An analytic approach to identifying the sources of the low-frequency round window cochlear response

Abstract

The cochlear microphonic, traditionally thought of as an indication of electrical current flow through hair cells, in conjunction with suppressing high-pass noise or tones, is a promising method of assessing the health of outer hair cells at specific locations along the cochlear partition. We propose that the electrical potential recorded from the round window in gerbils in response to low-frequency tones, which we call cochlear response (CR), contains significant responses from multiple cellular sources, which may expand its diagnostic purview. In this study, CR is measured in the gerbil and modeled to identify its contributing sources. CR was recorded via an electrode placed in the round window niche of sixteen Mongolian gerbils and elicited with a 45 Hz tone burst embedded in 18 high-pass filtered noise conditions to target responses from increasing regions along the cochlear partition. Possible sources were modeled using previously-published hair cell and auditory nerve response data, and then weighted and combined using linear regression to produce a model response that fits closely to the mean CR waveform. The significant contributing sources identified by the model are outer hair cells, inner hair cells, and the auditory nerve. We conclude that the low-frequency CR contains contributions from several cellular sources.

Figure 1.

A comparison of the 45 Hz toneburst stimulus recorded at the level of the ear canal (top) and the response in the quiet condition (bottom) from individual animals (grey) and mean response (black).

Figure 2.

Transducer functions for outer hair cells (a, c, e) recreated from data recorded from an apical OHC stimulated with an 800 Hz tone (800Hz-OHC_LF; Dallos, 1986), and two basal OHCs stimulated with an 600 Hz tone (600Hz-OHC_HF; Cody & Russell, 1987; Russell et al., 1986). Data points from these publications are represented by filled circles. A first-order Boltzmann equation fit to each transducer function (f_Tr)was solved using the derivative of the recorded 45 Hz, 80 dB SPL stimulus. The resulting simulated response waveforms for each hair cell are plotted to the right of their respective transducer curve (b, d, f). The dashed line is at amplitude = 0, to illuminate asymmetries in the transducer curves and the resulting DC shift in the response waveform.

Figure 3.

Transducer functions (f_Tr) for inner hair cells (a, c, e) recreated from data recorded from an apical IHC stimulated with an 800 Hz tone (800Hz-IHC_LF; Dallos, 1986), an apical IHC stimulated with a 700 Hz tone (700Hz-IHC_LF; Dallos & Cheatham, 1989), and a basal IHC stimulated with a 600 Hz tone (600Hz-IHC_HF; Russell et al., 1986). Data points from these publications are represented by filled circles. A first order Boltzmann equation fit to each transducer curve was solved using the second derivative of the recorded 45 Hz, 80 dB SPL stimulus. The resulting simulated response waveforms for each hair cell are plotted to the right of their respective transducer curve (b, d, f). The dashed line is at amplitude = 0, to illuminate asymmetries in the transducer curves and the resulting DC shift in the response waveform.

Figure 4.

The CR waveform of one animal high-pass filtered at 300 Hz (top). The most prominent APs are located at ± 0.707(peak) for each cycle of the waveform (AP+ & AP−). AP+ was simulated (bottom) to include in the regression model.

Figure 5.

FFTs of the stimulus recorded at the level of the ear canal (top) and CR of one animal (bottom). The second and third harmonics were measured as the magnitude of the peak at 90 and 135 Hz (dashed lines). Triangle indicated 225 Hz.

Figure 6.

Magnitudes of 90 Hz (blue) and 135 Hz (red) peaks in the FFT of the CR to 45 Hz, 80 dB SPL toneburst as a function of high-pass noise cutoff frequency. These ampltidues were compared to the magnitudes of peak responses at 90 Hz to a 90 Hz, 50 dB SPL toneburst (dark grey) and 135 Hz to a 135 Hz, 47 dB SPL (light grey) toneburst.

Figure 7.

The mean CR waveform (black) compared to the model waveform (red) in both quiet (top) and high-pass noise with a cutoff frequency of 3700 Hz (bottom).

Figure 8.

Estimated contributions (thick solid line) and 95% CI (thin solid lines) of the four model covariates as a function of high-pass noise cutoff frequency. Contributions are the product of the initial peak amplitude (V) and the estimated coefficients (β). Dashed lines indicate zero line.

Figure 9.

The estimated 45 Hz, 80 dB SPL CAF using the magnitude and phase of the 45 Hz component from the OHCr and IHCr covariates (solid line) plotted alongside the real data from all animals (n=16).