Analysis of the cochlear microphonic to a low-frequency tone embedded in filtered noise
The top panel illustrates an example of the CM recorded in response to a 733 Hz tone at 80 dB SPL in the presence of filtered noise. The waveform labeled Apex was obtained with the noise filtered with a high-pass cutoff frequency of 594 Hz. Each successive waveform represents the CM in response to increasing the high-pass cutoff frequency of the filtered noise. The bottom panel is the amplitude (left axis) and phase (right axis) of the CM obtained from the waveforms in the top panel. Filter cutoff frequency was converted to cochlear distance using the equation from Müller (1996).
Top panels show the amplitude of the CM as a function of distance from apex for the 80 and 60 dB SPL tones. The bottom panels show the corresponding CM phases relative to the unsuppressed condition.
Cumulative amplitude functions (CAFs), defined as the noise-suppressed response relative to the unsuppressed response, for the 80 and 60 dB SPL conditions from all animals, along with the mean CAFs. The curve with filled symbols is an exponential function with a space constant of 1 mm (panel C). Bottom panels are the phase of the CM normalized by subtracting the individual mean from each waveform for all animals. The corresponding mean phase for 80 and 60 dB SPL are shown in the bottom right panel.
Spiral geometry of the gerbil cochlea obtained from the anatomical measurements and fitted equations described in the Appendix. The arrows indicate a possible path of the electric field from the source to the electrode.
Left and middle panels illustrate the CAFs (dots) from the 80 and 60 dB SPL conditions. The lines represent the estimated model fits showing growth similar to the animal data. The right panel is the solution to the logistic function estimating the percentage of OHCs contributing to the CAF for the 80 dB SPL (dashed line) and 60 dB SPL (solid line) conditions.
The solid thin line is the percentage of OHC present along the length of the cochlea. The dashed line indicates regions wherein OHCs were uncountable due to error/artifacts of the histologic preparation. The CAF from this animal (solid thick line) is abnormal through the cochlear region with missing OHCs, then the curve increases over the region where OHCs are present in the base. The dotted line is the average CAF computed across all of the animals.
Left panel is the derivative of the mean CAF with respect to distance for the 80 dB SPL (solid line) and 60 dB SPL conditions (dashed lines). The middle panel illustrates a two-dimensional view of the cochlear spiral with locations of the peaks for the 80 dB SPL condition from the differentiated CAF indicated by letters A–D. The right panel is the same two-dimensional cochlear view with the location where the phase is negative (light grey line) or positive (dark line) relative to the no-noise condition.
A 4× image of the basal turn of the gerbil cochlea obtained with a confocal microscope. The arrow points to the beginning of the cochlear partition but also serves as the radius of the cochlear partition rotated around angle theta (), located in the center of the modioulus (left panel). Middle panel shows the result of five tracings of the cochlear partition. The coordinates of these tracings were converted to polar coordinates and fit with a cubic spline (right panel).
The top two panels are the values of the x and y coordinates (thin grey lines) as a function of angle theta obtained from the cochlea. The solid dark lines are the fit of Eq. (A3). The value of the z coordinate from the cochlea and the corresponding fit of Eq. (A3) is illustrated in the bottom left panel. Plot of the parametric equation of thebasilar membrane is illustrated in the bottom right panel.
Parameters, coefficient values, standard errors of the estimated coefficients, and confidence intervals for the coefficients obtained from fitting Eq. (1) to the CAF data from the first trial at 80 dB SPL. Values in parentheses were obtained from fitting the equation to the second trial recordings.
Parameters, coefficient estimates, standard errors, and 95% confidence intervals obtained from fitting Eq. (1) to CAF data obtained at 60 dB SPL.
Values of the parameters for obtaining the Cartesian coordinates for the cochlear partition.
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