Proving that science is still unraveling the basics of the human body, MIT researchers found hitherto unknown method by which the ear hears and processes sound.
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.
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.
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.
Here is video captured by the research group showing waves traveling along the ear’s tectorial membrane:
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