Sharp vibration maximum in the cochlea without wave reflection

The recently discovered sharp vibration maximum of the basilar membrane at the best frequency is difficult to reconcile with the reflectionless traveling wave that is implied by the empirical data. The apparent paradox is resolved by representing the cochlea as a transmission line and investigating its characteristic impedance. The appropriate differential equation has been derived previously (Zwislocki, J. (1953); J. Acoust. Soc. Am. 25, 986-989). It is valid for both long and short wave lengths found in the cochlea. The investigation reveals that, in the presence of sufficiently short waves prevailing around the vibration maximum, the characteristic impedance remains practically constant, independent of the rapid variation of the impedance of the basilar membrane. Since wave reflection depends on variation of the characteristic impedance, no wave reflection should be expected. The constancy of the characteristic impedance also enhances the vibration maximum.     

Recent developments in cochlear physiology

Recent findings in cochlear physiology have caused many of our long held ideas about how sound is analyzed by the ear to be reevaluated. This article describes changes which have occurred in three classical ideas of cochlear transduction: (1) There is a gradient of frequency representation along the cochlea with high frequencies being represented at the base and lower frequencies represented progressively toward the apex. It is now known that the specific frequency which is represented at a given location along the cochlea is not invariant but changes systematically during the normal development of hearing. (2) The place code and frequency tuning along the cochlea are due to the conventional traveling wave of von Békésy and basilar membrane mechanics. Experiments in nonmammalian vertebrates which lack a traveling wave have shown that other mechanisms, including the mechanical resonance of hair cell stereocilia, may contribute to tonotopic organization and frequency tuning. It is possible that hair cell stereocilia also contribute to frequency representation and tuning in the mammalian cochlea. (3) The vibration of the basilar membrane to sound is determined by its passive mechanical properties. It is now known that the response of the basilar membrane, and that of the cochlear partition as a whole, is influenced by physiological processes which utilize metabolic energy. The active processes are likely expressed through the motile activity of outer hair cells.     

Human whole-nerve response to clicks of various frequency.

The averaged VIIIth nerve response to third-octave clicks at 500, 2000 and 8000 Hz was recorded from the promontory of 4 normal-hearing young adults. As click frequency is lowered, the N, latency increases in a manner consistent with the changes in velocity of the cochlear traveling wave. This finding suggests that clicks of different spectral content stimulate different regions of the basilar membrane. N amplitude shows a general increase with frequency; this observation appears related to the increased synchrony of neural firing that results from the higher velocity of the traveling wave in the more basal portions of the cochlea.     

Use of frequency modulation combined with amplitude change to elicit frequency-specific whole-nerve response

The averaged whole-nerve response to frequency modulation combined with amplitude change was recorded from the round window of the guinea pig. A downward shift of 20% in the frequency of a 500-Hz, 1-kHz or 2-kHz pure tone with an amplitude increase elicited the responses. The latency for each frequency was investigated at different intensities. The latency is greater for lower-frequency tones and this latency increase agrees well with the traveling wave delay for the intensity range used in this study. A downward shift with an amplitude decrease failed to yield the response. These findings suggest that the response to a downward shift of frequency with an amplitude increase results from new activation due to an apical extension of the envelope of the traveling wave and thus represents the activity in a restricted area of the basilar membrane. Thus, the frequency-modulated tone in downward direction with increased amplitude can be a frequency-specific stimulus.     

Backward Propagation of Otoacoustic Emissions

IntroductionsSounds impinging the eardrum are transmitted viamiddle ear ossicles to the oval window. Stapes vibra-tion creates pressure difference between the scala tym-pani and the scala vestibuli. This pressure differencecauses a movement of cochlear partition and adjacentcochlear fluids. Basilar membrane vibrations result indeflection of hair cell stereocilia, which gate ion chan-nels on their tips. This mechanical-to-electrical trans-duction process converts mechanical vibrations intoelec…     

Effects of lidocaine on basilar membrane vibration in the guinea pig.

The effects of lidocaine on basilar membrane (BM) vibration and compound action potential (CAP) were studied in guinea pigs in order to elucidate the site of lidocaine action in the cochlea. BM vibration was measured with a laser Doppler vibrometer through an opening made in the lateral bony wall of the scala tympani at the basal turn. Ten min after local administration of lidocaine (250 microg) into the scala tympani, the velocity of BM vibration and the CAP amplitude decreased significantly at around the characteristic frequency of the stimulus sound (p < 0.05). The maximum decreases were 4 dB in the velocity of the BM vibration and 40 dB in the CAP amplitude. In contrast, such changes were not observed after i.v. injection of lidocaine (1.5 mg/kg). These results suggest that when lidocaine is administered locally in the cochlea it acts not only on the cochlear nerve but also on the outer hair cells.     

Effects of Lidocaine on Basilar Membrane Vibration in the Guinea Pig

The effects of lidocaine on basilar membrane (BM) vibration and compound action potential (CAP) were studied in guinea pigs in order to elucidate the site of lidocaine action in the cochlea. BM vibration was measured with a laser Doppler vibrometer through an opening made in the lateral bony wall of the scala tympani at the basal turn. Ten min after local administration of lidocaine (250 &#119 g) into the scala tympani, the velocity of BM vibration and the CAP amplitude decreased significantly at around the characteristic frequency of the stimulus sound ( p <0.05). The maximum decreases were 4 dB in the velocity of the BM vibration and 40 dB in the CAP amplitude. In contrast, such changes were not observed after i.v. injection of lidocaine (1.5 mg kg). These results suggest that when lidocaine is administered locally in the cochlea it acts not only on the cochlear nerve but also on the outer hair cells.     

Amplification in the apical turn of the cochlea with negative feedback

The apical turn of the anesthetized guinea pig cochlea was opened to examine the basilar membrane optically through the intact Reissner's membrane. Vibrations of the outer Hensen's cell and the basilar membrane (BM) adjacent to and about 130 microm below the level of the Hensen's cell were measured. Outer Hensen's cell vibration at the characteristic frequency was up to 900 times higher compared to the BM amplitude. After sacrifice BM vibration increased while Hensen's cell vibration decreased. The magnitude and sequence of change after sacrifice can best be explained by the presence of negative feedback between reticular lamina and BM. In other experiments using ototoxic drugs that damage outer hair cells, similar changes in Hensen's cell and BM vibration were observed. These results show that the apical turn behavior is different from that observed by other investigators in the basal turn. The potential benefits of the negative feedback are discussed. The presence of negative feedback would explain the linearity at the fundamental frequency observed in the apical turn of cochlea.     

A comparison of OAEs arising from different generation mechanisms in guinea pig

Otoacoustic emissions provide unambiguous evidence that the cochlea supports energy propagation both towards, and away from, the stapes. The standard wave model for energy transport and cochlear mechanical amplification provides for compressional and inertial waves to transport this energy, the compressional wave through the fluids and the inertial wave along the basilar membrane via fluid coupling. It is generally accepted that energy propagation away from the stapes is dominated by a traveling wave mechanism along the basilar membrane. The mechanism by which energy is predominantly transported back to the stapes remains controversial. Here, we compared signal onset delay measurements and rise/steady-state/fall times for SFOAEs and 2f1-f2 OAEs (f2/f1=1.2) obtained using a pulsed-tone paradigm in guinea pig. Comparison of 2f1-f2 OAE signal onset delay for the OAE arising from the f2 region with SFOAE signal onset delay (matched to the f2 stimulus frequency) based on signal onset occurring at 10% of the peak signal amplitude was suggestive of a bi-directional traveling wave mechanism. However, significant variability in signal onset delay and signal rise, steady-state duration, and fall times for both the 2f1-f2 OAE and SFOAE was found, qualifying this interpretation. Such variability requires explanation, awaiting further studies.     

Two-tone distortion at different longitudinal locations on the basilar membrane

When listening to two tones at frequency f1 and f2 (f2>f1), one can hear pitches not only at f1 and f2 but also at distortion frequencies f2-f1, (n+1)f1-nf2, and (n+1)f2-nf1 (n=1,2,3...). Such two-tone distortion products (DPs) also can be measured in the ear canal using a sensitive microphone. These ear-generated sounds are called otoacoustic emissions (OAEs). In spite of the common applications of OAEs, the mechanisms by which these emissions travel out of the cochlea remain unclear. In a recent study, the basilar membrane (BM) vibration at 2f1-f2 was measured as a function of the longitudinal location, using a scanning laser interferometer. The data indicated a forward traveling wave and no measurable backward wave. However, this study had a relatively high noise floor and high stimulus intensity. In the current study, the noise floor of the BM measurement was significantly decreased by using reflective beads on the BM, and the vibration was measured at relatively low intensities at more than one longitudinal location. The results show that the DP phase at a basal location leads the phase at an apical location. The data indicate that the emission travels along the BM from base to apex as a forward traveling wave, and no backward traveling wave was detected under the current experimental conditions.     

Detailed f 1, f 2 Area Study of Distortion Product Otoacoustic Emissions in the Frog

Distortion product otoacoustic emissions (DPOAEs) are weak sounds emitted from the ear when it is stimulated with two tones. They are a manifestation of the nonlinear mechanics of the inner ear. As such, they provide a noninvasive tool for the study of the inner ear mechanics involved in the transduction of sound into nerve fiber activity. Based on the DPOAE phase behavior as a function of frequency, it is currently believed that mammalian DPOAEs are the combination of two components, each generated by a different mechanism located at a different location in the cochlea. In frogs, instead of a cochlea, two separate hearing papillae are present. Of these, the basilar papilla (BP) is a relatively simple structure that essentially functions as a single auditory filter. A two-mechanism model of DPOAE generation is not expected to apply to the BP. In contrast, the other hearing organ, the amphibian papilla (AP), exhibits a tonotopic organization. In the past it has been suggested that this papilla supports a traveling wave in its tectorial membrane. Therefore, a two-mechanism model of DPOAE generation may be applicable for DPOAEs from the AP. In the present study we report on the amplitude and phase of DPOAEs in the frog ear in a detailed f₁, f₂ area study. The result is markedly different from that in the mammalian cochlea. It indicates that DPOAEs generated by neither papilla agree with the two-mechanism traveling wave model. This confirms our expectation for the BP and does not support the hypothesized presence of a mechanical traveling wave in the AP.     

Basilar membrane tuning properties in the specialised cochlea of the CF-bat, Rhinolophus ferrumequinum

The greater horseshoe bat has greatly expanded frequency mapping, and morphological specialisations, in the first half turn of its cochlea and a sudden transition to normal mapping. Amplitude and phase of vibration have been measured on various structures in the expanded and normal regions and have not revealed any sharply tuned responses. Amplitudes are much lower than those found in other species and frequently show a deep notch in the 77–84 kHz region. The high-frequency cut-off frequencies are tonotopically organised but deviate from the Bruns map, so that hair-cell tuning appears to occur at a frequency at which basilar membrane vibration is small. In the basal region, phase differences were frequently found between the inner and outer parts of the basilar membrane. These appear to be due to interaction between two components of motion and are probably not indicative of a further filtering mechanism. There is no evidence for reflection of the travelling wave at the transition.     

Effects of Low-Frequency Biasing on Otoacoustic and Neural Measures Suggest that Stimulus-Frequency Otoacoustic Emissions Originate Near the Peak Region of the Traveling Wave

Stimulus-frequency otoacoustic emissions (SFOAEs) have been used to study a variety of topics in cochlear mechanics, although a current topic of debate is where in the cochlea these emissions are generated. One hypothesis is that SFOAE generation is predominately near the peak region of the traveling wave. An opposing hypothesis is that SFOAE generation near the peak region is deemphasized compared to generation in the tail region of the traveling wave. A comparison was made between the effect of low-frequency biasing on both SFOAEs and a physiologic measure that arises from the peak region of the traveling wave—the compound action potential (CAP). SFOAE biasing was measured as the amplitude of spectral sidebands from varying bias tone levels. CAP biasing was measured as the suppression of CAP amplitude from varying bias tone levels. Measures of biasing effects were made throughout the cochlea. Results from cats show that the level of bias tone needed for maximum SFOAE sidebands and for 50% CAP reduction increased as probe frequency increased. Results from guinea pigs show an irregular bias effect as a function of probe frequency. In both species, there was a strong and positive relationship between the bias level needed for maximum SFOAE sidebands and for 50% CAP suppression. This relationship is consistent with the hypothesis that the majority of SFOAE is generated near the peak region of the traveling wave.     

The sharpening of cochlear frequency selectivity in the normal and abnormal cochlea.

In the normal (anaesthetized) animal cochlea, the frequency threshold curves for single primary fibres are up to an order of magnitude sharper than the analogous function derived from various reported measurements of the basilar membrane amplitude of vibration. This enhanced neural frequency selectivity is found in the same species and under conditions similar to those in which the mechanical measurements are taken. The sharpening process (at least near threshold) appears to be linear and is not dependent upon lateral inhibitory mechanisms. The variability of the neural frequency selectivity and its vulnerability to metabolic, chemical and pathological influences suggests the hypothesis that the sharpening is due to some form of "second filter" subsequent to the relatively broadly tuned basilar membrane. All fibres recorded from in the cochlear nerve in the normal cochlea show this enhanced frequency selectivity; in contrast, in pathological cochleas, all fibres, or a substantial proportion, have high-threshold, broadly tuned characteristics, approximating to those of the basilar membrane.The frequency selectivity of normal cochlear fibres is adequate to account for the analogous psychophysical measures of hearing. It is proposed that loss of this normal frequency selectivity occurs in deafness of cochlear origin, accounting for widening of the critical band. A new hypothesis for recruitment is proposed on this basis.     

Implications from cochlear implant insertion for cochlear mechanics

It is usually thought that the displacements of the two inner ear windows induced by sound stimuli lead to pressure differences across the basilar membrane and to a passive mechanical traveling wave progressing along the membrane. However, opening a hole in the sealed inner ear wall in experimental animals is surprisingly not accompanied by auditory threshold elevations. It has also been shown that even in patients undergoing cochlear implantation, elevation of threshold to low-frequency acoustic stimulation is often not seen accompanying the making of a hole in the wall of the cochlea for insertion of the implant. Such threshold elevations would be expected to result from opening the cochlea, reducing cochlear impedance, altering hydrodynamics. These considerations can be taken as additional evidence that it may not be the passive basilar membrane traveling wave which elicits hearing at low sound intensities, but rather factors connected with cochlear fluid pressures and fluid mechanics.     

Travelling wave motion along the pigeon basilar membrane

The basilar membrane (BM) motion in the pigeon was measured using the M?ssbauer technique. Tonotopic frequency mapping and travelling wave motion were observed over the basal 35% of the BM. The sensitivity and sharpness of the BM tuning depended on the physiological condition of the cochlea. The observed amplitude responses did not match the frequency threshold tuning curves of single primary auditory fibers.     

耳蜗中阶记录总和电位（SP）频率选择性的实验观察

用不同频率的短纯音（０．５、１、２、４、８、１０ｋＨｚ）诱发，从豚鼠不同耳蜗圈的中阶记录总和电位（ＳＰ），同时记录蜗内直流电位（ＥＰ），观察其ＳＰ幅度变化，结果提示，在基底圈记录时，ＳＰ的最大幅度出现在８ｋＨｚ，在第二回、第三回记录时，ＳＰ的最大振幅分别出现在４ｋＨｚ、２ｋＨｚ，似说明ＳＰ变化在耳蜗内遵循Ｂｅｋｅｓｙ行波原理进行，ＳＰ具有较好的频率选择性     

New clicklike stimuli for hearing testing

The click stimulus generally used for newborn hearing screening generates a traveling wave along the basilar membrane, which excites each of the frequency bands in the cochlea, one after another. Due to the lack in synchronization of the excitations, the summated response amplitude is low. A repetitive click-like stimulus can be set up in the frequency domain by adding a high number of cosines, the frequency intervals of which comply with the desired stimulus repetition rate. Straight-forward compensation of the cochlear traveling wave delay is possible with a stimulus of this type. As a result, better synchronization of the neural excitation can be obtained so that higher response amplitudes can be expected. The additional introduction of a frequency offset enables the use of a q-sample test for response detection. The results of investigations carried out on a large group of normal-hearing test subjects have confirmed the enhanced efficiency of this stimulus design. The new stimuli lead to significantly higher response SNRs and thus higher detection rates and shorter detection times. Using band-limited stimuli designed in the same manner, a "frequency-specific" hearing screening seems to be possible.     

A short history of hearing research. IV: Physiology]

In 1863, Hensen concluded from measurements of the width of the basilar membrane that tones of high and low pitch were represented at the base and apex of the cochlea, respectively. According to his calculations on the tonotopic representation of sound stimuli in the cochlea Helmholtz proposed additional resonators that would transmit the amplified signal to the afferent nerve endings. He speculated that the pillar cells of the tunnel of Corti or strands of the basilar membrane might be these proposed resonators. The resonance theory was contradicted by Wien in 1905. However, further experiments by Held and Kleinknecht in 1927 and by Békésy in 1928 demonstrated that Helmholtz's ideas on the tonotopic dispersion of the vibration of the basilar membrane were correct. Békésy measured the vibration of the cochlear partition in human and animal cadavers and discovered the travelling-wave of the basilar membrane. At the turn of the century Ter Kuile noted that the vibration of the cochlear partition caused a deflection of the sensory hairs of the hair cells, the auditory receptor cells. Wever and Bray described in 1930 stimulus-evoked electrical currents near the cochlea with a wave form similar to that of the original sound stimulus. It was Adrian who later coined the term "cochlear microphonics" for this phenomenon. According to calculations of Gold (1948) and others active mechanical amplification would be required for such a sharp tuning in the cochlea. The first to measure action potentials of the afferent auditory nerve was Tasaki (1954).(ABSTRACT TRUNCATED AT 250 WORDS)     

Longitudinal Coupling in the Basilar Membrane

A systematic and detailed study of the longitudinal coupling exhibited by the basilar membrane (BM) was performed in the excised gerbil cochlea. Contrary to the notion that the adjacent regions of the BM are decoupled from each other, the data indicate that: (a) the BM exhibits longitudinal coupling; (b) the length of the coupled region increases from base to apex of the cochlea; and (c) the cells of the organ of Corti (OC) increase the overall coupling exhibited by the BM. Modeling results show that, at a given location, longitudinal coupling increases the effective stiffness of the OC near the characteristic frequency. Therefore, the effect of longitudinal coupling cannot be neglected in the region of the peak of the traveling wave.