Hot topics in physiological acoustics: The adaptive cochlea

There are several areas of hot topics in physiological acoustics. However, from the point of view of acoustics, the cochlea that both transmits sound and transduces it into electrochemical events that initiate the digitized neural code must be the closest to an acoustician’s heart. The cochlea has been a hot topic since the discovery in the late 60’s that a live cochlea acted in a different way from a passive post-mortem one. Subsequent discoveries revealed that it was able to emit sound and that its mechano-electrical transducers of one type, the outer hair cells, were bidirectional and also acted as electromechanical transducers, providing in this way a feedback loop. Now it has been found that neither the cochlear transfer functions nor its input–output functions are invariant, but that they change with the intensity and duration of sound. These nonlinear dynamics suggest explanations for the nonlinear growth of loudness with sound intensity and for the shift between the frequency of a damaging sound and the frequency of maximum hearing loss. They also show that the location of maximum cochlear excitation cannot be a code for subjective pitch.