The researchers have found that an electrically powered amplification mechanism in the cochlea of the ear is critical to the acute hearing of humans and other mammals.

The findings will enable better understanding of how hearing loss can result from malfunction of this amplification machinery due to genetic mutation or overdose of drugs such as aspirin.

Sound entering the cochlea is detected by the vibration of tiny, hair-like cilia that extend from cochlear hair cells. While the cochlea's "inner hair cells" are only passive detectors, the so-called "outer hair cells" amplify the sound signal as it transforms into an electrical signal that travels to the brain's auditory center.

Without such amplification, hearing would be far less sensitive, since sound waves entering the cochlea are severely diminished as they pass through the inner ear fluid.

In the research, the team has sought to establish the mechanism by which outer hair cells produce such amplification.

Specifically, they wanted to distinguish between two amplification theories-called "stereociliary motility" and "somatic motility"-that have resulted from previous studies of the auditory machinery.

The stereociliary theory holds that amplification is produced by intricate vibrations of the bundles of cilia extending from the outer hair cells.