Using Cochlear Microphonic Potentials to Localize Peripheral Hearing Loss

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.

Using electrocochleography to detect sensory and neural damages in a gerbil model

“…To extract more precise frequency specific information about sensory hearing loss from the eCochG, it is better to consider not the responses to the stimulus tones directly, but to use them to evoke a nonlinear cochlear response that is location specific, related to the stimulus CL. This strategy was demonstrated 13 by exploiting the phenomenon of cochlear two-tone suppression to identify and localize cochlear deficiencies from the RW-eCochG. Here we used another nonlinear, cochlear phenomenon, the generation of DPs (Figs.…”

“…However, location-specific information is available in the CM, and it can be extracted when the evaluated responses rely on CL-specific response properties of the cochlea. For example, Charaziak et al 13 demonstrated that CM responses can be used to detect and localize sensory hearing loss by exploiting the local, cochlear phenomenon of two-tone suppression.…”

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

“…The CM is an indication of electrical current flow through OHCs and has shown potential to be developed into a diagnostic measure (Withnell, 2001;Cheatham et al, 2011). Research by Chertoff and colleagues (2012Chertoff and colleagues ( , 2014Chertoff and colleagues ( , 2015, Kamerer et al (2016), and Charaziak et al (2017) has provided evidence in rodent models that CM can be used to assess OHCs in a locationspecific manner.…”

“…This could confound attempts to diagnose the health of OHCs in the apex (Dallos, 1969;Patuzzi et al, 1989). Several studies have developed methods of resolving this issue, using high-pass noise to suppress responses from OHCs outside a targeted region of the cochlear partition (Chertoff et al, 2012b(Chertoff et al, , 2014Kamerer et al, 2016;Charaziak et al, 2017). A third inherent issue with RW recording is the addition of AN activity in the response (Patuzzi et al, 1989;Henry, 1995;He et al, 2012;Lichtenhan et al, 2014).…”