Characterizing non-linearity in the cochlear microphonic using the instantaneous frequency

Effects of low-frequency biasing on spontaneous otoacoustic emissions: frequency modulation

It was previously reported that low-frequency biasing of cochlear structures can suppress and modulate the amplitudes of spontaneous otoacoustic emissions (SOAEs) in humans [Bian, L. and Watts, K. L. (2008). "Effects of low-frequency biasing on spontaneous otoacoustic emissions: Amplitude modulation," J. Acoust. Soc. Am. 123, 887-898]. In addition to amplitude modulation, the bias tone produced an upward shift of the SOAE frequency and a frequency modulation. These frequency effects usually occurred prior to significant modifications of SOAE amplitudes and were dependent on the relative strength of the bias tone and a particular SOAE. The overall SOAE frequency shifts were usually less than 2%. A quasistatic modulation pattern showed that biasing in either positive or negative pressure direction increased SOAE frequency. The instantaneous SOAE frequency revealed a "W-shaped" modulation pattern within one biasing cycle. The SOAE frequency was maximal at the biasing extremes and minimized at the zero crossings of the bias tone. The temporal modulation of SOAE frequency occurred with a short delay. These static and dynamic effects indicate that modifications of the mechanical properties of the cochlear transducer could underlie the frequency shift and modulation. These biasing effects are consistent with the suppression and modulation of SOAE amplitude due to shifting of the cochlear transducer operating point.

Characterizing cochlear mechano-electric transduction with a nonlinear system identification technique: the influence of the middle ear

Previously a third-order polynomial equation characterizing mechano-electric transduction was obtained from a nonlinear system identification procedure applied to an ear canal acoustic signal and cochlear microphonic (CM/AC). In this paper, we examine the influence of the linearity and frequency response of the intervening middle ear on the nonlinearity, frequency response, and coherence of the third-order polynomial model of mechano-electric transduction (MET). Ear canal sound pressure (AC), cochlear microphonics (CM), and stapes velocity (SV) were simultaneously recorded from Mongolian gerbils. Linear and nonlinear transfer and coherence functions relating stapes velocity to the acoustic signal (SV/AC), CM to the acoustic signal (CM/AC), and CM to the stapes velocity (CM/SV) were computed. The results showed that SV/AC was linear while CM/AC and CM/SV were not, indicating that the nonlinearity of CM/AC was not due to nonlinearity of the middle ear. The frequency response of the linear term of CM/AC was similar to that of ST/AC but differed from that of CM/SV while the cubic term of CM/AC was similar to that of CM/SV. This indicates that the frequency dependence of CM/AC was due to both the middle ear and frequency dependence of the inner ear. Finally the fit of the polynomial model of MET without the middle ear (CM/SV) did not improve from the fit including the middle ear (CM/AC). A cochlear model of the CM indicated that the lack of improvement was due to the limitations of a third-order polynomial equation characterizing the hair cell transducer function.

The influence of inner hair cell loss on the instantaneous frequency of the cochlear microphonic

The cochlear microphonic (CM) is produced by a change in standing currents during the motion of the cochlear partition. The motion of the partition and associated hair cell transduction processes are nonlinear and are reflected in the variation of the instantaneous frequency (IF) of the CM. Although the CM is dominated from receptor currents from outer hair cells (OHCs), receptor currents from inner hair cells (IHCs) may contribute to the fluctuation in the IF. In this paper we examine the influence of IHCs on the variation of the IF of the CM. A 75 mg/kg intraperitoneal (i.p.) dose of carboplatin reduced the IHC population by approximately 40%. The reduction in IHCs did not substantially affect the amplitude of the CM. The amplitude of the IF, however, was reduced at high signal levels (90 and 100 dB peak SPL). A phenomenological model of the CM indicated that the contribution of IHC receptor currents to the IF was small and that changes in OHC transducer characteristics may have a greater impact on the IF.

Mathematical decomposition of the round window potential

We present an algorithm called the median transform which can be used to decompose the round window auditory potential into AC and DC components. The first of these is identified with the cochlear microphonic, and the second with the combined summating and compound action potentials. Elsewhere in this volume, the algorithm is employed as an intermediate step in obtaining the instantaneous frequency of the CM. Since the algorithm is easily implemented and operates entirely in the time domain, it may prove useful to clinicians as well as researchers.