The Group Delay and Suppression Pattern of the Cochlear Microphonic Potential Recorded at the Round Window

Abstract

BackgroundIt is commonly assumed that the cochlear microphonic potential (CM) recorded from the round window (RW) is generated at the cochlear base. Based on this assumption, the low-frequency RW CM has been measured for evaluating the integrity of mechanoelectrical transduction of outer hair cells at the cochlear base and for studying sound propagation inside the cochlea. However, the group delay and the origin of the low-frequency RW CM have not been demonstrated experimentally.Methodology/Principal FindingsThis study quantified the intra-cochlear group delay of the RW CM by measuring RW CM and vibrations at the stapes and basilar membrane in gerbils. At low sound levels, the RW CM showed a significant group delay and a nonlinear growth at frequencies below 2 kHz. However, at high sound levels or at frequencies above 2 kHz, the RW CM magnitude increased proportionally with sound pressure, and the CM phase in respect to the stapes showed no significant group delay. After the local application of tetrodotoxin the RW CM below 2 kHz became linear and showed a negligible group delay. In contrast to RW CM phase, the BM vibration measured at location ∼2.5 mm from the base showed high sensitivity, sharp tuning, and nonlinearity with a frequency-dependent group delay. At low or intermediate sound levels, low-frequency RW CMs were suppressed by an additional tone near the probe-tone frequency while, at high sound levels, they were partially suppressed only at high frequencies.Conclusions/SignificanceWe conclude that the group delay of the RW CM provides no temporal information on the wave propagation inside the cochlea, and that significant group delay of low-frequency CMs results from the auditory nerve neurophonic potential. Suppression data demonstrate that the generation site of the low-frequency RW CM shifts from apex to base as the probe-tone level increases.

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

“…For low frequency tones, however, the phase locking from auditory nerve fibers (i.e., auditory neurophonic) may contribute to the response (Henry, 1995;Lichtenhan et al, 2012). In gerbil, Henry (1995) and others (He et al, 2012) showed that the auditory neurophonic contributes to the recording at the round window for lowfrequency tones presented at low signal levels. At high signal levels such as 80 dB SPL, a difference between responses with and without TTX is not observable.…”

The potential use of low-frequency tones to locate regions of outer hair cell loss

“…The first is that the literature suggests that a significant proportion of the RW response is generated from auditory nerve potentials in addition to OHCs, especially for low-frequency stimuli (Henry, 1995; Patuzzi et al, 1989; He et al, 2012; Lichtenhan et al, 2014). In order to quantify the proportion of neural and OHC potentials present in the RW response, or cochlear response (CR), Chertoff et al (2014) damaged the auditory nerve with 10 mM of an ototoxic drug, ouabain, and recorded the CR before and after an acute application to the RW niche.…”

An analysis of cochlear response harmonics: Contribution of neural excitation

“…If basilar membrane displacement to a low frequency tone is not everywhere constant and in phase along the cochlear spiral, electrical responses of differing polarity from many cells will sum and consequently smooth the amplitude of the cochlear response. Moreover, slow biasing of cochlear mechanics with a low-frequency tone has largely ignored the contribution of neural excitation that is predominately phase locked to low-frequency tones (Henry, 1995;He et al, 2012;Lichtenhan et al, 2014;Stankovic and Guinan, 1999). That is to say, low frequency tones evoke both hair cell and neural responses.…”