In this lecture, I want to talk about the type of stimuli that we would use
to record the evoke potentials that we are looking at.
From the cochlear microphonic through to the summating potential,
dendritic potential and the compound action potential.
Here's the hair cell transfer curve for the outer hair cell.
It is symmetric, it has an operating point of 50%,
which means that the displacement of the hair cell, hair bundles
will create an alternating current which you can see here.
The cochlear microphonic waveform is an alternating current waveform.
You want a single polarity stimulus
because if you have an alternating polarity stimulus
and you average it over time, of course you can average it out to 0.
Your single polarity stimulus should be of low frequency
to avoid contaminating the response with inner hair cell activity.
The inner hair cells are velocity, not displacement-coupled.
So, the lower the frequency of the tone that is presented,
the less likely the inner hair cells will contribute.
The other thing to remember is that the round-window electrode sits
right near the base of the cochlea.
It's consistent with the high frequency region
of the cochlear, or of the hair cells.
The loudness of the sound must be high enough
to generate activity in these basal hair cells.
Not only do you need a low frequency single polarity stimulus,
you need it to be loud enough to generate basal hair cell activity.
Under pathological conditions, outer hair cells can do many things.
For example, the operating point might change due to
a static displacement on the basilar membrane.
Look at the first example, point A, which shows
a normal physiological cochlea.
We have the operating point at 50%.
You can see this lovely, symmetric cochlear microphonic
or current that occurs in the outer hair cells.
On the other hand, if we have a static displacement
of the basilar membrane down to what is called timpani,
this is something that we see with Ménière's Disease for example.
We have an endolymphatic hydrops of the scala media,
which pushes through pressure a displacement down
on the basilar membrane.
This causes a biais of the outer hair cells
since it's displacement-coupled.
In fact, what we see is an asymmetric current
that would saturate on one side before the other side.
You can see now how you might look at the cochlear microphonic
to differently diagnose what's happening with the outer hair cells.
On the other hand, if something were to occur,
that caused your basilar membrane to move up towards scala vestibuli.
No particular pathology would create this.
But just because we can't think of what it might be
doesn't mean it doesn't happen.
You get a saturation on the other side of the cochlear microphonic.
So, you can see how the cochlear microphonic
might enable us to see what
the operating point of the outer hair cells could be.
On the other hand, instead of a change in the operating point,
mechano-electrical transduction channels
might be closed off or inactivated due to things like
loud noise, ototoxic drugs,
or any type of blockage that can occur.
In this case, there would be a reduction in the maximum
of the saturation currents.
You'd still get a symmetric-looking cochlear microphonic
but the maximum amplitude of the cochlear microphonic would be reduced.
That is cochlear microphonic and how to use it with electrocochleography,
how we might differentially diagnose a problem there.
Let's look at the stimulus we need to measure the summating potential,
the compound action potential and the dendritic potential.
In this case, we want to get rid of the cochlear microphonic.
To do that, we need an alternating tone burst.
We also want to capture the activity from the inner hair cells
and from the neurons.
We need a high frequency, alternating tone burst that gives us
the best representation of the activity at the base of the cochlea,
which is where the round-window electrode is situated.
In this case, for humans,
the activity at the round window of the cochlea
is consistent with a characteristic frequency
of about 8 kHz.
We want to present a high frequency,
roughly 8 to 10 kHz tone burst that is alternating in nature.
In that way, when we average, we're averaging out not just the noise
but also a cochlear microphonic.
On the left-hand side, for the human, we can see the summating potential
and a compound action potential.
On the right hand side, it is very similar to what we see
in the animal model, in the guinea pig.
Here, we see the response that we will get with a single polarity
versus an alternating polarity click.
It is a click, it's not a long duration tone burst.
The cochlear microphonic's quality is not incredible
but you can still see it in the bottom trace.
If we look at the top trace, you can see a rarefaction stimulus
and a condensation stimulus that create the response.
Rarefaction plus condensation gets rid of the cochlear microphonic.
So we get rid of any alternating component.
We're left with the summating potential,
followed by the compound action potential.
On the other hand, if we subtract the rarefaction
and the condensation responses,
we find that the cochlear microphonic is maximised or amplified,
We also get a slight compound action potential
due to the difference in magnitude of the response
produced by the rarefaction versus condensation response.
Simply, from this diagram, it shows that if you add the two responses,
you null the cochlear microphonic.
If you substract the two responses you magnify it.
The next point is:
How do you measure the amplitude of the response you obtain?
Here is an example of a high frequency alternating tone burst
that's been used, in fact a click has been used,
with an 80dBnHL sound.
The response shows the summating potential
followed by the action potential.
There are two ways you can look at the magnitude of the response.
The first one is that you measure from baseline
what the magnitude of the summating potential is
and then, from the tip of the summating potential,
from the greatest point of the summating potential,
you can measure what the magnitude of the action potential is.
In this example, the summating potential
and the action potentials are in a negative direction.
But for a high-frequency tone burst, for reasons of polarity
and direction of currents, etc. the summating potential is often
a positive response compared to the action potential.
The left shows a summating potential followed by an action potential.
But on the right, both potentials are measured
in amplitude from the baseline.
Either could be correct but you need to clearly articulate
how you've measured each.
This final example shows the adaptation
that occurs when you are presenting click trains.
This example comes from a paper by ?? Santarelli,
which is talking about auditory neuropathy.
But this slide depicts what happens in control subjects,
in normal hearing, non-pathological subjects.
If you have a look at the click train that is presented – 11 clicks.
They present one, followed by a pause of about 15 ms
and then they present a series of 10 click trains.
You can see, if you look at part C that the summating potential
is not particularly affected by how rapid
the click stimuli are presented.
On the other hand, the compound action potential
is highly adapted by this rapid presentation of stimuli.
So when you are measuring the compound action potential,
bear in mind how much time you need to allow for recovery of the response
before you measure it again.
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