Efferent neural control of cochlear mechanics? Olivocochlear bundle stimulation affects cochlear biomechanical nonlinearity (original) (raw)
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The bottom-up signal pathway, which starts from the outer ear and leads to the brain cortices, gives the classic image of the human sound perception. However, there have been growing evidences in the last six decades for existence of a functional descending network whereby the central auditory system can modulate the early auditory processing, in a top-down manner. The medial olivocochlear efferent fibers project from the superior olivary complex at the brainstem into the inner ear. They are linked to the basal poles of the hair cells by forming synaptic cisterns. This descending network can activate nicotinic cholinergic receptors (nAChR) that increase the membrane conductance of the outer hair cells and thereby modify the magnitude of the active force generated inside the cochlea. The aim of the presented work is to quantitatively investigate how the changes in the biomechanics of the outer hair cells, caused by the efferent activation, manipulate the cochlear responses. This is done by means of a frequency-domain biophysical model of the cochlea [12] where the parameters of the model convey physiological interpretations of the human cochlear structures. The simulations manifest that a doubling of the outer hair cell conductance, due to efferent activation, leads to a frequency-dependent gain reduction along the cochlear duct with its highest effect at frequencies between 1 kHz and 3.5 kHz and a maximum of approximately 10 dB gain reduction at 2 kHz. This amount of the gain inhibition and its frequency dependence reasonably agrees with the experimental data recorded from guinea pig, cat and human cochleae where the medial olivococlear efferents had been elicited by broad-band stimuli. The simulations also indicate that the efferent-induced increase of the outer hair cell conductance increases the best frequency of the cochlear responses, in the basal region. The presented simulations quantitatively confirm that activation of the medial olivocochlear efferents can biomechanically manipulate the cochlear responses, in a top-down manner, by inhibiting the gain of the cochlear amplifier as well as altering the frequency-position map (tuning pattern) of the cochlea.
Some observations on cochlear mechanics
Journal of The Acoustical Society of America, 1978
A set of experiments was conducted using the MSssbauer effect to determine the vibratory characteristics of the basilar membrane, Reissner's membrane, the malleus, incus, and oval window in squirrel monkey.
Functional role of the olivo-cochlear bundle: A motor unit control system in the mammalian cochlea
Hearing Research, 1989
A fiber optic lever is applied to the measurement of the motion of the basilar membrane motion in guinea pigs. In response to intense tones from either ear, the motion includes a substantial summating shift in the mean position in addition to a travelling wave originally described by von B&&y. His stroboscopic technique and most techniques used since have been concentrated upon measuring vibrations of the basilar membrane synchronous with the stimulus and have been insensitive to variations in the baseline position such as a summating component of motion analogous to the extracellular summating potential. In addition to the role of the outer hair cells in providing normal hearing sensitivity, they evidently play a role in regulating the mean position of the basilar membrane. For a fixed frequency, the polarity of the mean position varies systematically with sound level and place and summates with time since onset. Since these cells are the target cells for the olivocochlear bundle, homeostasis in the cochlea would appear to be linked efferent function and involve cochlear mechanics. The negative damping hypothesis asserts that hair cell activity is necessary for low thresholds. The results presented here demonstrate that OHC activity exists independent of neural thresholds. The discussion develops the concept that threshold losses are due to a mismatch of opposing tonic forces which normally maintain the mean position of the basilar membrane. Structure is examined in relation to function and the group of outer hair cells innervated by a single medial efferent neuron is identified as a motor unit. Implications of central control of individual motor units include peripheral involvement in selective attention tasks.
Efferent Insights into Cochlear Mechanics
AIP Conference Proceedings, 2011
Medial olivocochlear efferent (MOCE) effects on tone-evoked basilar membrane (BM) vibrations are analyzed in terms of the vector differences between BM responses with and without MOCE stimulation. The technique is sensitive to rapid (i.e., "fast", τ ≤ 100 ms) changes in both the amplitude and the phase of the BM responses, and reveals MOCE effects over much wider frequency and intensity ranges (for a given BM location) than previously envisioned. The findings confirm and extend previous suggestions that MOCE effects are brought about by at least two different, outerhair-cell based mechanisms. The effects on BM responses to characteristic frequency (CF) tones are consistent with suggestions that the MOCEs affect both the stiffness and the damping of the cochlear partition (damping effects dominating at low levels, and stiffness effects dominating at high levels). The analyses also indicate that MOCE activity can enhance, rather than inhibit, BM responses to low frequency tones (well below CF)-albeit by miniscule amounts. If one assumes that all of the mechanical effects of MOCE activation are brought about via gain changes in a single "cochlear amplifier" [4], these results seem to reveal that this amplifier exhibits a frequencydependent transition from negative feedback (below CF) to positive feedback (near CF). This scenario is reminiscent of the type of amplification proposed by Mountain et al. a long, long time ago [9].
Cochlear Mechanical Distortion Products for Complex Stimuli in the Chinchilla Basal Region
2003
Nonlinearities in cochlear mechanics have been shown to largely explain two-tone distortion products (DPs) that are perceived psychophysically. N-component stimuli (N = 2, 3, 5, and 7) were presented while measuring basilar-membrane vibration in the 6-8 kHz region of the chinchilla cochlea using a laser displacement sensitive interferometer. When N=2, stimuli consist o f two sinewaves with frequencies =fi and f2 (= fi+A), the most prominent distortion product is 2 fi -f2, both perceptually and in basilar membrane mechanics. Two-tone distortion behaved in a manner similar to that reported by several investigators. Multiple DPs were seen both above and below the primary frequencies. The largest DPs occurred for small ratios of the primary frequencies ({7/ fi <1.1), while the largest human perceptual effects are seen for i-rf fi ~1.2 possibly due to species differences. With three equal amplitude sinewaves, the basilar membrane response consists of the three primaries, a DP above and below the primary response and pairs of DP components that decrease in amplitude with increasing separation from the primaries that are spaced by a missing component. Sideband complexes have a width that is equal to the width of the signal bandwidth. Sideband amplitude decreases at a rate that is a function of stimulus bandwidth and level with the largest rates for N=2 where the rates are as large as 250 dB/oct. Sidebands are seen for component frequency spacing less than 300 Hz while for larger frequency spacings sidebands are not present. Passing the stimulus through a compressive nonlinearity of ~0.3 dB/dB and the cochlear filter is sufficient to explain the results.
The Journal of the Acoustical Society of America, 2005
Despite the insights obtained from click responses, the effects of medial-olivocochlear (MOC) efferents on click responses from single-auditory-nerve (AN) fibers have not been reported. We recorded responses of cat single AN fibers to randomized click level series with and without electrical stimulation of MOC efferents. MOC stimulation inhibited (1) the whole response at low sound levels, (2) the decaying part of the response at all sound levels, and (3) the first peak of the response at moderate to high sound levels. The first two effects were expected from previous reports using tones and are consistent with a MOC-induced reduction of cochlear amplification. The inhibition of the AN first peak, which was strongest in the apex and middle of the cochlea, was unexpected because the first peak of the classic basilar-membrane (BM) traveling wave receives little or no amplification. In the cochlear base, the click data were ambiguous, but tone data showed particularly short group delays in the tail-frequency region that is strongly inhibited by MOC efferents. Overall, the data support the hypothesis that there is a motion that bends inner-hair-cell stereocilia and can be inhibited by MOC efferents, a motion that is present through most, or all, of the cochlea and for which there is no counterpart in the classic BM traveling wave.
Journal of The Acoustical Society of America, 1986
Low-frequency stimuli (40-to 1000-Hz tones) have been used to correlate the motion of the 8to 9-kHz place of the chinchilla basilar membrane with the eochlear microphonics recorded at the round window and with the responses of auditory nerve fibers with appropriate characteristic frequency. At the lowest stimulus frequencies, maximum displacement of the basilar membrane toward scala tympani occurs in near synchrony with maximum rarefaction at the eardrum and maximum negativity at the round window; at higher frequencies, the mechanical and microphonic response phases progressively lag rarefaction, reaching -240 deg at 1000 Hz. At most frequencies (40-1000 Hz) near-threshold neural responses, once corrected for neural travel-time and synaptic delays, somewhat lead (by some 40 deg) -maximal scala tympani displacement and maximal negativity of the round window microphonics. The variation of sensitivity with frequency is similar for basilar membrane displacement and microphonic responses: Under open-bulla conditions, sensitivity is constant for frequencies between 100 and I000 Hz; below 100 Hz, sensitivity decreases at rates close to 12 dB/oct toward lower frequencies. Neural response sensitivity matches BM displacement more closely than BM velocity. expected to vibrate in phase. The near-threshold low-frequency response phases segregate into two groups separated by as much as 180ø: Neurons with CFs lower than about 2--4 kHz respond at near-threshold levels during condensation at the eardrum, where, as neurons with higher CFs respond at near-threshold levels during rarefaction. In addition, at suprathreshold levels, there may be more than one preferred response phase per stimulus cycle. One Problem in relating these results to BM motion was that there existed no direct measurements of the BM response in chinchilla. Thus we were forced to rely on an indirect estimate of BM displacement, namely, round window cochlear microphonics (RW CM). In the chinchilla, in contrast with the guinea pig and the kangaroo rat, RW CM responses to low-frequency tones are maximally positive with maximal inward stapes displacement (Dallos, 1970; Ruggero and Rich, 1983). Therefore, on the assumption that positivity of RW CM corresponds to depolarization of outer hair cells and maximum displacement of the BM toward scala vestibuli, we interpreted our neural data as indicating that basal neurons respond at near-threshold levels somewhat leading maximal BM displacement toward scala tympani, while more apical neurons (CFs below 2 kHz) respond approximately in phase with displacement or velocity toward scala vestibuli. This'interpretation, however, remained tentative until CM could be checked against BM mechanical measurements. Recently, we applied the Mfissbauer technique to the study of BM motion at the base of the chinchilla cochlea (Robles etal . In the present work, we use this preparation to explore the relationship between BM motion, RW CM and 1375 . "Cochlear mechanies: Nonlinear behavior in two-tone responses as reflected in cochlear-nerve-fiber responses and in ear-canal sound pressure," J. Acoust. Sac. Am. 67, 1704-1721. Konishi, T., and Nielsen, D. W. (197;5). "Temporal relationship between motion of the basilar membrane and initiation of nerve impulses in the auditory nerve fibers," J. Acoust. SOc. Am. 53, 325. Konishi, T., and Nielsen, D. W. (197•}. "The temporal relationship between basilar membrane motion and nerve impulse initiation in auditory nerve fibers of guinea pigs," Jap. J. Physiol. 28, 291-307. Liberman, M. C. (1978). "Auditory-nerve responses from eats raised in a low-noise chamber," J. Aconst. So=. Am. 63, 442-455. Liberman, M. C., and Kiang, N.Y. S. (1984). "ingle-neuron labeling and chronic cochlear pathology. IV. Stereocilia damage and alterations in rate-and phase-level functions," Hear. Res. 16, 75-90. Lynch, T.
Hearing Research, 2000
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 highfrequency 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.