Sounds in the unwrapped inner ear.

Sounds in the unwrapped inner ear

The details of how the inner ear converts sound waves into electrical pulses for transmission to the brain have long puzzled researchers. As one step in studying this process, mathematicians have been working with physiologists to develop computer models that simulate how various parts of the ear work. Such information would aid in the design of hearing aids and speech recognition systems. One recently developed model of the cochlea, a spiral-shaped, fluid-filled organ in the inner ear, illuminates how sound waves of different frequencies excite fibers in the basilar membrane, a platform running down the cochlea.

The difficulty in building such a model, says Mark H. Holmes of the Rensselaer Polytechnic Institute in Troy, N.Y., is in matching the mathematics with experimental data from physiological studies. Holmes' model of the cochlea extends his earlier work on eardrum motions (SN: 5/17/86, p.311). In his animated film, the cochlea is shown cut open and unrolled to make it easier to see what happens to the basilar membrane as the frequency of the incoming sound wave changes (see illustration). "As you change frequencies, the area of stimulation changes," says Holmes.

Now, Holmes and his colleagues are developing mathematical equations that describe how the ear canal leading to the eardrum influences sound waves and how specialized hair cells embedded in the basilar membrane convert mechanical movement into electrical signals. "We feel we have the major components of the system," says Holmes, "although we haven't yet put them together into a whole-ear model."