Super hearing in flies may improve hearing aid technology

The secret to better hearing aid technology may lie in the "super" hearing abilities of parasitic flies, according to researchers at the University of Toronto and Cornell University.

In a study to be published in the April 5th issue of Nature, the researchers report that Ormia ochracea, a parasitic fly measuring less than one centimetre in length, can determine the direction of a sound within a range of two degrees - a feat previously ascribed to only keen-eared owls, cats and humans. "Their sense of hearing is remarkable because their ears are so close together that directional hearing would be impossible in any other animal," says Andrew Mason, a zoology professor in the Division of Life Sciences at the University of Toronto at Scarborough and lead author of the study. The study's other authors are Michael Oshinsky, postdoctoral fellow at Thomas Jefferson University and Ron Hoy, professor in the department of Neurobiology and Behaviour at Cornell University.

Most flies have no sense of hearing at all, explains Mason. But Ormia is a specialist at locating singing crickets, on which they deposit tiny larvae that grow into large maggots by eating the crickets from the inside out. Ormia can detect singing crickets using a unique set of eardrums that are located behind the head as reported in Science by Hoy and his post doctoral fellow, Daniel Robert in 1992.

The flies' unique mechanism of directional hearing has inspired a new generation of hearing aid technology and nanoscale microphones. Miniature listening devices based on the Ormia ear are under development at several industrial and engineering labs in the U.S. This new technology will enable the manufacture of directional hearing aids that are smaller, simpler and cheaper than currently available devices.

But the researchers didn't suspect how well this system could work until they directly measured the flies' ability to track changes in the location of a sound source. "Our results demonstrate that in terms of directional hearing, the flies are as good as any system known," says Mason, who carried out the experiments while working as a postdoctoral fellow at Cornell. "These flies possess one of the most accurate and efficient auditory systems in the animal kingdom which is critical to their survival."

To measure Ormia's ability to pinpoint a sound source, Mason and his colleagues tethered flies on a spherical treadmill made from a ping-pong ball. The ball was floated on an air-stream to reduce friction so that the flies' walking movements were translated into rotations of the ball. When the researchers played artificial cricket sounds from various locations using a movable speaker, the flies walked towards the sound and the computer recorded their walking path. They found that the flies could detect changes in sound-source location smaller than two degrees. "Even humans trying to detect who is speaking in a crowded room can't do better than that," notes Mason. See www.scar.utoronto.ca/~amason/movie/flyball2.mpg for a demonstration.

The flies have only about 100 nerve cells in each ear, compared with thousands in the human ear. Other experiments by Mason, Oshinsky and Hoy used electrodes to demonstrate how the simpler nervous system of the fly can process sound information so accurately. "Beyond the mechanical processes of the ear drum movement, we now know how the nervous system converts movement of the eardrums into information on sound direction," says Mason.

The fly's ears act essentially as an extremely small, simple and sensitive directional microphone. Hearing aids that incorporate directional microphones would help the hearing impaired filter out background noise and tune in to one sound - a task that would enable them to better decipher what one person is saying in a crowd of people. "Hearing aids made with directional microphones would make selective listening much easier," says Mason. But he cautions that more research needs to be completed before this potential can be realized. This study was funded by the U.S. National Institutes of Health.