Imaging the Cochlea to Learn More about Hearing

So the cochlea is part of your hearing organ.
It is the inner ear. It is the piece which actually transduces the sound wave in to an
electrical signal which leads to signaling the auditory nerve, which is your hearing.
And so, some pressure wave comes in, it moves portions of the cochlea, and that motion gets
transduced into an electrical signal which makes it to the auditory nerve. So the cochlea
is in all animals, at least all mammals for certain, is buried behind the densest bone
in the body. And so in humans, that is a couple centimeters of bone to get through so it’s
very difficult obviously to get through that bone to do imaging. In the mouse where we’ve
been working, it’s about a couple hundred microns thick of dense bone, so you have to
be able to see through that in order to measure the soft tissues on the inside, which is what
you want to measure to understand the mechanics of the ear. So the challenge is getting past
that bone and measuring the vibrations, and on top of that the vibrations are very small,
in the order of a nanometer in scale – so very small vibrations, very behind a strongly
scattering dense bone. What we are using is a technology based on optical coherence tomography
which is currently used clinically to image in the eye and to do intravascular imaging
for measuring atherosclerosis. What we are doing is applying that to imaging in the ear.
Specifically it can image through the bone of the ear and is able to measure very small
scale vibrations by looking at the interferometric phase. It can have a phase sensitivity or
a motion sensitivity on the order of tens to hundreds of picometers. And so that lets
us make these kind of really fine, very precise measurements of the mouse cochlea. And so
we make these measurements at the same time as we are playing sounds, and by doing that
we can measure out the function of the ear, and then we can look at different pathologies,
different diseases in the mouse, different genetic mutations and try to understand how
those things change the mechanics and morphology of the ear. Okay so first of all we’re generating
some of the very first mechanical or functional measurements across all the soft tissues of
the Organ of Corti, which is part of the inner ear. And so before that there was no detailed
vibrational measurements of the entire organ inside an unopened cochlea. By unopened, that
means the bone is not opened up. So that is the first thing we’re measuring. In addition,
one of the things we have found is that the mammalian cochlea, which is fairly standard
across all mammals, there’s two unique things, or at least two unique things from the mammalian
hearing that is it has gain, in other words there’s some processes in it which produce
movement, and there’s also the narrowing of the frequency bandwidth, which gives you
sort of a narrow band of hearing. And previously those things have been thought to be strongly
related, but what we found by making the measurements inside the Organ of Corti is that the narrowing
of the frequency band and the gain happen in sort of two different areas. And where
the gain is part of the active processes, the narrowing of the frequency band is more
for the passive mechanics, the passive processes. One of the challenges with the ears was that
its buried deep inside bone so it’s hard to get good measurements on it, especially
because it’s a functional thing, so it moves, the movement is part of its function. And
so there’s not a lot of information available for the functioning of even a healthy cochlea,
and so let alone pathology or diseases. And so before we can come up with cures or therapies
for hearing loss, we really need to understand in detail how it works both in a healthy cochlea
as well as a diseased cochlea. That’s just an essential part of trying to figure out
how to develop the right kind of therapies.

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