Sunday, October 14, 2007

Better understanding of mammalian hearing

I had the impression that the hows and whys of hearing was pretty much known, at least when it came to mammals, but a new discovery shows that I was wrong about that.

ScienceDaily reports on it.

New Hearing Mechanism Discovered

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 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.

The article explains in some detail how this discovery differentiates from our former knowledge.

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.

It's always fascinating when scientists expand on our knowledge, especially so when it's on our knowledge of how the body functions.

The research has been published in Proceedings of the National Academy of Sciences (with the graduate student as the first-name author), and it's a featured article, so it's available for free.

Longitudinally propagating traveling waves of the mammalian tectorial membrane

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