October 16, 2007

Recent research at MIT found that the tectorial membrane selectively pick ups and transmits energy to different parts of the cochlea via a kind of wave that is different from that commonly associated with hearing. The findings fundamentally change scientific understanding of the inner ear and could help lead to processes that will restore human hearing.

The research described in the advance online issue of the Proceedings of the National Academy of Sciences, was the product of MIT professor Dennis M. Freeman, MIT graduate student Roozbeh Ghaffari and research scientist Alexander J. Aranyosi.

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 more than 50 years 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 team’s discovery may have a direct impact on how cochlear mechanisms are modeled. "In the long run, this could affect the design of hearing aids and cochlear implants," says Ghaffari.

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.

Source: MIT