Abstract
The cochlear microphonic (CM) is created primarily by the receptor currents of outer hair cells (OHCs) and may therefore be useful for identifying cochlear regions with impaired OHCs. However, the CM measured across the frequency range with round-window or ear-canal electrodes lacks place-specificity as it is dominated by cellular sources located most proximal to the recording site (e.g., at the cochlear base). To overcome this limitation, we extract the “residual” CM (rCM), defined as the complex difference between the CM measured with and without an additional tone (saturating tone, ST). If the ST saturates receptor currents near the peak of its excitation pattern, then the rCM should reflect the activity of OHCs in that region. To test this idea, we measured round-window CMs in chinchillas in response to low-level probe tones presented alone or with an ST ranging from 1 to 2.6 times the probe frequency. CMs were measured both before and after inducing a local impairment in cochlear function (a 4-kHz notch-type acoustic trauma). Following the acoustic trauma, little change was observed in the probe-alone CM. In contrast, rCMs were reduced in a frequency-specific manner. When shifts in rCM levels were plotted vs. the ST frequency, they matched well the frequency range of shifts in neural thresholds. These results suggest that rCMs originate near the cochlear place tuned to the ST frequency and thus can be used to assess OHC function in that region. Our interpretation of the data is supported by predictions of a simple phenomenological model of CM generation and two-tone interactions. The model indicates that the sensitivity of rCM to acoustic trauma is governed by changes in cochlear response at the ST tonotopic place rather than at the probe place. The model also suggests that a combination of CM and rCM measurements could be used to assess both the site and etiology of sensory hearing loss in clinical applications.
Highlights
The practical application of anticipated pharmacological and genetic treatments for hearing loss will require diagnostic tests that can differentiate between sites and etiologies of the damage
Because the phase of the local cochlear microphonic (CM) sources excited by PT follows the phase of the BM displacement, the model predicts that rCMST1 tends to be lower in level than rCMST2.1 due to destructive interference between the CM sources located near the probe CF place
We demonstrated that remote (e.g., round window (RW)) measurements of cochlear-microphonic potentials may serve as sensitive indicators of the reduction in outer hair cells (OHCs)-dependent cochlear gain induced by acoustic trauma
Summary
The practical application of anticipated pharmacological and genetic treatments for hearing loss will require diagnostic tests that can differentiate between sites and etiologies of the damage. The CM depends on the position of the recording electrode relative to the CM sources: both electrical attenuation of the cochlear potentials with distance from the source as well as the spiral shape and complex electroanatomy of the cochlea can affect the measured response (e.g., von Békésy, 1951; Chertoff et al, 2012) Together, these factors limit the CM’s place-specificity (i.e., the ability to assess the function of OHCs located near the CF place of the stimulus). The ablation of the apical turn of the cochlea had little effect on the CM measured at the round window (RW) in response to a low-frequency tone that would normally have peaked near the apical end These limitations have hindered the clinical application of the CM, which serves primarily as a gross indicator of OHC function across the cochlea (e.g., Gibson and Sanli, 2007; Radeloff et al, 2012)
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