Some current concepts of cochlear mechanics.

The development of the current concepts is reviewed in historical perspective. Helmholtz's hypothesis of basilar-membrane resonance was partially confirmed and partially defeated by Békésy's experiments on models and postmortem cochlear preparations. He discovered that sound was propagated in the cochlea in the form of traveling waves which reached a flat maximum at a frequency-dependent location. Mathematical theory explained this type of sound propagation as a special case of surface waves. Johnstone and his coworkers discovered that the maximum of cochlear vibration in living animals was much sharper than postmortem, and more recently Khanna and Johnstone independently determined the maximum to be nearly as sharp as the tuning curves of the inner hair cells and the auditory-nerve fibers. These findings, together with the work at the Massachusetts Institute of Technology on alligator lizards, have led to new concepts of cochlear mechanics which include hypothesized micromechanical processes in the organ of Corti. These concepts deal not only with the sharpness of basilar-membrane tuning but also with the details of the basilar-membrane amplitude and phase characteristics, as well as with the hair cell and neural tuning curves and response phases. They suggest that some sharpening of the tuning curves occurs between the basilar membrane and hair-cell responses. Such sharpening has been demonstrated in lizards, but in the mammalian ear, the relation is less clear.