus 22 and the
Nucleus 24
The Daily Check/Trouble Shooting Guides are available in 2 formats. Due to the
limitations of web pages, this guide is best viewed and printed in PDF Format.
PDF Format - Adobe Acrobat Reader required - if you don't have it,
download it free from Adobe
Acrobat.
Web Pages
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Please note that this article is the intellectual property of the
author, Dave Sindrey. It's presence on
this web page is with permission and does not place it in the public domain. Mr. Sindrey
retains all rights to this article. Feel free to use the information but please do not
include it in any published material without first obtaining his permission.
