Left: The cochlear partition, showing the path taken by the basilar membrane through the interior of the cochlea. A wave is shown traveling through a portion of the basilar membrane, at bottom. Right: The arrangement of the basilar membrane (BM), outer hair cells (OHC), inner hair cells (IHC) and tectorial membrane (TM).

Among biosensors, the ear rates highly as an exquisitely sensitive and sophisticated device, arguably on a par with the eye. The human ear is capable of responding to frequencies from about 20 Hz to 20 kHz with fine pitch discrimination and is sensitive to the slightest whisper. Much regarding how the ear itself actually works, however, remains unknown. NIDCD researchers Richard Chadwick and Daphne Manoussaki are trying to better understand the workings of the cochlea (inner ear). Although their research is not directly related to a NASA project, it's a good example of the marriage of the physical and biological sciences NASA director Daniel Goldin advocates (see story, page 1) and represents a step toward understanding how sophisticated sensing systems function.

Daphne Manoussaki and Richard Chadwick

Chadwick, acting chief of the auditory mechanics section in the Laboratory of Cellular Biology, explains that his section's research aims to "understand the mechanical and electrical properties of hearing." Manoussaki, whose background is in applied mathematics, joined the section last year as a fellow and has focused on producing a computational model of the mechanical properties of the cochlea—a daunting task.

The cochlea is a coiled, fluid-filled structure that receives its input from a tiny bone called the stapes, or "stirrup." Vibrations of the eardrum are transmitted through the ossicles in the middle ear—through the "hammer" and "anvil" to the stirrup, which strikes the membranous "oval window" of the cochlea, sending waves through its fluid interior. Winding through the cochlear interior is the "organ of Corti," consisting of the basilar membrane, atop which sit the outer and inner hair cells, bounded at their apical surfaces by the tectorial membrane. Vibrations of the basilar membrane cause the hair cells to deform against the tectorial membrane, stimulating the neurons that innervate the hair cells.

"There are many basic, interesting things about normal hearing that are puzzling, things we just don't understand," says Chadwick. For example, "The transduction process should be too slow—it should take time to charge up the cell membranes—somehow, there's something there that bypasses that." Another curious phenomenon he points to is that of "otoacoustic emissions"—sounds produced by the ear itself in response to an acoustic stimulus. He notes that such events are not rare, though typically not noticed, and that detection of these sounds is used clinically to assess normal auditory function in infants.

A more obvious feature of the cochlea is its geometry, something that initially caught the eye of Manoussaki. "When I came here, I didn't know much about the cochlea, and the first thing that intrigued me was the shape," she says. She explains that prevailing ideas concerning the cochlea's coiled shape pointed to more efficient use of space, or to a presumed benefit related to the innervation pattern it produced. However, no one had adequately dealt with the effect of coiling on wave propagation within the cochlea; most attempts made by others to model the inner ear simply treated it as a long rectangular tube. But Manoussaki "just couldn't believe" the coiled shape would not contribute critical properties to the cochlea. After all, she observed, water flowing through a coiled hose will force it to uncoil, a result that would not be produced in a straight hose.

To test her idea, Manoussaki produced a coiled model of the cochlea, the complex geometry of which, Chadwick suggests, may account for others' preference to treat the system in simpler terms. Their findings, presented in a poster at the Research Festival, suggest that coiling amplifies traveling waves in the cochlea. Manoussaki is working to achieve a more detailed, realistic physical model of the cochlea to better discern what happens as sound waves move through its interior. Chadwick observes that even cochlear implant technology, which bypasses cochlear mechanics to stimulate auditory nerve branches, would benefit from a finer understanding of normal hearing.

 — Doug Loftus