Loud sound-induced changes in cochlear mechanics

Spectral Ripples in Round-Window Cochlear Microphonics: Evidence for Multiple Generation Mechanisms

The cochlear microphonic (CM) results from the vector sum of outer hair cell transduction currents excited by a stimulus. The classical theory of CM generation-that the response measured at the round window is dominated by cellular sources located within the tail region of the basilar membrane (BM) excitation pattern-predicts that CM amplitude and phase vary little with stimulus frequency. Contrary to expectations, CM amplitude and phase-gradient delay measured in response to low-level tones in chinchillas demonstrate a striking, quasiperiodic pattern of spectral ripples, even at frequencies > 5 kHz, where interference with neurophonic potentials is unlikely. The spectral ripples were reduced in the presence of a moderate-level saturating tone at a nearby frequency. When converted to the time domain, only the delayed CM energy was diminished in the presence of the saturator. We hypothesize that the ripples represent an interference pattern produced by CM components with different phase gradients: an early-latency component originating within the tail region of the BM excitation and two delayed components that depend on active cochlear processing near the peak region of the traveling wave. Using time windowing, we show that the early, middle, and late components have delays corresponding to estimated middle-ear transmission, cochlear forward delays, and cochlear round-trip delays, respectively. By extending the classical model of CM generation to include mechanical and electrical irregularities, we propose that middle components are generated through a mechanism of "coherent summation" analogous to the production of reflection-source otoacoustic emissions (OAEs), while the late components arise through a process of internal cochlear reflection related to the generation of stimulus-frequency OAEs. Although early-latency components from the passive tail region typically dominate the round-window CM, at low stimulus levels, substantial contributions from components shaped by active cochlear processing provide a new avenue for improving CM measurements as assays of cochlear health.

Using Cochlear Microphonic Potentials to Localize Peripheral Hearing Loss

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.

Basilar membrane and tectorial membrane stiffness in the CBA/CaJ mouse

The mouse has become an important animal model in understanding cochlear function. Structures, such as the tectorial membrane or hair cells, have been changed by gene manipulation, and the resulting effect on cochlear function has been studied. To contrast those findings, physical properties of the basilar membrane (BM) and tectorial membrane (TM) in mice without gene mutation are of great importance. Using the hemicochlea of CBA/CaJ mice, we have demonstrated that tectorial membrane (TM) and basilar membrane (BM) revealed a stiffness gradient along the cochlea. While a simple spring mass resonator predicts the change in the characteristic frequency of the BM, the spring mass model does not predict the frequency change along the TM. Plateau stiffness values of the TM were 0.6 ± 0.5, 0.2 ± 0.1, and 0.09 ± 0.09 N/m for the basal, middle, and upper turns, respectively. The BM plateau stiffness values were 3.7 ± 2.2, 1.2 ± 1.2, and 0.5 ± 0.5 N/m for the basal, middle, and upper turns, respectively. Estimations of the TM Young's modulus (in kPa) revealed 24.3 ± 25.2 for the basal turns, 5.1 ± 4.5 for the middle turns, and 1.9 ± 1.6 for the apical turns. Young's modulus determined at the BM pectinate zone was 76.8 ± 72, 23.9 ± 30.6, and 9.4 ± 6.2 kPa for the basal, middle, and apical turns, respectively. The reported stiffness values of the CBA/CaJ mouse TM and BM provide basic data for the physical properties of its organ of Corti.

ATP-gated ion channels mediate adaptation to elevated sound levels

The sense of hearing is remarkable for its auditory dynamic range, which spans more than 10(12) in acoustic intensity. The mechanisms that enable the cochlea to transduce high sound levels without damage are of key interest, particularly with regard to the broad impact of industrial, military, and recreational auditory overstimulation on hearing disability. We show that ATP-gated ion channels assembled from P2X2 receptor subunits in the cochlea are necessary for the development of temporary threshold shift (TTS), evident in auditory brainstem response recordings as sound levels rise. In mice null for the P2RX2 gene (encoding the P2X2 receptor subunit), sustained 85-dB noise failed to elicit the TTS that wild-type (WT) mice developed. ATP released from the tissues of the cochlear partition with elevation of sound levels likely activates the broadly distributed P2X2 receptors on epithelial cells lining the endolymphatic compartment. This purinergic signaling is supported by significantly greater noise-induced suppression of distortion product otoacoustic emissions derived from outer hair cell transduction and decreased suprathreshold auditory brainstem response input/output gain in WT mice compared with P2RX2-null mice. At higher sound levels (≥95 dB), additional processes dominated TTS, and P2RX2-null mice were more vulnerable than WT mice to permanent hearing loss due to hair cell synapse disruption. P2RX2-null mice lacked ATP-gated conductance across the cochlear partition, including loss of ATP-gated inward current in hair cells. These data indicate that a significant component of TTS represents P2X2 receptor-dependent purinergic hearing adaptation that underpins the upper physiological range of hearing.

Mapping auditory nerve firing density using high-level compound action potentials and high-pass noise masking

Future implementation of regenerative treatments for sensorineural hearing loss may be hindered by the lack of diagnostic tools that specify the target(s) within the cochlea and auditory nerve for delivery of therapeutic agents. Recent research has indicated that the amplitude of high-level compound action potentials (CAPs) is a good predictor of overall auditory nerve survival, but does not pinpoint the location of neural damage. A location-specific estimate of nerve pathology may be possible by using a masking paradigm and high-level CAPs to map auditory nerve firing density throughout the cochlea. This initial study in gerbil utilized a high-pass masking paradigm to determine normative ranges for CAP-derived neural firing density functions using broadband chirp stimuli and low-frequency tonebursts, and to determine if cochlear outer hair cell (OHC) pathology alters the distribution of neural firing in the cochlea. Neural firing distributions for moderate-intensity (60 dB pSPL) chirps were affected by OHC pathology whereas those derived with high-level (90 dB pSPL) chirps were not. These results suggest that CAP-derived neural firing distributions for high-level chirps may provide an estimate of auditory nerve survival that is independent of OHC pathology.

Bandwidth dependency of cochlear centrifugal pathways in modulating hearing desensitization caused by loud sound

Centrifugal olivocochlear (OC) pathways modulate cochlear hearing desensitization induced by loud sounds, but there is a null point, determined by sound bandwidth, for this effect. In a previous study, using loud sounds from the region of greatest hearing sensitivity in cats, OC pathways did not affect desensitization induced by 2-kHz wide noise, but did to narrower bandwidth (tones) or broader bandwidth (3.5 kHz-wide or 5 kHz-wide noise) trauma from the same cochlear region. The bandwidth null-point effect occurred in three very different conditions in which OC pathways modulated losses to narrower or broader bandwidth traumata, confirming the robustness of this phenomenon, and was also true for sub-component OC pathways: neither crossed nor uncrossed OC pathways individually modulated desensitization to that 2 kHz-wide noise. The medial olivocochlear system (MOCS) that is most likely to have modulated desensitization in that study, varies in its cochlear distribution; in cats, densest innervation is in the region of greatest hearing sensitivity and the decrease away from that region means MOCS effects there may not translate to other regions. This hypothesis was now tested in lower- (around 4 kHz) and higher- (around 18 kHz) frequency cochlear regions. Across this fairly large cochlear swath, no OC modulation of desensitization occurred to 2-kHz-wide bandwidth sounds, but did to broader bandwidth; thus the bandwidth dependency was constant across this swath. However, when OC effects did occur, the pattern of effects of OC sub-components could be idiosyncratic to sound bandwidth and cochlear region even for similar net OC effects.

Vibration-induced hearing loss: mechanical and physiological aspects

HYPOTHESIS

The sensorineural hearing loss (HL) after middle ear surgery has been explained by the noise generated by drilling, without considering the vibration generated by the burr.

BACKGROUND

The role of temporal bone vibration in the etiology of the HL was evaluated.

METHODS

An electromagnetic shaker was used to vibrate the bony external ear canal of guinea pigs at different frequencies ranging from 32 to 1,000 Hz and at intensities ranging from 4.2 to 18.8 m/s for 15 minutes. The hearing threshold was measured with auditory evoked responses. A total of 30 animals were tested.

RESULTS

After vibration, 60% of the guinea pigs developed a threshold shift (TS) exceeding 10 dB at two frequencies, with average TS of 8.8 dB across all frequencies and animals. The exposure to vibration at higher frequencies (range, 500-1,000 Hz) produced stronger TS than did the exposure to lower frequencies (range, 32-250 Hz). The vibration-induced TS showed prominent recovery so that after 7 days, TS was 2.4 dB on average and 27 of 30 animals had recovered. After 14 days, the TS was 1.3 dB. The vibration excitation measurements showed that at lower frequencies, the vibration transmission into the skull was significantly greater than at higher frequencies, at which the transmission was heavily attenuated. There were no acoustic resonances detected in the skull. The frequency of vibration and the hearing frequency in auditory brainstem response were significant determinants in the model explaining the vulnerability of vibration on hearing. Hearing loss primarily occurred at higher frequencies. The HL was mostly reversible, consistent with the results observed after human temporal bone surgery.

CONCLUSION

We conclude that in the guinea pig model, the temporal bone vibration at higher frequencies produced a more severe HL than did the vibration at lower frequencies, although the vibration at higher frequencies caused less efficient transmission from the vibrating probe to the temporal bone. The guinea pig model may be useful in the development of surgical techniques and in the understanding of temporal bone pathology.

Bandwidth determines modulatory effects of centrifugal pathways on cochlear hearing desensitization caused by loud sound

Centrifugal olivocochlear (OC) pathways modulate cochlear hearing losses induced in cats by loud sounds varying in bandwidth from tones to clicks and noise bands, in a variety of conditions. The general effect, always to reduce hearing damage, can be a net effect resulting from complex interactions between OC subcomponents (crossed and uncrossed OC pathways). The interactions between these subcomponents vary with type of loud sound, suggesting that sound bandwidth may be important in determining how OC pathways modulate loud sound-induced hearing loss. This dependency was examined and here it is reported that OC pathways do not alter cochlear hearing losses caused by loud noise with a 2-kHz-wide bandwidth intermediate between the loud sounds of previous studies. Increasing stimulus bandwidth even slightly more, to use a loud 3.5-kHz-wide bandwidth noise as the damaging sound, once again revealed OC modulation of cochlear hearing loss. The fact that OC pathways do not modulate cochlear hearing losses induced by loud 2-kHz-wide noise was demonstrated in three very different test conditions in which OC pathways modulate hearing losses caused by narrower or broader bandwidth sounds. This confirmed that the absence of centrifugal modulation of hearing loss to this particular sound was a robust phenomenon not related to test condition. The absence of overall centrifugal effects was also true at the level of subcomponent pathways; neither crossed nor uncrossed OC pathways individually modulated cochlear hearing losses to the loud 2-kHz-wide noise. This surprising frequency dependency has general implications for centrifugal modulation of cochlear responses.

Contextual modulation of cochlear hearing desensitization depends on the type of loud sound trauma

In ears in which cochlear efferent pathways were cut and with testing done under anaesthetic conditions that preclude middle ear muscle activity (so as to examine the "intrinsic" effects of loud sound on the cochlea without any confounding effect of efferent pathways to the auditory periphery), atraumatic background white noise (WN) increases cochlear hearing loss (temporary threshold shifts, TTSs) induced by a traumatic pure tone but reduces TTSs caused by traumatic 5-kHz wide narrow band (NB) sound. The short-duration moderately intense traumata used in these studies most likely cause TTSs by affecting cochlear mechanics and these WN modulatory effects, exerted directly on the cochlea's intrinsic susceptibility to TTSs, are not predicted by any current description of cochlear mechanics. Here it is demonstrated that background WN reduces trauma-induced TTSs with even a relatively small increase in trauma bandwidth beyond that of a pure tone, discounting the alternative that contextual modulatory effects transition systematically along a continuum as trauma bandwidth increases from a pure tone to a broader bandwidth (albeit 2 kHz-wide NB) trauma. These results have implications for cochlear mechanics as the TTSs due to the traumatic sound of this study are most likely due to changes in cochlear mechanics but are not easily explained by what is currently known of cochlear mechanics.

Organ of Corti potentials and the motion of the basilar membrane

During sound stimulation, receptor potentials are generated within the sensory hair cells of the cochlea. Prevailing theory states that outer hair cells use the potential-sensitive motor protein prestin to convert receptor potentials into fast alterations of cellular length or stiffness that boost hearing sensitivity almost 1000-fold. However, receptor potentials are attenuated by the filter formed by the capacitance and resistance of the membrane of the cell. This attenuation would limit cellular motility at high stimulus frequencies, rendering the above scheme ineffective. Therefore, Dallos and Evans (1995a) proposed that extracellular potential changes within the organ of Corti could drive cellular motor proteins. These extracellular potentials are not filtered by the membrane. To test this theory, both electric potentials inside the organ of Corti and basilar membrane vibration were measured in response to acoustic stimulation. Vibrations were measured at sites very close to those interrogated by the recording electrode using laser interferometry. Close comparison of the measured electrical and mechanical tuning curves and time waveforms and their phase relationships revealed that those extracellular potentials indeed could drive outer hair cell motors. However, to achieve the sharp frequency tuning that characterizes the basilar membrane, additional mechanical processing must occur inside the organ of Corti.

Contextual modulation of olivocochlear pathway effects on loud sound-induced cochlear hearing desensitization

This study shows that the cochlear hearing losses [temporary threshold shifts (TTSs)] induced by traumatic sound and the effect of olivocochlear (OC) pathways to the cochlea on these hearing losses depend on the context of the sound. Background atraumatic white noise (WN) has been shown to 1) exacerbate loud-pure-tone-induced TTSs, and 2) promote the modulation of TTSs by the uncrossed OC (UOC) pathways additional to the action on TTSs, elicited by binaural loud tones themselves, by the crossed OC (COC) pathway. Here the same atraumatic WN reduced TTSs caused by loud narrow band sound. It also reduced TTS modulation by OC pathways. The UOC no longer exerted any effects on TTSs, and COC effects were significantly reduced in two discrete frequency bands: low frequencies within the narrow band ("within-band" frequencies) and high frequencies outside the band ("high-side" frequencies). COC effects were unchanged at high frequencies within the band. Despite these reductions in OC effects, because the WN itself reduced TTSs, the total effect of OC pathways and background WN now produced larger TTS reductions, especially at higher frequencies. Thus the modulatory effects of the OC pathways on TTSs depend on how background WN modulates cochlear state. It is postulated that the WN background and the OC pathways both modulate TTSs by acting on the outer hair cells, in a way that promotes the reduction of TTSs caused by the narrow band sound trauma. This joint promotion of a protective end-effect on TTSs to narrow band sound trauma contrasts against the effects seen with pure tone trauma where the same background WN exacerbated TTSs at high-side frequencies.