Scientists discover new hearing mechanism
By David Chandler
Editor: Here's some exciting news about a newly-discovered hearing
mechanism within the cochlea. It helps explain the ear's amazing
sensitivity and could potentially lead to new treatments for hearing loss.
Here's MIT's press release.
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October 2007
Discovery could lead to improved hearing aids
MIT researchers have discovered a hearing mechanism that fundamentally
changes the current understanding of inner ear function. This new
mechanism could help explain the ear's remarkable ability to sense and
discriminate sounds. Its discovery could eventually lead to improved
systems for restoring hearing.
The research is described in the advance online issue of the
Proceedings of the National Academy of Sciences the week of October 8.
MIT Professor Dennis M. Freeman, working with graduate student Roozbeh
Ghaffari and research scientist Alexander J. Aranyosi, found that the
tectorial membrane, a gelatinous structure inside the cochlea of the ear,
is much more important to hearing than previously thought. It can
selectively pick up and transmit energy to different parts of the cochlea
via a kind of wave that is different from that commonly associated with
hearing.
Ghaffari, the lead author of the paper, is in the Harvard-MIT Division
of Health Sciences and Technology, as is Freeman. All three researchers
are in MIT's Research Laboratory of Electronics. Freeman is also in MIT's
Department of Electrical Engineering and Computer Science and the
Massachusetts Eye and Ear Infirmary.
It has been known for over half a century that inside the cochlea sound
waves are translated into up-and-down waves that travel along a structure
called the basilar membrane. But the team has now found that a different
kind of wave, a traveling wave that moves from side to side, can also
carry sound energy. This wave moves along the tectorial membrane, which is
situated directly above the sensory hair cells that transmit sounds to the
brain. This second wave mechanism is poised to play a crucial role in
delivering sound signals to these hair cells.
In short, the ear can mechanically translate sounds into two different
kinds of wave motion at once. These waves can interact to excite the hair
cells and enhance their sensitivity, "which may help explain how we hear
sounds as quiet as whispers," says Aranyosi. The interactions between
these two wave mechanisms may be a key part of how we are able to hear
with such fidelity - for example, knowing when a single instrument in an
orchestra is out of tune.
"We know the ear is enormously sensitive" in its ability to
discriminate between different kinds of sound, Freeman says. "We don't
know the mechanism that lets it do that." The new work has revealed "a
whole new mechanism that nobody had thought of. It's really a very
different way of looking at things."
The tectorial membrane is difficult to study because it is small (the
entire length could fit inside a one-inch piece of human hair), fragile
(it is 97 percent water, with a consistency similar to that of a
jellyfish), and nearly transparent. In addition, sound vibrations cause
nanometer-scale displacements of cochlear structures at audio frequencies.
"We had to develop an entirely new class of measurement tools for the nano-scale
regime," Ghaffari says.
The team learned about the new wave mechanism by suspending an isolated
piece of tectorial membrane between two supports, one fixed and one
moveable. They launched waves at audio frequencies along the membrane and
watched how it responded by using a stroboscopic imaging system developed
in Freeman's lab. That system can measure nanometer-scale displacements at
frequencies up to a million cycles per second.
The team's discovery has implications for how we model cochlear
mechanisms. "In the long run, this could affect the design of hearing aids
and cochlear implants," says Ghaffari. The research also has implications
for inherited forms of hearing loss that affect the tectorial membrane.
Previous measurements of cochlear function in mouse models of these
diseases "are consistent with disruptions of this second wave," Aranyosi
adds.
Because the tectorial membrane is so tiny and so fragile, people "tend
to think of it as something that's wimpy and not important," Freeman says.
"Well, it's not wimpy at all." The new discovery "that it can transport
energy throughout the cochlea is very significant, and it's not something
that's intuitive."
This research was funded by the National Institutes of Health.
