Transmission of infrasonic pressure waves from cerebrospinal to intralabyrinthine fluids through the human cochlear aqueduct: Non-invasive measurements with otoacoustic emissions

The cochlear aqueduct connecting intralabyrinthine and cerebrospinal fluids (CSF) acts as a low-pass filter that should be able to transmit infrasonic pressure waves from CSF to cochlea. Recent experiments have shown that otoacoustic emissions generated at 1kHz respond to pressure-related stapes impedance changes with a change in phase relative to the generator tones, and provide a non-invasive means of assessing intracochlear pressure changes. In order to characterize the transmission to the cochlea of CSF pressure waves due to respiration, the distortion-product otoacoustic emissions (DPOAE) of 12 subjects were continuously monitored around 1kHz at a rate of 6.25epochs/s, and their phase relative to the stimulus tones was extracted. The subjects breathed normally, in different postures, while thoracic movements were recorded so as to monitor respiration. A correlate of respiration was found in the time variation of DPOAE phase, with an estimated mean amplitude of 10 degrees , i.e. 60mm water, suggesting little attenuation across the aqueduct. Its phase lag relative to thoracic movements varied between 0 degrees and -270 degrees . When fed into a two-compartment model of CSF and labyrinthine spaces, these results suggest that respiration rate at rest is just above the resonance frequency of the CSF compartment, and just below the corner frequency of the cochlear-aqueduct low-pass filter, in line with previous estimates from temporal bone and intracranial measurements. The fact that infrasonic CSF waves can be monitored through the cochlea opens diagnostic possibilities in neurology.     

Stapes Vibration in the Chinchilla Middle Ear: Relation to Behavioral and Auditory-Nerve Thresholds

The vibratory responses to tones of the stapes and incus were measured in the middle ears of deeply anesthetized chinchillas using a wide-band acoustic-stimulus system and a laser velocimeter coupled to a microscope. With the laser beam at an angle of about 40 ° relative to the axis of stapes piston-like motion, the sensitivity-vs.-frequency curves of vibrations at the head of the stapes and the incus lenticular process were very similar to each other but larger, in the range 15–30 kHz, than the vibrations of the incus just peripheral to the pedicle. With the laser beam aligned with the axis of piston-like stapes motion, vibrations of the incus just peripheral to its pedicle were very similar to the vibrations of the lenticular process or the stapes head measured at the 40 ° angle. Thus, the pedicle prevents transmission to the stapes of components of incus vibration not aligned with the axis of stapes piston-like motion. The mean magnitude curve of stapes velocities is fairly flat over a wide frequency range, with a mean value of about 0.19 mm^.(s Pa⁻¹), has a high-frequency cutoff of 25 kHz (measured at −3 dB re the mean value), and decreases with a slope of about −60 dB/octave at higher frequencies. According to our measurements, the chinchilla middle ear transmits acoustic signals into the cochlea at frequencies exceeding both the bandwidth of responses of auditory-nerve fibers and the upper cutoff of hearing. The phase lags of stapes velocity relative to ear-canal pressure increase approximately linearly, with slopes equivalent to pure delays of about 57–76 μs.     

Reverse Transmission along the Ossicular Chain in Gerbil

In a healthy cochlea stimulated with two tones f (1) and f (2), combination tones are generated by the cochlea's active process and its associated nonlinearity. These distortion tones travel "in reverse" through the middle ear. They can be detected with a sensitive microphone in the ear canal (EC) and are known as distortion product otoacoustic emissions. Comparisons of ossicular velocity and EC pressure responses at distortion product frequencies allowed us to evaluate the middle ear transmission in the reverse direction along the ossicular chain. In the current study, the gerbil ear was stimulated with two equal-intensity tones with fixed f (2)/f (1) ratio of 1.05 or 1.25. The middle ear ossicles were accessed through an opening of the pars flaccida, and their motion was measured in the direction in line with the stapes piston-like motion using a laser interferometer. When referencing the ossicular motion to EC pressure, an additional amplitude loss was found in reverse transmission compared to the gain in forward transmission, similar to previous findings relating intracochlear and EC pressure. In contrast, sound transmission along the ossicular chain was quite similar in forward and reverse directions. The difference in middle ear transmission in forward and reverse directions is most likely due to the different load impedances-the cochlea in forward transmission and the EC in reverse transmission.     

Basilar Membrane Vibrations Near the Round Window of the Gerbil Cochlea

Using a laser velocimeter, responses to tones were measured at a basilar membrane site located about 1.2 mm from the extreme basal end of the gerbil cochlea. In two exceptional cochleae in which function was only moderately disrupted by surgical preparations, basilar membrane responses had characteristic frequencies (CFs) of 34–37 kHz and exhibited a CF-specific compressive nonlinearity: Sensitivity near the CF decreased systematically and the response peaks shifted toward lower frequencies with increasing stimulus level. Response phases also changed with increases in stimulus level, exhibiting small relative lags and leads at frequencies just lower and higher than CF, respectively. Basilar membrane responses to low-level CF tones exceeded the magnitude of stapes vibrations by 54–56 dB. Response phases led stapes vibrations by about 90° at low stimulus frequencies; at higher frequencies, basilar membrane responses increasingly lagged stapes vibration, accumulating 1.5 periods of phase lag at CF. Postmortem, nonlinearities were abolished and responses peaked at ~0.5 octave below CF, with phases which lagged and led in vivo responses at frequencies lower and higher than CF, respectively. In conclusion, basilar membrane responses near the round window of the gerbil cochlea closely resemble those for other basal cochlear sites in gerbil and other species.     

Nonlinearity in intracochlear pressure

Nonlinearity exists in intracochlear pressure responses close to the cochlea's sensory tissue. Its characteristics are much like those of basilar membrane motion nonlinearity. Here several aspects of the pressure nonlinearity in the base of the gerbil cochlea are illustrated.     

Three-dimensional stapes footplate motion in human temporal bones

The literature provides conflicting information on whether the motion of the stapes footplate is piston-like or some other type of motion, such as rotational or rocking. Examination of the three-dimensional (3D) motion of the stapes footplate appears to be an excellent way to understand this complicated motion. Five microsphere reflective targets were placed on the stapes footplate in ten fresh human cadaver temporal bone preparations, and their vibration measured through an extended facial recess approach using a laser Doppler vibrometer. The five target sites on the stapes footplate were center, anterior, posterior, superior and inferior. The stimulus was a sound input of 80-120 dB SPL at the tympanic membrane over a frequency range of 0.1 to 10 kHz. The 3D motion of the stapes footplate was calculated using the velocity amplitude and phase obtained for each target. For frequencies up to 1.0 kHz the vibration of the stapes footplate was primarily piston-like; this motion became complex at higher frequencies, with rotary motion along both the long and short axis of the footplate. When the cochlea was drained, stapes footplate motion became essentially piston-like for all frequencies.     

On Cochlear Impedances and the Miscomputation of Power Gain

In their article, “Measurement of cochlear power gain in the sensitive gerbil ear,” Ren et al. (Nat Commun 2:216, 2011) claim to provide “the first direct experimental evidence of power amplification in the sensitive living cochlea.” While we recognize the technical challenges of the experiments and appreciate the beauty of the data, the authors’ analysis and interpretation of the measurements are invalid. We review the concept of impedance (i.e., the ratio of pressure to velocity) as it applies to cochlear mechanics and show that Ren et al. mistakenly equate the impedances near the basilar membrane and stapes with the impedance characteristic of an infinite, uniform tube of fluid. As a consequence of this error, Ren et al.’s measurements and analysis provide no evidence for power amplification in the cochlea. Compelling evidence for power amplification has, however, been previously provided by others.     

Simultaneous Intracochlear Pressure Measurements from Two Cochlear Locations: Propagation of Distortion Products in Gerbil

Sound energy propagates in the cochlea through a forward-traveling or slow wave supported by the cochlear partition and fluid inertia. Additionally, cochlear models support traveling wave propagation in the reverse direction as the expected mechanism for conveying otoacoustic emissions out of the cochlea. Recently, however, this hypothesis has been questioned, casting doubt on the process by which otoacoustic emissions travel back out through the cochlea. The proposed alternative reverse travel path for emissions is directly through the fluids of the cochlea as a compression pressure in the form of a fast wave. In the present study, a custom-made micro-pressure sensor was used in vivo in the gerbil cochlea to map two-tone-evoked pressure responses at distinct longitudinal and vertical locations in both the scala tympani and scala vestibuli. Analyses of the magnitude and phase of intracochlear pressure responses at the primary tone and distortion product frequencies were used to distinguish between fast and slow waves in both the forward- and reverse-propagation directions. Results demonstrated that distortion products may travel in both forward and reverse directions post-generation and the existence of both traveling and compression waves. The forward-traveling component appeared to duplicate the process of any external tone, tuned to the local characteristic-frequency place, as it increased compressively and nonlinearly with primary-tone levels. A compression wave was evidenced at frequencies above the cutoff of the recording site. In the opposite direction, a reverse-traveling wave played the major role in driving the stapes reversely and contributed to the distortion product otoacoustic emission. The compression wave may also play a role in reverse propagation when distortion products are generated at a region close to the stapes.     

Subharmonic Distortion in Ear Canal Pressure and Intracochlear Pressure and Motion

When driven at sound pressure levels greater than ~110 dB stimulus pressure level, the mammalian middle ear is known to produce subharmonic distortion. In this study, we simultaneously measured subharmonics in the ear canal pressure, intracochlear pressure, and basilar membrane or round window membrane velocity, in gerbil. Our primary objective was to quantify the relationship between the subharmonics measured in the ear canal and their intracochlear counterparts. We had two primary findings: (1) The subharmonics emerged suddenly, with a substantial amplitude in the ear canal and the cochlea; (2) at the stimulus level for which subharmonics emerged, the pressure in scala vestibuli/pressure in the ear canal amplitude relationship was similar for the subharmonic and fundamental components. These findings are important for experiments and clinical conditions in which high sound pressure level stimuli are used and could lead to confounding subharmonic stimulation.     

The effect of superior semicircular canal dehiscence on intracochlear sound pressures

Semicircular canal dehiscence (SCD) is a pathological opening in the bony wall of the inner ear that can result in conductive hearing loss. The hearing loss is variable across patients, and the precise mechanism and source of variability are not fully understood. Simultaneous measurements of basal intracochlear sound pressures in scala vestibuli (SV) and scala tympani (ST) enable quantification of the differential pressure across the cochlear partition, the stimulus that excites the cochlear partition. We used intracochlear sound pressure measurements in cadaveric preparations to study the effects of SCD size. Sound-induced pressures in SV and ST, as well as stapes velocity and ear canal pressure were measured simultaneously for various sizes of SCD followed by SCD patching. Our results showed that at low frequencies (<600 Hz), SCD decreased the pressure in both SV and ST, as well as differential pressure, and these effects became more pronounced as dehiscence size was increased. Near 100 Hz, SV decreased by about 10 dB for a 0.5-mm dehiscence and by 20 dB for a 2-mm dehiscence, while ST decreased by about 8 dB for a 0.5-mm dehiscence and by 18 dB for a 2-mm dehiscence. Differential pressure decreased by about 10 dB for a 0.5-mm dehiscence and by about 20 dB for a 2-mm dehiscence at 100 Hz. In some ears, for frequencies above 1 kHz, the smallest pinpoint dehiscence had bigger effects on the differential pressure (10-dB decrease) than larger dehiscences (less than 10-dB decrease), suggesting larger hearing losses in this frequency range. These effects due to SCD were reversible by patching the dehiscence. We also showed that under certain circumstances such as SCD, stapes velocity is not related to how the ear can transduce sound across the cochlear partition because it is not directly related to the differential pressure, emphasizing that certain pathologies cannot be fully assessed by measurements such as stapes velocity.     

Contribution of various frequency bands to ABR in dolphins

Auditory brainstem responses (ABR) to clicks and noise bursts of various frequency bands and intensities were recorded in two bottlenosed dolphins, Tursiops truncatus. The purpose was to assess contributions of various parts of the cochlear partition to ABR and travelling wave velocity in the cochlea. At band-pass filtered stimuli (1-0.25 oct wide), ABR amplitude increased with increasing stimulus frequency, thus indicating higher contribution of basal cochlear parts. At high-pass and low-pass filtered stimuli, ABR amplitude increased with passband widening. However, the sum of all narrow-band contributions was a waveform of higher amplitude than the real ABR evoked by the wide-band stimulus. Applying a correction based on an assumption that the 'internal spectrum' is about 0.4 oct wider than the nominal stimulus spectrum resulted in the sum of narrow-band contributions equal to the wide-band ABR. The travelling wave velocity was computed based on ABR latencies and assigned a frequency of 128 kHz to the basal end of the cochlea. The computation gave values from 38.2 oct/ms at the proximal end of the basilar membrane to 4.0 oct/ms at a distance of 3.25 oct (13.5 kHz).     

Spontaneous Basilar-Membrane Oscillation (SBMO) and Coherent Reflection

In a previous report (in JARO) we have described a relatively high-frequency (15 kHz) spontaneous oscillation of the basilar membrane (SBMO) in a guinea pig ear; this oscillation was accompanied by a spontaneous otoacoustic emission (SOAE) at the same frequency. During the spontaneous oscillation and after it had subsided, the mechanical frequency response of the basilar membrane was measured by way of a wide-band random-noise stimulus, and it showed a number of spectral peaks, one of which having the frequency of the original oscillation. This pattern of peaks cannot be explained by assuming a single place of reflection in the cochlea. In this paper the process of ‘coherent reflection’ is artificially evoked in a three-dimensional model of the cochlea by imposing random place-fixed irregularities to the basilar-membrane impedance. It is shown that in the model a series of peaks arises in the frequency spectrum of the basilar-membrane response which phenomenon resembles the one found in the experimental animal. It is also shown that these peaks are actually due to superposition of the primary wave and a wave resulting from ‘coherent reflection’ which is reflected at the stapes. When the intensity of the acoustic stimulus signal is increased, the relative sizes of these peaks in the simulation diminish in about the same way as in the experiment. It is concluded that coherent reflection most likely is the cause of the ‘extra peaks’, and that this concept can also explain the observed level dependence of these peaks. The findings of this study lead to a minor refinement regarding the actual requirements for coherent reflection to arise.     

Differential Intracochlear Sound Pressure Measurements in Normal Human Temporal Bones

We present the first simultaneous sound pressure measurements in scala vestibuli and scala tympani of the cochlea in human cadaveric temporal bones. The technique we employ, which exploits microscale fiberoptic pressure sensors, enables the study of differential sound pressure at the cochlear base. This differential pressure is the input to the cochlear partition, driving cochlear waves and auditory transduction. In our results, the sound pressure in scala vestibuli (P _SV) was much greater than scala tympani pressure (P _ST), except for very low and high frequencies where P _ST significantly affected the input to the cochlea. The differential pressure (P _SV − P _ST) is a superior measure of ossicular transduction of sound compared to P _SV alone: (P _SV−P _ST) was reduced by 30 to 50 dB when the ossicular chain was disarticulated, whereas P _SV was not reduced as much. The middle ear gain P _SV/P _EC and the differential pressure normalized to ear canal pressure (P _SV − P _ST)/P _EC were generally bandpass in frequency dependence. At frequencies above 1 kHz, the group delay in the middle ear gain is about 83 μs, over twice that of the gerbil. Concurrent measurements of stapes velocity produced estimates of cochlear input impedance, the differential impedance across the partition, and round window impedance. The differential impedance was generally resistive, while the round window impedance was consistent with compliance in conjunction with distributed inertia and damping. Our technique of measuring differential pressure can be used to study inner ear conductive pathologies (e.g., semicircular dehiscence), as well as non-ossicular cochlear stimulation (e.g., round window stimulation and bone conduction)—situations that cannot be completely quantified by measurements of stapes velocity or scala vestibuli pressure by themselves.     

Acoustic responses of the human middle ear

Measurements on human cadaver ears are reported that describe sound transmission through the middle ear. Four response variables were measured with acoustic stimulation at the tympanic membrane: stapes velocity, middle-ear cavity sound pressure, acoustic impedance at the tympanic membrane and acoustic impedance of the middle-ear cavity. Measurements of stapes velocity at different locations on the stapes suggest that stapes motion is predominantly ‘piston-like’, for frequencies up to at least 2000 Hz. The measurements are generally consistent with constraints of existing models. The measurements are used (1) to show how the cavity pressure and the impedance at the tympanic membrane are related, (2) to develop a measurement-based middle-ear cavity model, which shows that the middle-ear cavity has only small effects on the motion of the tympanic membrane and stapes in the normal ear, although it may play a more prominent role in pathological ears, and (3) to show that inter-ear variations in the impedance at the tympanic membrane and the stapes velocity are not well correlated.     

Three-Dimensional Vibration of the Malleus and Incus in the Living Gerbil

In previous studies, 3D motion of the middle-ear ossicles in cat and human was explored, but models for hearing research have shifted in the last few decades to smaller mammals, and gerbil, in particular, has become a popular hearing model. In the present study, we have measured with an optical interferometer the 3D motion of the malleus and incus in anesthetized gerbil for sound of moderate intensity (90-dB sound pressure level) over a broad frequency range. To access the ossicles, the pars flaccida was removed exposing the neck and head of the malleus and the incus from the malleus-incus joint to the plate of the lenticular process. Vibration measurements were done at six to eight points per ossicle while the angle of observation was varied over approximately 30 ° to enable calculation of the 3D rigid-body velocity components. These components were expressed in an intrinsic reference frame, with one axis along the anatomical suspension axis of the malleus-incus block and a second axis along the stapes piston direction. Another way of describing the motion that does not assume an a priori rotation axis is to calculate the instantaneous rotation axis (screw axis) of the malleus/incus motion. Only at frequencies below a few kilohertz did the screw axis have a maximum rotation in a direction close to that of the ligament axis. A slight slippage in the malleus-incus joint developed with increasing frequency. Our findings are useful in determining the sound transfer characteristics through the middle ear and serve as a reference for validation of mathematical middle-ear models. Last but not least, comparing our present results in gerbil with those of previously measured species (human and cat) exposes similarities and dissimilarities among them.     

Developmental Changes of Mechanics Measured in the Gerbil Cochlea

This report describes stiffness and best frequency measurements obtained in vitro from the basilar membrane of the gerbil cochlea at the onset of hearing, during hearing maturation, and after hearing has matured. Our stiffness data constitute the first direct experimental evidence of developmental stiffness changes in the basal and middle turns. Stiffness changes by a factor of 5.5 in the basal turn between postnatal day 11 and adult, and the difference from adult is statistically significant for all ages measured up to postnatal day 16. For the middle turn, stiffness changes by a factor of 1.6 between postnatal day 11 and adult. Whereas for postnatal day 12 and beyond there is no statistically significant difference from adult, our data suggest that there may be a significant difference of stiffness between day 11 and adult in the middle turn. For the basal turn, our motion measurements confirm a passive component to the developmental best frequency shift. For the middle turn, changes in best frequency are not statistically significant. Best frequency was determined by stimulating the tissue at audio frequencies with a glass paddle and measuring motion with a computer-based imaging system. Tissue stiffness was measured with a piezoelectric-based sensor system. Tissue stiffness changes have previously been postulated to contribute to the best frequency shift observed in the cochlear base. Incorporating our data into a simple spring-mass resonance model demonstrates that our experimentally measured stiffness change can account for the change of best frequency. These results suggest that a stiffness change is, in fact, a critical component of the best frequency shift observed in the basal turn of the gerbil cochlea after the onset of hearing.     

Multi-frequency tympanometry in experimentally-induced cochlear lesions in chinchillas and guinea pigs

OBJECTIVES: To establish that susceptance-conductance tympanograms at a probe-tone frequency of 2 kHz reflects the status of the annular ligament (AL) and through it of the cochlea. METHODS: Experimental study in 5 chinchillas and 22 guinea pigs. Six validating experiments were used: blockages of the stapes and of the round window membrane (RWM), fistula of the RWM, fluid removal from the cochlea, injection of saline in the scala tympani (ST) and acoustic trauma (AT). Quantitative data (mean values of Y226, FR, Y2000, G2000 and B2000) and shape of the curves were analyzed before and immediately after lesions were done. RESULTS: Guinea pig was the most convenient provided bulla was vented and the same tip was used along the experiments. Only the shape of the curves are discriminant: 1/a supplementary sharp peak, centered around negative pressures, is observed in Y/G tympanograms in every case of RWM fistulas and in some case of AT. 2/injection of saline into ST induces immediate and reproducible Y2000, G2000, et B2000 curves modifications. 3/RWM and stapes blockages provoke foreseeable stiffening and sharpening of the tympanograms at 2 kHz. 4/on the contrary, fluid removal from the cochlea induces multiple peaks curves. CONCLUSIONS: Experimentally-induced modifications at the AL either direct (stapes blockage) or indirect by AT or decrease/increase of pressure load at the cochlear interface at the footplate result in noticeable, constant, reproducible changes of curves registered at 2 kHz. The stapes behaves both as the plotter of the curves and the interpreter of the inner ear pressure.     

Measurements of Wide-Band Cochlear Reflectance in Humans

The total sound pressure measured in the ear canal may be decomposed into a forward- and a reverse-propagating component. Most of the reverse-propagating component is due to reflection at the eardrum. However, a measurable contribution to the reverse-propagating component comes from the cochlea. Otoacoustic emissions (OAEs) are associated with this component and have been shown to be important noninvasive probes of cochlear function. Total ear-canal reflectance (ECR) is the transfer function between forward and reverse propagating components measured in the ear canal. Cochlear reflectance (CR) is the inner-ear contribution to the total ECR, which is the measured OAE normalized by the stimulus. Methods are described for measuring CR with a wide-band noise stimulus. These measurements offer wider bandwidth and minimize the influence of the measurement system while still maintaining features of other OAEs (i.e., frequency- and level-dependent latency). CR magnitude decreases as stimulus level increases. Envelopes of individual band-limited components of the time-domain CR have multiple peaks with latencies that persist across stimulus level, despite a shift in group delay. CR has the potential to infer cochlear function and status, similar to other OAE measurements.     

Response Characteristics in the Apex of the Gerbil Cochlea Studied Through Auditory Nerve Recordings

In this study, we analyze the processing of low-frequency sounds in the cochlear apex through responses of auditory nerve fibers (ANFs) that innervate the apex. Single tones and irregularly spaced tone complexes were used to evoke ANF responses in Mongolian gerbil. The spike arrival times were analyzed in terms of phase locking, peripheral frequency selectivity, group delays, and the nonlinear effects of sound pressure level (SPL). Phase locking to single tones was similar to that in cat. Vector strength was maximal for stimulus frequencies around 500 Hz, decreased above 1 kHz, and became insignificant above 4 to 5 kHz. We used the responses to tone complexes to determine amplitude and phase curves of ANFs having a characteristic frequency (CF) below 5 kHz. With increasing CF, amplitude curves gradually changed from broadly tuned and asymmetric with a steep low-frequency flank to more sharply tuned and asymmetric with a steep high-frequency flank. Over the same CF range, phase curves gradually changed from a concave-upward shape to a concave-downward shape. Phase curves consisted of two or three approximately straight segments. Group delay was analyzed separately for these segments. Generally, the largest group delay was observed near CF. With increasing SPL, most amplitude curves broadened, sometimes accompanied by a downward shift of best frequency, and group delay changed along the entire range of stimulus frequencies. We observed considerable across-ANF variation in the effects of SPL on both amplitude and phase. Overall, our data suggest that mechanical responses in the apex of the cochlea are considerably nonlinear and that these nonlinearities are of a different character than those known from the base of the cochlea.