Effects of electrical biasing on electrically-evoked otoacoustic emissions

Long-term effects of acoustic trauma on electrically evoked otoacoustic emission

Electrically evoked otoacoustic emissions (EEOAEs) are sounds measured in the ear canal when alternating current (AC) stimulation is passed into the cochlea. These sounds are attributed to the motile responses of outer hair cells (OHCs). The EEOAE has characteristic amplitude, phase, and fine structure. Multicomponent analysis of the EEOAE shows short (SDC) and long delay components (LDC) that are thought to originate from OHCs near the AC stimulating site and from OHCs at more remote locations, respectively. We measured the effects of various loud noise exposures on the EEOAE and the cochlear whole-nerve action potential (CAP) in animals chronically implanted with a scala tympani electrode. Noise exposures that produced permanent (PTS) or temporary threshold shifts (TTS) were associated with frequency-specific changes in CAP thresholds, EEOAE fine structure, and reductions in the amplitude of the LDC. A frequent observation in this study was an increase in the overall EEOAE amplitude after the noise exposure. The increase was correlated with increased SDC amplitude. The SDC was present in animals chemically treated with ototoxic drugs and mechanical damage to the cochlea. The SDC was eliminated after disarticulation of the ossicular chain. The presence of EEOAE fine structure in the postexposure response is an indicator of TTS in advance of CAP recovery. The results suggest that the EEOAE might be used to differentiate the mechanisms associated with TTS and PTS.

Cochlear compression: effects of low-frequency biasing on quadratic distortion product otoacoustic emission

Distortion product otoacoustic emissions (DPOAEs) are generated from the nonlinear transduction n cochlear outer hair cells. The transducer function demonstrating a compressive nonlinearity can be estimated from low-frequency modulation of DPOAEs. Experimental results from the gerbils showed that the magnitude of quadratic difference tone (QDT, f2-f1) was either enhanced or suppressed depending on the phase of the low-frequency bias tone. Within one period of the bias tone, QDT magnitudes exhibited two similar modulation patterns, each resembling the absolute value of the second derivative of the transducer function. In the time domain, the center notches of the modulation patterns occurred around the zero crossings of the bias pressure, whereas peaks corresponded to the increase or decrease in bias pressure. Evaluated with respect to the bias pressure, modulated QDT magnitude displayed a double-modulation pattern marked by a separation of the center notches. Loading/unloading of the cochlear transducer or rise/fall in bias pressure shifted the center notch to positive or negative sound pressures, indicating a mechanical hysteresis. These results suggest that QDT arises from the compression that coexists with the active hysteresis in cochlear transduction. Modulation of QDT magnitude reflects the dynamic regulation of cochlear transducer gain and compression.

Sound-evoked efferent effects on cochlear mechanics of the mustached bat

The influence of the crossed medial efferent system on cochlear mechanics of the mustached bat was tested by measuring delayed evoked otoacoustic emissions (DEOAEs), cochlear microphonics, distortion product otoacoustic emissions (DPOAEs) and stimulus frequency otoacoustic emissions. Contralaterally delivered sinusoids, broadband noise and bat echolocation calls were used for acoustic stimulation of the efferent system. With all four measures we found a level-dependent suppression under stimulation with both broadband noise and echolocation calls. In addition, the sharply tuned cochlear resonance of the mustached bat which is involved in processing echolocation signals at 61 kHz shifted upward in frequency by several 100 Hz. Presentation of sinusoids did not have any significant effect. DEOAEs and DPOAEs were in some cases enhanced during contralateral presentation of the bat calls at moderate intensities. The most important function of the efferent system in the mustached bat might be the control of the extraordinarily fine-tuned resonator of this species, which is close to instability as evident from the very pronounced evoked otoacoustic emissions which sometimes convert into spontaneous otoacoustic emissions of high level.

Interaction between adenosine triphosphate and mechanically induced modulation of electrically evoked otoacoustic emissions

It was shown previously that electrically evoked otoacoustic emissions (EEOAEs) can be amplitude modulated by low-frequency bias tones and enhanced by application of adenosine triphosphate (ATP) to scala media. These effects were attributed, respectively, to the mechano-electrical transduction (MET) channels and ATP-gated ion channels on outer hair cell (OHC) stereocilia, two conductance pathways that appear to be functionally independent and additive in their effects on ionic current through the OHC. In the experiments described here, the separate influences of ATP and MET channel bias on EEOAEs did not combine linearly. Modulated EEOAEs increased in amplitude, but lost modulation at the phase and frequency of the bias tone (except at very high sound levels) after application of ATP to scala media, even though spectral components at the modulation sideband frequencies were still present. Some sidebands underwent phase shifts after ATP. In EEOAEs modulated by tones at lower sound levels, substitution of the original phase values restored modulation to the waveform, which then resembled a linear summation of the separate effects of ATP and low-frequency bias. While the physiological meaning of this procedure is not clear, the result raises the possibility that a secondary effect of ATP on one or more nonlinear stages in the transduction process, which may have caused the phase shifts, obscured linear summation at lower sound levels. In addition, "acoustic enhancement" of the EEOAE may have introduced nonlinear interaction at higher levels of the bias tones.

Effects of 4-aminopyridine on electrically evoked cochlear emissions and mechano-transduction in guinea pig outer hair cells

Stimulation of the cochlea with alternating current produces sound in the ear canal. These electrically evoked oto-acoustic emissions (EEOAEs) are attributed to electro-motility of outer hair cells (OHCs). Earlier work suggested EEOAEs were sensitive to the open probability of OHC mechano-electrical transduction (MET) channels. They were attenuated by 4-aminopyridine (4-AP) and amplitude-modulated by low frequency sound, consistent with current gaining access to a motility source via the MET conductance. However, inconsistencies in the behaviour as well as physical considerations argued against this simple interpretation. In this study the behaviour of EEOAEs in the presence of 4-AP in scala media was examined along with OHC transfer functions derived from low frequency cochlear microphonic (CM) waveforms. Both the level and the modulation of the EEOAEs were reduced by 4-AP, but disproportionately more so than the 4-AP-induced loss of CM. In addition, the modulation as well as the level of the EEOAEs recovered more rapidly than the CM. Both these results indicated that 4-AP modified the process of EEOAE generation independently of its effect on the gross receptor current through the MET conductance. Changes in the derived OHC transfer functions, specifically shifts in the estimated operating bias of the MET channels, indicated the effects of 4-AP applied to the endolymphatic surface of OHCs were complex. It is suggested that both direct and indirect consequences of a 4-AP blockade may have contributed. 4-AP was ineffective when applied to scala tympani.

Tinnitus suppression by electrical promontory stimulation (EPS) in patients with sensorineural hearing loss

OBJECTIVE

Almost 10-15% of the population suffer from chronic tinnitus. There are clinical indications that a 'pathological sound' may be induced by any level of the auditory pathways. Theraupeutical difficulties and many hypotheses about tinnitus and places from which they originate might indicate various methods of treatment. Electrostimulation tinnitus suppression was achieved by many authors from 22 (Graham, Hazell) to 87% (Portman). The aim of our study was to define the usefulness of electrostimulation in treatment of persistent noise induced cochlear lesion tinnitus (group I - 43 men) and compare the results with the non noise induced tinnitus group II (68 patients).

METHODS

Otolaryngological and audiological examination of the patients was made before and after electrostimulation, and at 90 days. The stimulation system consisted of a prototype tinnitus suppressor, the active platino-iridium needle electrode and silver surface electrode located on the forehead. Transtympanal electrical stimulation was performed using positive polarity direct current. Parameters of electrical impulse were individually different and depended on tinnitus parameters and patients sensation. The current levels ranged from 20 to 600 microA and frequency ranged from 60 to 10000 Hz. Average time of EPS was 60 s.

RESULTS

The control examination 90 days after stimulation in group I showed subjective improvement in 18 (41.9%) patients, 22 (51.2%) did not notice any change and tinnitus deteriorated in 3 (6.9%) of the patients. In the comparative group II improvement was occurred in 34 (50%) persons including 2 (17.6%) in whom tinnitus was abolished, in 30 (44.1%) tinnitus was unchanged and 4 (0.6%) became worse. In both groups our method did not have a destructive influence on hearing. Electrical stimulation to relieve tinnitus has been used for nearly 200 years, but it is unclear how electrical stimulation works to suppress tinnitus.

CONCLUSION

In our opinion electrical stimulation by using positive DC changes the spontaneous activity of cochlear nerve fibres. According to our results it is suggested that the mechanism of beneficial effects is due to increased microcirculation in part of the auditory pathways. Poorer results in patients with noise induced tinnitus could be explained by greater damage of the cochlea outer hair cells. In our opinion EPS could be a method of treatment for persistent tinnitus in cases which fail to respond to other methods.

Effects of acoustic trauma on acoustic enhancement of electrically evoked otoacoustic emissions

Moderate acoustic trauma results in decreased cochlear sensitivity and frequency selectivity. This decrease is believed to be caused by damage to the cochlear amplifier that is associated with outer hair cells (OHCs) and their nonlinear electromechanical characteristics. A consequence of OHC nonlinearity is the acoustic enhancement effect, in which low-frequency electrically evoked otoacoustic emissions are enhanced by a simultaneous tone. The present study found that acoustic trauma reduced the acoustic enhancement effect and this reduction is correlated with the N1 threshold at the electrode site. This result is consistent with the theory that trauma affects the mechanoelectric transduction process, thus affecting cochlear mechanical nonlinearity. Acoustic trauma also reduced the cochlear microphonic in a way that suggests that the number of functioning tension-gated channels and the stiffness of the gating springs were decreased. In some cases, the electromechanical transduction process was also found to be affected by acoustic trauma.

Evidence for active, nonlinear, negative feedback in the vibration response of the apical region of the in-vivo guinea-pig cochlea

The transverse vibration response of the organ of Corti near the apical end of the guinea-pig cochlea was measured in vivo. For cochleae in good physiological condition, as ascertained with threshold compound action potentials and the endocochlear potential, increasing amounts of attenuation and phase lag were found as the intensity was decreased below 80 dB SPL. These nonlinear phenomena disappeared post mortem. The data suggest that an active, nonlinear damping mechanism exists at low intensities at the apex of the cochlea. The phase nonlinearity, evident at all frequencies except at the best frequency (BF), was limited to a total phase change of 0.25 cycles, implying negative feedback of electromechanical force from the outer hair cells into a compliant organ of Corti. The amplitude nonlinearity was largest above BF, possibly due to interaction with a second vibration mode. The high-frequency flank of the amplitude response curve was shifted to lower frequencies by as much as 0.6 octave (oct) for a 50-dB reduction of sound intensity; the reduction of BF was 0.3 oct, but there was no change of relative bandwidth (Q(10 dB)). Detailed frequency responses measured at 60 dB SPL were consistent with non-dispersive, travelling-wave motion: travel time to the place of BF (400 Hz at 60 dB SPL) was 2.9 ms, Q(10 dB) was 1.0; standing-wave motion occurred above 600 Hz. Based on comparison with neural and mechanical data from the base of the cochlea, amplitudes at the apex appear to be sufficient to yield behavioural thresholds. It is concluded that active negative feedback may be a hallmark of the entire cochlea at low stimulus frequencies and that, in contrast to the base, the apex does not require active amplification.

Enhancement of electrically evoked oto-acoustic emissions associated with low-frequency stimulus bias of the basilar membrane towards scala vestibuli

Electrically evoked oto-acoustic emissions (EEOAEs) are sounds present in the ear canal when ac current is passed into the cochlea. EEOAEs are attributed to the activation of fast electromotile responses in outer hair cells (OHCs). An interesting property of EEOAEs is the phenomenon of "acoustic enhancement," where the emission amplitude is increased by moderate-level sound [D. C. Mountain and A. E. Hubbard, Hear. Res. 42, 195-202 (1989)]. In this report a form of enhancement is described which occurs with displacements of the basilar membrane toward scala vestibuli, during amplitude modulation of the EEOAE waveform by low-frequency tones. This "SV-bias enhancement" possibly consists of two components: (i) a low-level component induced by sound at levels which produce nonlinear growth of the cochlear microphonic and which may be equivalent to the "acoustic enhancement" described previously, and (ii) a high-level component which occurs at sound levels well above those which cause saturation of the cochlear microphonic. The low-level component could be explained by either an increased access of the extrinsically applied current to a membrane-based source of OHC motility, perhaps coupled with a reduction in negative feedback, or an increase in electromotile output during scala vestibuli displacements, but the origin of the high-level component is obscure.

Cochlear electrically evoked emissions modulated by mechanical transduction channels

Cochlear outer hair cells are capable of both mechanical-to-electrical and electrical-to-mechanical transduction. Vibration of their stereocilia by sound is believed to stimulate somatic motility via a receptor potential developed across the basolateral membrane, thereby enhancing the mechanical vibration and increasing the sensitivity and frequency selectivity of the ear. Extrinsic electrical currents, applied at the tops of the cells, also appear to activate motility in vivo, presumably after entering the cell. Earlier experiments suggested such currents might enter through the transduction channels themselves, but an alternative shunt pathway through the membrane capacitance seems more likely on physical grounds. We therefore recorded electrically evoked oto-acoustic emissions while modulating the transduction channels by driving them with low-frequency sound. Recordings of the low-frequency cochlear microphonic provided a measure of the mean electrical conductance through the channels during sound stimulation. Emissions increased during displacement of the basilar membrane toward scala vestibuli, when the channels were biased open, and decreased on the opposite phase, and the modulation of the emission was in direct proportion to the cochlear microphonic. The results are the strongest evidence yet that electrically evoked emissions are generated directly by mechanisms related to cochlear transduction and lead to the surprising conclusion that, for frequencies up to at least 12 kHz, extrinsic electrical currents enter the hair cell predominantly by the resistive pathway through the transduction channels. Alternatively, the results might be consistent with direct modulation of a motility source driven by capacitive currents but whose output depends on the state of the channels.

4-aminopyridine in scala media reversibly alters the cochlear potentials and suppresses electrically evoked oto-acoustic emissions

Iontophoresis of 4-aminopyridine into scala media of the guinea pig cochlea caused elevation of the thresholds of the compound action potential of the auditory nerve, loss of amplitude of the extracellular cochlear microphonic response (CM), increase in the endocochlear potential (EP) and reduction in the amplitude of electrically evoked oto-acoustic emissions (EEOAEs). These changes were reversible over 10-20 min. The reciprocity of the changes in the CM and the EP was consistent with an interruption of both DC and AC currents through outer hair cells (OHCs), probably by blockade of mechano-electrical transduction (MET) channels in OHCs. Reductions in EEOAEs were consistent with the extrinsically applied generating current entering the OHC via the MET channels. Implications for the activation of OHC electromotility in vivo are discussed.

The acoustic two-tone distortions 2f1-f2 and f2-f1 and their possible relation to changes in the operating point of the cochlear amplifier

Acoustic two-tone distortions are generated during non-linear mechanical amplification in the cochlea. Generation of the cubic distortion 2f1-f2 depends on asymmetric components of a non-linear transfer function whereas the difference tone f2-f1 relies on symmetric components. Therefore, a change of the operating point and hence the symmetry of the cochlear amplifier could be strongly reflected in the level of the f2-f1 distortion. To test this hypothesis, low-frequency tones (5 Hz) were used to bias the position of the cochlear partition in the gerbil. Phase-correlated changes of f2-f1 occurred at bias tone levels where there were almost no effects on 2f1-f2. Higher levels of the bias tone induced pronounced changes of both distortions. These results are qualitatively in good agreement with the results of a simulation in which the operating point of a Boltzman function was shifted. This function is similar to those used to describe outer hair cell (OHC) transduction. To influence OHC motility, salicylate was injected. It caused a decrease of the 2f1-f2 level and an increase in the level of f2-f1. Such reciprocal changes of both distortions, again, can be interpreted in terms of a shift of the operating point of the cochlear amplifier along a non-linear transfer characteristic. To directly influence the cochlear amplifier, DC current was injected into the scala media. Large negative currents (> -2 microA) caused a pronounced decrease of 2f1-f2 (> 15 dB) and positive currents had more complex effects with increasing and/or decreasing 2f1-f2 distortion level. The effects were time and primary level dependent. Changes of f2-f1 for DC currents > magnitude of mu 2A were in most cases larger compared to 2f1-f2 and reversed for certain primary levels. The current effects probably result from a combination of changing the endocochlear potential and shifting the operating point along a non-linear transfer function.

Electromotile hearing: evidence from basilar membrane motion and otoacoustic emissions

Electrical stimulation of the cochlea is known to cause auditory sensations in humans and other animals. It also has been shown to produce emissions of sound from the inner ear. In the current study we investigate the relationship between electrically induced motion of the basilar membrane (BM) and the production of otoacoustic emissions. We test the hypothesis that electrical current-induced movements of the outer hair cell (OHC electromotility) result in intracochlear acoustic pressure which causes traveling waves on the BM. Our results demonstrate that the dominant response of the guinea pig inner ear to electric stimulation, at the round window membrane (RW) or across the cochlear duct, is a mechanical response of the organ of Corti. We observed that electrical stimulation of the cochlea produced traveling wave activity on the BM, measured with a laser Doppler velocimeter. The BM motion was accompanied by sound emitted by the cochlea for frequencies up to at least 25 kHz. Furthermore, bipolar rectangular current stimulation produced steady, bipolar displacements of the BM (to 2 nm), indicating functional elongation or contraction of OHCs occurs depending on the polarity of the current pulse. All of the evoked responses were absent after drug treatments eliminated the OHCs. Our data indicate that OHCs undergo electrically evoked displacements capable of producing high-fidelity, high-frequency acoustic energy. The electrically evoked intra-cochlear energy results in conventional traveling waves within the cochlea, as well as emissions of sound from the cochlea. These data provide direct support for a mechanism of cochlear sensitivity and tuning involving high-frequency OHC electromotility. Moreover, the data also indicate that any intra- or extracochlear electric current which affects the electric polarization of OHCs could induce BM traveling waves and cause 'electromotile hearing'. This form of hearing would be one component under the more general definition of the electrophonic effect.