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M Kössl

Publications and source records attributed to M Kössl.

At least 37 records · Page 2Linked to original sources
PubMed1996

Analysis of non-linear cochlear mechanics in the marsupial Monodelphis domestica: ancestral and modern mammalian features.

The acoustic distortion product 2f1-f2 was measured in the ear canal of the gray short-tailed opossum. Monodelphis domestica, during stimulation with two pure tone stimuli of the frequencies f1 and f2 (f1 < f2). From 2f1-2f growth functions for different primary frequencies from 4 to 85 kHz, a distortion threshold curve was obtained describing the relative threshold of non-linear mechanical processes in the cochlea. Distortion products could be measured over the entire investigated frequency range and the distortion audiogram proved sensitive (thresholds < 20 dB SPL) over a wide range from 8 to 55 kHz with a broad minimum around 17.5 kHz. Thresholds steeply increased for frequencies below 8 kHz which can be regarded as an original mammalian feature (Heffner and Masterton, 1980). To assess the tuning characteristics of the cochlea, suppression tuning curves of 2f1-2f were measured. The shape and tuning sharpness of these curves resembled that of modern eutherian mammals with a shallower low-frequency and a steeper high-frequency flank and Q10dB values between 2.4 and 7.12. The optimum primary ratio f2/f1 (best-ratio) was on average 1.172. For f2 frequencies between 20 and 30 kHz the ration curves displayed multiple peaks in periodic intervals. The same peaks were also evident for higher-order distortions (3f1-2f2, 4f1-3f2). It is discussed to which extent this finding is consistent with the second filter theory of Brown et al. (1992) and Allen and Fahey (1993) and how in Monodelphis domestica this filter could work differently, possibly representing a less evolved stage, than in eutherian mammals.

Acoustic Stimulation↗
PubMed1996

Further studies on the mechanics of the cochlear partition in the mustached bat. I. Ultrastructural observations on the tectorial membrane and its attachments.

From semithin and ultrathin sections of the mustached bat cochlea, baso-apical gradients in ultrastructural composition, shape and attachment site of the tectorial membrane (TM) were determined in relation to gradients in hair cell size and stereocilia size. These provide a data base for estimates of the mechanical properties of the organ of Corti as they relate to specialized aspects of the cochlear frequency map (Kössl and Vater, 1996). As in other mammals, the TM is composed to type A and type B protofibrils. Measurements of the packing density of type A protofibrils reveal gradients in both the radial and longitudinal direction. Distinct variations in packing density of type A protofibrils across the radial extent of the TM allow the definition of more subregions than in other mammals. Throughout the cochlea, packing density is highest in the 'stripe' region located close to the spiral limbus. The centrally located 'core' region of the middle zone contains distinctly fewer type A protofibrils than the laterally located 'mantle' region of the middle zone. The TM in the specialized basal turn (first and second half-turns) features a higher packing density of type A protofibrils in the 'mantle' than the TM in the apical cochlea (upper third to fifth half-turns), and in incorporation of longitudinally directed type A protofibrils in the marginal zone. Among cochlear turns, there are pronounced changes in cross-sectional area of the TM and the extent of its limbal attachment site. Within the densely innervated second half-turn that contains an expanded representation of the 60 kHz constant frequency (CF) component of the echolocation signal, both the cross-sectional area (see also Henson and Henson, 1991) and the attachment site of the TM are enlarged. An extended limbal attachment site is also observed in the densely innervated region of the lower first half-turn that represents the upper harmonics of the call. Within the sparsely innervated region of the upper first half-turn, the limbal attachment site of the TM is significantly diminished. Size of outer hair cells (OHC) ranges between 12 and 13 microns throughout the basal 80% of cochlear length and reaches maximal values of 20 microns in the apex. Size of OHC stereocilia ranges between 0.7 and 0.8 microns throughout the basal 60% of cochlear length and reaches a maximal size of 2.2 microns in the apex. These data corroborate and extend previous notions that morphological specializations of the TM in concert with specializations of the basilar membrane and perilymphatic spaces play an integral role in creating specialized cochlear tuning in the mustached bat.

Animals↗
PubMed1996

Further studies on the mechanics of the cochlear partition in the mustached bat. II. A second cochlear frequency map derived from acoustic distortion products.

It has been proposed that acoustic 2f1-f2 distortions reflect the frequency characteristics of a secondary cochlear filter mechanism (Brown et al., 1992; Allen and Fahey, 1993). This concept was used to construct a second cochlear frequency map that may represent aspects of tectorial membrane (TM) tuning. Within the frequency range of 15-105 kHz, for a given f2 frequency, f1 was varied and the frequency ratio f2/f1 determined that produced maximum levels of the 2f1-f2 distortion (best ratio). The second cochlear frequency map was derived by plotting the distortion frequency that corresponded to the best ratio f2/f1 against the cochlear place of f2 which was obtained from the HRP-frequency map of Pteronotus (Kössl and Vater, 1985b). Minimum best ratios of 1.0005 and hence practically identical characteristic frequencies of the putative tuning of basilar membrane (HRP) and TM (2f1-f2) were found at about 45% distance from the base, a point at which 62 kHz are represented on the BM. This frequency is associated with strong cochlear resonance and large evoked and spontaneous otoacoustic emissions. Between 45% and 20% distance from the base, the basilar membrane (BM) tuning progressively increases to about 70 kHz whereas the calculated TM tuning remains constant at a frequency close to 62 kHz. The range of constant TM tuning coincides with the sparsely innervated cochlear region of Pteronotus where BM thickness is maximal and TM mass and limbal attachment are reduced (Vater and Kössl, 1996). We suggest that here the TM oscillates strongly at 62 kHz and may carry most of the energy of cochlear resonance which is transferred into movement of the organ of Corti at and apical to the 45% location where the BM is tuned to 62 kHz.

Acoustic Stimulation↗
PubMed1996

Auditory enhancement at the absolute threshold of hearing and its relationship to the Zwicker tone.

Auditory enhancement describes an improvement in the detection of a tonal signal in a broad-band masker with a spectral gap at the signal frequency if the signal is delayed in its onset relative to the masker. This auditory enhancement may be based on an increase of the effective signal level instead of a decline in the effective masker level. In order to evaluate whether this signal enhancement also exists at the threshold of hearing, we measured the absolute threshold for pure-tone pulses of different frequencies with and without preceding band-rejected noise. Such noise also causes the sensation of the Zwicker tone-a faint pure tone lasting for a few seconds immediately after the noise presentation. The pitch of this sensation is a complex function of the noise parameters but always lies at a frequency within the rejected band. During the Zwicker tone sensation, auditory sensitivity for tone pulses at frequencies adjacent to the Zwicker tone was improved by up to 13 dB instead of being reduced which might be expected due to the presence of the simultaneously audible Zwicker tone. The failure to influence this threshold shift with low-frequency tones and measurements of the ear's acoustical response indicate that this threshold improvement may be produced through neuronal disinhibition rather than through a release from mechanical suppression in the cochlea.

Acoustic Stimulation↗
PubMed1995

Basilar membrane resonance in the cochlea of the mustached bat.

The mustached bat (Pteronotus parnellii) detects the wing beats of prey insects by resolving small frequency modulations in the return echoes to the constant frequency component of its echolocation call at about 61 kHz. The fine frequency discrimination required for this task is due largely to the mechanical properties of the cochlea, as indicated indirectly by the presence of large otoacoustic emissions at 61 kHz. A laser diode interferometer was used to measure basilar membrane displacements in the basal turn of the cochlea at a location that is 4-6 mm basal to the 61-kHz place on the basilar membrane. The region of the basilar membrane from which the measurements were made was tuned not only to those characteristic of this location (88-98 kHz) but also very sharply tuned to 61 kHz. The 61-kHz tuning was labile and could be modeled as a simple resonance. The resonance is not restricted to the 61-kHz place but involves more basal regions. We also provide direct evidence to show that otoacoustic emissions from the ear propagate along the basilar membrane.

Acoustic Stimulation↗
PubMed1995

The shape of 2f1-f2 suppression tuning curves reflects basilar membrane specializations in the mustached bat, Pteronotus parnellii.

Iso-suppression tuning curves (STCs) of the 2f1-f2 distortion product (dp) were measured over a primary frequency range of 20 to 93 kHz in mustached bats, Pteronotus parnellii parnellii. Primary levels were chosen to produce dp levels between 0 and 7 dB SPL. At frequencies outside the ranges of 60-72 kHz and 90-93 kHz the shapes of the STCs were symmetrical or asymmetrical with a steep high frequency slope. In the vicinity of 61 kHz where a strong stimulus frequency otoacoustic emission (SFOAE) is present, the asymmetry of the STCs was inverted with a very steep low frequency slope (max. -89 dB/kHz) and a shallow high frequency slope. The inverted STCs resemble neuronal tuning curves of the same species with best frequencies at about 61 kHz. Close to 61 kHz the STCs were sharply tuned with Q10dB values up to 177. The STC-thresholds were about 20 dB above the neuronal thresholds. Thickenings of the basilar membrane located just basal to the cochlear place of the SFOAE frequency are probably involved in creating the asymmetric STCs. Cochlear resonance at the SFOAE frequency and an increased longitudinal coupling within the thickened basilar membrane region are thought to contribute to the specialized STC shape. In the range of 90-93 kHz, the STCs are also sharply tuned with inverted asymmetry which is probably not due to an harmonic effect of the specialized cochlear mechanics in the 60 kHz region but may be caused by an independent mechanism.

Acoustic Stimulation↗
PubMed1994

Electromotility of outer hair cells from the cochlea of the echolocating bat, Carollia perspicillata.

Isolated outer hair cells (OHCs) and explants ot the organ of Corti were obtained from the cochlea of the echolocating bat, Carollia perspicillata, whose hearing range extends up to about 100 kHz. The OHCs were about 10-30 microns long and produced resting potentials between -30 to -69 mV. During stimulation with a sinusoidal extracellular voltage field (voltage gradient of 2 mV/microns) cyclic length changes were observed in isolated OHCs. The displacements were most prominent at the level of the cell nucleus and the cuticular plate. In the organ of Corti explants, the extracellular electric field induced a radial movement of the cuticular plate which was observed using video subtraction and photodiode techniques. Maximum displacements of about 0.3-0.8 microns were elicited by stimulus frequencies below 100 Hz. The displacement amplitude decreased towards the noise level of about 10-30 nm for stimulus frequencies between 100-500 Hz, both in apical and basal explants. This compares well with data from the guinea pig, where OHC motility induced by extracellular electrical stimulation exhibits a low pass characteristic with a corner frequency below 1 kHz. The data indicate that fast OHC movements presumably are quite small at ultrasonic frequencies and it remains to be solved how they participate in amplifying and sharpening cochlear responses in vivo.

Animals↗
PubMed1994

Otoacoustic emissions from the cochlea of the 'constant frequency' bats, Pteronotus parnellii and Rhinolophus rouxi.

During stimulation with continuous pure tones, the cochlea of each individual of the mustached bat, Pteronotus parnellii, produces a strong evoked stimulus-frequency otoacoustic emission (SFOAE) at about 62 kHz. The SFOAEs were on average 480 Hz above the dominant constant frequency component of the echolocation call (resting frequency, RF). In two out of nine individuals of Pteronotus the SFOAEs changed into spontaneous otoacoustic emissions of 25-40 dB SPL. In the rufuous horseshoe bat, Rhinolophus rouxi spontaneous emissions were not detected and only in two out of seven animals were there weak SFOAEs about 300 Hz above the RF of 78 kHz. This difference may be due to a stronger damping of underlying resonant processes in Rhinolophus (Henson et al., 1985a). Acoustic distortion products behaved quite similar in both species. The first lower sideband distortion 2f1-f2 was measurable over a wide frequency range between 10 and 100 kHz. The optimum frequency separation delta f of the two primary tones to evoke maximum 2f1-f2 distortion was 0.8 to 5.8 kHz in Pteronotus and 1 to 7 kHz in Rhinolophus for frequencies outside the range of the constant frequency components of the call. This corresponds to ratios f2/f1 of about 1.03 to 1.2. At the frequency of the SFOAE in Pteronotus (480 Hz above the RF) and about 300 Hz above the RF in Rhinolophus the optimum delta f decreased sharply to values of 31-63 Hz in Pteronotus (ratio f2/f1 of 1.0005-1.001), and to 39-590 Hz in Rhinolophus (ratio f2/f1 of 1.0005-1.007). In Pteronotus a second minimum of delta f was found at about 90 kHz (values of 180-620 Hz, ratios f2/f1 of 1.002-1.007). In both bat species, the respective minima of delta f are located at or close to frequencies where neuronal tuning sharpness is exceptionally high. This indicates a mechanical origin of enhanced tuning. After adjusting the frequency of f2 to match the optimum delta fs, 2f1-f2 threshold curves were obtained. The distortion product threshold approximately parallels neuronal data and is in both species characterized by a pronounced insensitivity at the RF followed by a steep threshold minimum at frequencies 0.3-3 kHz above the RF. These features may be involved in reducing the cochlear response to the call such that the bats are able to focus on the Doppler-shifted echos which are slightly higher in frequency and thus within the range of the threshold minimum.

Acoustic Stimulation↗
PubMed1994

Evidence for a mechanical filter in the cochlea of the 'constant frequency' bats, Rhinolophus rouxi and Pteronotus parnellii.

To investigate the function of basilar membrane (BM) thickenings in the cochlea of bats which use constant frequency (CF) echolocation calls, acoustic distortion products were measured while placing the primary stimuli f1 and f2 at frequencies which are represented in the thickened BM regions. In Rhinolophus, for primary stimuli between about 80-100 kHz, pronounced maxima of the level of distortion products (2f1-f2, 3f1-2f2, 4f1-3f2) can be measured if the frequency separation between the two primary tones is chosen so that the resulting distortion frequency matches the dominant CF frequency (resting frequency, RF). The distortion maxima extend from the individual RF down to frequencies which are 2-4 kHz lower. The data indicate that the thickened BM region in the basal halfturn of the cochlea strongly oscillates at the bats' RF and slightly below. The hearing threshold, however, is at a maximum at the RF (see Kössl 1993). Therefore, the RF oscillations are thought to be involved in absorbing the respective frequency such that the more apically located frequency place of the RF is insensitive and a mechanical notch filter is established. In Pteronotus, there are maxima of the level of 2f1-f2 at distortion frequencies around the RF of about 61.5 kHz up to the frequency of a strong stimulus-frequency otoacoustic emission (SFOAE) which is a few hundred Hz higher. Pronounced distortions in the RF range can only be elicited when the stimulus frequencies are between about 62 to 72 kHz. Similar to the situation in Rhinolophus, this frequency band is represented on a stretch of thickened BM.

Acoustic Stimulation↗
PubMed1992

Nonlinear mechanical responses of mouse cochlear hair bundles.

The stiffness of sensory hair bundles of both inner (IHC) and outer (OHC) hair cells was measured with calibrated silica fibres in mouse cochlear cultures to test the hypothesis that the mechanical properties of the hair bundle reflect processes underlying mechanotransduction. For OHCs, the displacement of the hair bundle relaxed with time constants of 6 ms for displacements which open transducer channels and 4 ms for displacements which close the channels. The corresponding values of the time constants for IHCs were 10 ms and 8 ms, respectively. A displacement-dependent change in the stiffness of the hair bundle was not observed when the bundle was displaced orthogonally to the direction of excitation. The stiffness of the hair bundle as a function of nanometre displacements from the resting position was remarkably nonlinear. The stiffness declined to a minimum from the resting stiffness by about 12% for OHCs and 20% for IHCs when the hair bundle was displaced by about 20 nm in the excitatory direction, and it increased by a similar amount when the bundle was displaced by 20 nm in the inhibitory direction. The displacement at which the stiffness reached a minimum was within the most sensitive region of the hair-cell transducer function (receptor potential as a function of hair-bundle displacement), and the displacement at which the stiffness reached a maximum was at the point of saturation of the transducer function in the inhibitory direction. The nonlinear displacement-dependent compliance change is reversibly abolished, and the time constant of relaxation of the bundle for excitatory displacements is reversibly reduced, when mechanotransduction is blocked by the addition of either neomycin sulphate or cobalt chloride to the solution bathing the hair cells. The displacement-dependent compliance change was not apparently reduced when the receptor potential was attenuated through the substitution of sodium in the bathing solution with a less permeant cation, tetraethylammonium. These findings suggest that the nonlinear mechanical properties of the hair bundle are associated with aspects of the hair-cell mechanotransducer process. The mechanical properties of the hair bundle are discussed in relation to the 'gating-spring' hypothesis of hair-cell transduction.

Animals↗
PubMed1992

GAD- and GABA-immunoreactivity in the ascending auditory pathway of horseshoe and mustached bats.

A comparative study of the immunostain to antibodies directed against glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in the ascending auditory pathway was carried out in horseshoe bats (Rhinolophus rouxi) and mustached bats (Pteronotus parnellii). In both species GAD/GABA-positive puncta (presumed axonal boutons) and GAD/GABA-positive cells were found in the cochlear nucleus, the superior olivary complex, the nuclei of the lateral lemniscus the inferior colliculus, and the medial geniculate body. General features of the immunostaining pattern in the auditory pathway agree with observations in other mammals. Quantitative analysis of puncta distribution shows that many auditory centers are characterized by subregional differences in puncta density and distribution. This indicates local differences in putatively inhibitory input related to connectivity and tonotopic organization. The following species characteristic features were found: 1) The dorsal non-laminated portion of the dorsal cochlear nucleus in horseshoe bats lacks the GAD/GABA-immunoreactive cells typical for the ventral laminated portion and the dorsal cochlear nucleus of other species. Clearly, a cytoarchitectonic specialization is accompanied by a loss of putatively GABAergic local inhibitory circuits. 2) The ventral division of the medial geniculate body of the mustached bat lacks GAD/GABA-immunopositive cells. Such cells are present in the horseshoe bat and other mammals. This finding implies functional differences in the organization of the medial geniculate body within the same mammalian order.

Animals↗
PubMed1992

Sensory transduction and frequency selectivity in the basal turn of the guinea-pig cochlea.

Receptor potentials recorded from outer hair cells (OHC) and inner hair cells (IHC) in the basal high-frequency turn were compared. The DC component of the IHC receptor potential is maximized to ensure that IHCS can signal a voltage response to high-frequency tones. The OHC DC component is minimized so that OHCS transduce in the most sensitive region of their operating range. The phase and magnitude of OHC receptor potentials were recorded as an indicator of the magnitude and phase of the energy which is fed back to the basilar membrane to provide the basis for the sharp tuning and fine sensitivity of the cochlea to tones. IHC receptor potentials were recorded to assess the net effect of the feedback on the mechanics of the cochlea. It was concluded that OHCS generate feedback which enhances the IHC responses only at the best frequency. At frequencies below CF, IHC DC responses are elicited only when the OHC AC responses begin to saturate.

Acoustic Stimulation↗
PubMed1992

Voltage responses to tones of outer hair cells in the basal turn of the guinea-pig cochlea: significance for electromotility and desensitization.

Voltage responses were recorded from outer hair cells (OHCS) in the basal coil of the guinea-pig cochlea in response to tones at frequencies above the characteristic frequency (CF) presented together with a 100 Hz tone at 80 dB or 85 dB sound pressure level (SPL). The amplitude and polarity of voltage responses to a 100 Hz, 85 dB SPL tone were altered when presented together with tones at frequencies above CF according to the frequency and level of the high-frequency tone, OHC phasic (ac) (greater than 500 microV) but not tonic (dc) voltage responses were elicited by the high-frequency tone. Thus the responses of OHCS to low-frequency tones can be altered when presented together with a high-frequency tone without an apparent dc change in membrane potential. Recordings were made from an OHC during cochlear desensitization through exposure to an intense tone. The maximum voltage response to high-level low-frequency tones remained unchanged, although the OHC response to high-frequency tones became less sensitive to low-level stimuli and more linear as a function of level. It is suggested that desensitization is associated with a change in the mechanical properties of the cochlea, possibly associated with the OHCS themselves, and not with inactivation of the transducer channels. The amplitude of the OHC ac voltage response was measured at neural threshold, and the consequences of these measurements on hair cell electromotility are considered.

Acoustic Stimulation↗
PubMed1992

The functional role of GABA and glycine in monaural and binaural processing in the inferior colliculus of horseshoe bats.

The functional role of GABA and glycine in monaural and binaural signal analysis was studied in single unit recordings from the central nucleus of the inferior colliculus (IC) of horseshoe bats (Rhinolophus rouxi) employing microiontophoresis of the putative neurotransmitters and their antagonists bicuculline and strychnine. Most neurons were inhibited by GABA (98%; N = 107) and glycine (92%; N = 118). Both neurotransmitters appear involved in several functional contexts, but to different degrees. Bicuculline-induced increases of discharge activity (99% of cells; N = 191) were accompanied by changes of temporal response patterns in 35% of neurons distributed throughout the IC. Strychnine enhanced activity in only 53% of neurons (N = 147); cells exhibiting response pattern changes were rare (9%) and confined to greater recording depths. In individual cells, the effects of both antagonists could markedly differ, suggesting a differential supply by GABAergic and glycinergic networks. Bicuculline changed the shape of the excitatory tuning curve by antagonizing lateral inhibition at neighboring frequencies and/or inhibition at high stimulation levels. Such effects were rarely observed with strychnine. Binaural response properties of single units were influenced either by antagonization of inhibition mediated by ipsilateral stimulation (bicuculline) or by changing the strength of the main excitatory input (bicuculline and strychnine).

Animals↗
PubMed1992

High frequency distortion products from the ears of two bat species, Megaderma lyra and Carollia perspicillata.

In two echolocating bat species, Megaderma lyra and Carollia perspicillata 2f1-f2 distortion products were measured acoustically in the outer ear canal for f1 frequencies between 5 and 95 kHz in an attempt to study nonlinear cochlear processes at high frequencies. Similar to other mammals, the input/output (I/O) functions of 2f1-f2 show non-monotonicities and notches at f1 levels between 40-70 dB SPL. The slope of the initial rise of the 2f1-f2 I/O functions increased with f1 frequency from 0.73 (5 kHz) to 1.41 (95 kHz) in Megaderma and from 0.66 (7.5 kHz) to 1.47 (95 kHz) in Carollia. With increasing f1 frequency the optimum frequency ratio f2/f1 in order to evoke maximum distortion level decreases from 1.18 (5 kHz) to 1.09 (95 kHz) in Megaderma and from 1.21 (7.5 kHz) to 1.11 (95 kHz) in Carollia. This is taken as indication of a general increase of the quality of tuning of the distortion generating mechanism with frequency. The f1 levels that were sufficient to elicit distortion levels of -10 dB SPL were used to construct iso-distortion threshold curves which lay on average 37.2 dB (Megaderma) and 33.9 dB (Carollia) above the neuronal threshold curves [Rübsamen et al., J. Comp. Physiol. A 163 (1988); Sterbing et al., Proc. 18th Göttingen Neurobiol. Conf. Thieme Verlag, Stuttgart (1990)] and roughly paralleled these. Highest distortion levels (40-50 dB SPL with f1 levels of 80 dB SPL) and lowest distortion thresholds were measured in the f1 frequency range of 10-30 kHz (Megaderma) and 15-30 kHz (Carollia).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

The phase and magnitude of hair cell receptor potentials and frequency tuning in the guinea pig cochlea.

Voltage responses to tones were recorded intracellularly from inner (IHC) and outer (OHC) hair cells in the basal turn of the guinea pig cochlea. Tone-evoked voltage responses were also recorded extracellularly from fluid-filled spaces adjacent to the hair cells and from supporting cells. The AC component of the OHC voltage responses to tones at frequencies between 8 and 24 kHz and those recorded extracellularly were remarkably similar with respect to phase as a function of sound level, but the magnitude of the AC response was 2-10 times larger when recorded intracellularly from an OHC. At frequencies more than half an octave below the characteristic frequency (CF), the phase of OHC AC response was independent of level, and the slope of the magnitude/level functions was 1 dB/db. At levels exceeding about 70 dB SPL, the slopes became less steep and depolarizing IHC and OHC DC responses appeared. At frequencies one-half an octave below CF and at frequencies between one-third and one-half an octave above CF, notches were present in the AC/level function between 70-100 dB SPL that were accompanied by a sudden phase lag of -180 degrees. These frequency- and level-dependent characteristics were also present in relatively insensitive preparations and were attributed to a change in the phase of OHC excitation due to level-dependent changes in the relative stiffness of the mechanical components of the cochlear partition. At CF the detection thresholds of the OHC AC response and IHC DC response and slopes of the response/level functions were similar. At sound levels around 60 dB SPL, the AC signal began to phase lead, amounting to approximately 90 degrees at 70 dB SPL. Within the same range of levels, the OHC DC potentials first appeared and the IHC DC response began to saturate. At frequencies just above the CF, the phase of the AC component increased with level to a lead of about 180 degrees. OHC and IHC tuning curves are comparable in the tip region, but they differ in that the low- and high-frequency shoulders of the OHC AC tuning curves are more sensitive by 10-30 dB SPL. On the basis of the frequency- and level-dependent characteristics of the IHC and OHC responses, it is proposed that OHC AC potentials provide a measure of the phase and magnitude of the proposed electromechanical feedback of the cochlear partition that enhance frequency tuning in the cochlea.

Acoustic Stimulation↗