Analysis of the cochlear microphonic to a low-frequency tone embedded in filtered noise

Abstract

The cochlear microphonic was recorded in response to a 733 Hz tone embedded in noise that was high-pass filtered at 25 different frequencies. The amplitude of the cochlear microphonic increased as the high-pass cutoff frequency of the noise increased. The amplitude growth for a 60 dB SPL tone was steeper and saturated sooner than that of an 80 dB SPL tone. The growth for both signal levels, however, was not entirely cumulative with plateaus occurring at about 4 and 7 mm from the apex. A phenomenological model of the electrical potential in the cochlea that included a hair cell probability function and spiral geometry of the cochlea could account for both the slope of the growth functions and the plateau regions. This suggests that with high-pass-filtered noise, the cochlear microphonic recorded at the round window comes from the electric field generated at the source directed towards the electrode and not down the longitudinal axis of the cochlea.

Figure 1

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).

Figure 2

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.

Figure 3

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.

Figure 4

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.

Figure 5

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.

Figure 6

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.

Figure 7

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.

Figure 8

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 (θ), 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).

Figure 9

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.