New Hearing Mechanism Discovered

[Guest guest]

Source:

Massachusetts

Institute Of Technology

Date:

October 12, 2007

New Hearing Mechanism Discovered

Science Daily —

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.

MIT Professor Dennis

Freeman, left, graduate student Roozbeh Ghaffari and research scientist

J. Aranyosi have found that the tectorial membrane, a gelatinous structure

inside the cochlea of the ear, is much more important to hearing than

previously thought. (Credit: Photo / Donna Coveney)

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MIT Professor Dennis M. Freeman, working with graduate student Roozbeh

Ghaffari and research scientist 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. "

The research is described in the advance online issue of the Proceedings of

the National Academy of Sciences the week of October 8.

This research was funded by the National Institutes of Health.

Note: This story has been adapted from material provided by

Massachusetts Institute Of Technology.

http://www.sciencedaily.com/releases/2007/10/071011140215.htm