Frequency glides in click responses of the basilar membrane and auditory nerve: their scaling behavior and origin in traveling-wave dispersion.

Abstract

Frequency modulations (or glides), reported in impulse responses of both the auditory nerve and the basilar membrane, represent a change over time in the instantaneous frequency of oscillation of the response waveform. Although the near invariance of glides with stimulus intensity indicates that they are not the consequence of nonlinear or active processes in the inner ear, their origin has remained otherwise obscure. This paper combines theory with experimental data to explore the basic phenomenology of glides. When expressed in natural dimensionless form, glides are shown to have a universal form nearly independent of cochlear location for characteristic frequencies (CFs) above approximately 1.5 kHz (the "scaling region"). In the apex of the cochlea, by contrast, glides appear to depend strongly on CF. In the scaling region, instantaneous-frequency trajectories are shown to be approximately equal to the "inverse group delays" of basilar-membrane transfer functions measured at the same locations. The inverse group delay, obtained by functionally inverting the transfer-function group-delay-versus-frequency curve, specifies the frequency component of a broadband stimulus expected to be driving the cochlear partition at the measurement point as a function of time. The approximate empirical equality of the two functions indicates that glides are closely related to cochlear traveling-wave dispersion and suggests that they originate primarily through the time dependence of the effective driving pressure force at the measurement location. Calculations in a one-dimensional cochlear model based on solution to the inverse problem in squirrel monkey [Zweig, J. Acoust. Soc. Am. 89, 1229-1254 (1991)] support this conclusion. In contrast to previous models for glides, which locate their origin in the differential build-up and decay of multiple micromechanical resonances local to each radial cross section of the organ of Corti, the model presented here identifies glides as the global consequence of the dispersive character of wave propagation in the cochlea.