Cochlear Tuning Research Articles

Phase of shear vibrations within cochlear partition leads to activation of the cochlear amplifier.

Since Georg von Bekesy laid out the place theory of the hearing, researchers have been working to understand the remarkable properties of mammalian hearing. Because access to the cochlea is restricted in live animals, and important aspects of hearing are destroyed in dead ones, models play a key role in interpreting local measurements. Wentzel-Kramers-Brillouin (WKB) models are attractive because they are analytically tractable, appropriate to the oblong geometry of the cochlea, and can predict wave behavior over a large span of the cochlea. Interest in the role the tectorial membrane (TM) plays in cochlear tuning led us to develop models that directly interface the TM with the cochlear fluid. In this work we add an angled shear between the TM and reticular lamina (RL), which serves as an input to a nonlinear active force. This feature plus a novel combination of previous work gives us a model with TM-fluid interaction, TM-RL shear, a nonlinear active force and a second wave mode. The behavior we get leads to the conclusion the phase between the shear and basilar membrane (BM) vibration is critical for amplification. We show there is a transition in this phase that occurs at a frequency below the cutoff, which is strongly influenced by TM stiffness. We describe this mechanism of sharpened BM velocity profile, which demonstrates the importance of the TM in overall cochlear tuning and offers an explanation for the response characteristics of the Tectb mutant mouse.

Efferent-mediated reduction in cochlear gain does not alter tuning estimates from stimulus-frequency otoacoustic emission group delays

The existence of efferent feedback from cortical and subcortical brain centers to the hair cells of the cochlea has been recognized for many years, but the role that efferent neurons play in hearing is not completely known. Stimulation of medial olivocochlear (MOC) efferent neurons suppresses sound-evoked basilar membrane responses and changes the tuning of single auditory nerve fibers in animal models. Both of these effects are linked to a MOC-induced reduction in the gain of the cochlear amplification provided by outer hair cells. To non-invasively examine the link between cochlear suppression and tuning in humans, stimulus-frequency otoacoustic emissions (SFOAEs) were recorded in conditions with and without contralateral acoustic stimulation (CAS) from 28 normal-hearing participants. SFOAEs were measured using clusters of closely-spaced probe-tone frequencies centered near 1.4 and 2.0kHz. An index of cochlear tuning, QERB, was calculated based on measures of SFOAE group delay at both 1.4 and 2.0kHz. A statistically significant (p<0.01) decrease in SFOAE levels acquired during CAS was detected only for the SFOAE cluster centered at 2kHz. No statistically significant differences in QERB were found between conditions with and without CAS at 1.4 and 2.0kHz. These findings suggest that in humans, tuning based on SFOAE group delay estimates is not appreciably altered at cochlear locations with MOC efferent-induced reductions in cochlear gain.

On the Controversy About the Sharpness of Human Cochlear Tuning

In signal processing terms, the operation of the mammalian cochlea in the inner ear may be likened to a bank of filters. Based on otoacoustic emission evidence, it has been recently claimed that cochlear tuning is sharper for human than for other mammals. The claim was corroborated with a behavioral method that involves the masking of pure tones with forward notched noises (NN). Using this method, it has been further claimed that human cochlear tuning is sharper than suggested by earlier behavioral studies. These claims are controversial. Here, we contribute to the controversy by theoretically assessing the accuracy of the NN method at inferring the bandwidth (BW) of nonlinear cochlear filters. Behavioral forward masking was mimicked using a computer model of the squared basilar membrane response followed by a temporal integrator. Isoresponse and isolevel versions of the forward masking NN method were applied to infer the already known BW of the cochlear filter used in the model. We show that isolevel methods were overall more accurate than isoresponse methods. We also show that BWs for NNs and sinusoids equate only for isolevel methods and when the levels of the two stimuli are appropriately scaled. Lastly, we show that the inferred BW depends on the method version (isolevel BW was twice as broad as isoresponse BW at 40 dB SPL) and on the stimulus level (isoresponse and isolevel BW decreased and increased, respectively, with increasing level over the level range where cochlear responses went from linear to compressive). We suggest that the latter may contribute to explaining the reported differences in cochlear tuning across behavioral studies and species. We further suggest that given the well-established nonlinear nature of cochlear responses, even greater care must be exercised when using a single BW value to describe and compare cochlear tuning.

Mechanisms and mechanics of auditory masking

It is rare to hear just one sound at a time. More often than not, our experience of listening to someone speak, or hearing a piece of music, is made more challenging, and possibly more annoying, by the presence of interfering sound, be it traffic noise, air-conditioning rumble, or other people talking at the same time. In extreme situations, such as in a noisy restaurant or bar, the background noise may prevent us from even hearing the conversation we are trying our best to follow. This phenomenon is known as masking, and it occurs in every sensory system. In this issue of The Journal of Physiology, a study by Recio-Spinoso & Cooper (2013) traces the origins of masking down to the first stages of electro-mechanical processing in the cochlea. When multiple sound sources are present at the same time, the variations in air pressure produced by each source combine to form a single pattern of rapidly varying air pressure at each of the listener's eardrums, causing them to vibrate. The auditory system must then decompose the patterns of vibrations at the eardrums to provide us with information about the individual sound sources, such as their identity and location. Given this daunting challenge, the question might be how we are ever able to hear out sounds from a mixture, rather than why our ability to do so sometimes fails. A crucial first step in this decomposition process is accomplished within the cochlea, where different frequencies within a complex sound maximally stimulate different places along the length of the basilar membrane – a structure that runs the length of the cochlea. This frequency-to-place mapping, or tonotopic organization, is a fundamental organizational principle of auditory coding and is maintained throughout lower structures of the auditory system, up to and including primary auditory cortex (Pickles, 2012). Masking tends to occur when the tonotopic representation of one sound interferes with that of another. The ‘classical’ masking studies in both physiological and perceptual (behavioural) experiments have involved embedding a tonal signal in a noise masker. Physiological correlates of masking have been identified at various levels of the auditory system beginning in the auditory nerve (Delgutte, 1990), but the current study by Recio-Spinoso & Cooper (2013) is the first to report the responses of the mammalian basilar membrane to this classical stimulus combination. The study shows that many of the non-linear neural phenomena produced by the interaction of tones and noise, such as frequency-specific suppression of tones by noise (and vice versa), have their origins in the mechanical response of the basilar membrane. In addition, some of the more linear aspects of auditory processing, such as the constant signal-to-noise ratio at threshold over a large dynamic range, provide an early physiological correlate of the perceptual invariance we experience when listening to the same sound over a wide range of stimulus levels. The new study demonstrates that many attributes of auditory masking, previously observed in both neural and psychophysical experiments, are established at the level of mechanical transduction in the cochlea. The data also provide a valuable resource with which to test and validate non-linear computational models of the peripheral auditory system, which in turn can be used as front ends for devices as diverse as low bit-rate audio codecs (such as those used for MP3 audio compression) and automatic speech recognizers. Important remaining questions include the extent to which cochlear responses to complex sounds are altered by efferent control (activated by prior sounds, or ‘top-down’ effects such as attention) in awake, behaving animals, and the still-controversial issue of how similar human cochlear tuning is to that of typical laboratory animals (Shera et al. 2010; Joris et al. 2011), such as the chinchilla and gerbil studied by Recio-Spinoso & Cooper (2013).

Contralateral efferent reflex effects on threshold and suprathreshold psychoacoustical tuning curves at low and high frequencies.

Medial olivocochlear efferent neurons can control cochlear frequency selectivity and may be activated in a reflexive manner by contralateral sounds. The present study investigated the significance of the contralateral medial olivocochlear reflex (MOCR) on human psychoacoustical tuning curves (PTCs), a behavioral correlate of cochlear tuning curves. PTCs were measured using forward masking in the presence and in the absence of a contralateral white noise, assumed to elicit the MOCR. To assess MOCR effects on apical and basal cochlear regions over a wide range of sound levels, PTCs were measured for probe frequencies of 500 Hz and 4 kHz and for near- and suprathreshold conditions. Results show that the contralateral noise affected the PTCs predominantly at 500 Hz. At near-threshold levels, its effect was obvious only for frequencies in the tails of the PTCs; at suprathreshold levels, its effects were obvious for all frequencies. It was verified that the effects were not due to the contralateral noise activating the middle-ear muscle reflex or changing the postmechanical rate of recovery from forward masking. A phenomenological computer model of forward masking with efferent control was used to explain the data. The model supports the hypothesis that the behavioral results were due to the contralateral noise reducing apical cochlear gain in a frequency- and level-dependent manner consistent with physiological evidence. Altogether, this shows that the contralateral MOCR may be changing apical cochlear responses in natural, binaural listening situations.

Processing pitch in a nonhuman mammal (Chinchilla laniger).

Whether the mechanisms giving rise to pitch reflect spectral or temporal processing has long been debated. Generally, sounds having strong harmonic structures in their spectra have strong periodicities in their temporal structures. We found that when a wideband harmonic tone complex is passed through a noise vocoder, the resulting sound can have a harmonic structure with a large peak-to-valley ratio, but with little or no periodicity in the temporal structure. To test the role of harmonic structure in pitch perception for a nonhuman mammal, we measured behavioral responses to noise-vocoded tone complexes in chinchillas (Chinchilla laniger) using a stimulus generalization paradigm. Chinchillas discriminated either a harmonic tone complex or an iterated rippled noise from a 1-channel vocoded version of the tone complex. When tested with vocoded versions generated with 8, 16, 32, 64, and 128 channels, responses were similar to those of the 1-channel version. Behavioral responses could not be accounted for based on harmonic peak-to-valley ratio as the acoustic cue, but could be accounted for based on temporal properties of the autocorrelation functions such as periodicity strength or the height of the first peak. The results suggest that pitch perception does not arise through spectral processing in nonhuman mammals but rather through temporal processing. The conclusion that spectral processing contributes little to pitch in nonhuman mammals may reflect broader cochlear tuning than that described in humans.

Moments of click-evoked otoacoustic emissions in human ears: Group delay and spread, instantaneous frequency and bandwidth

A click-evoked otoacoustic emission (CEOAE) has group delay and spread as first- and second-order temporal moments varying over frequency, and instantaneous frequency and bandwidth as first- and second-order spectral moments varying over time. Energy-smoothed moments were calculated from a CEOAE database over 0.5-15 kHz bandwidth and 0.25-20 ms duration. Group delay and instantaneous frequency were calculated without phase unwrapping using a coherence synchrony measure that accurately classified ears with hearing loss. CEOAE moment measurements were repeatable in individual ears. Group delays were similar for CEOAEs and stimulus-frequency OAEs. Group spread is a frequency-specific measure of temporal spread in an emission, related to spatial spread across tonotopic generation sites along the cochlea. In normal ears, group delay and spread increased with frequency and decreased with level. A direct measure of cochlear tuning above 4 kHz was analyzed using instantaneous frequency and bandwidth. Synchronized spontaneous OAEs were present in most ears below 4 kHz, and confounded interpretation of moments. In ears with sensorineural hearing loss, group delay and spread varied with audiometric classification and amount of hearing loss; group delay differed between older males and females. CEOAE moments reveal clinically relevant information on cochlear tuning in ears with normal and impaired hearing.

Effect of contralateral acoustic stimulation on cochlear tuning measured using stimulus frequency and distortion product OAEs

Objective: To study whether a change in cochlear tuning, measured using OAEs, could be detected due to contralateral activation of the efferent system using broadband noise. Design: Cochlear tuning measures based on SFOAE phase gradients and SFOAE-2TS ‘Q’ were used to test this hypothesis. SFOAE magnitude and phase gradient were measured using a pure-tone sweep from 1248 to 2496 Hz at 50 dB SPL. 2TS curves of SFOAE were recorded with a suppressor frequency swept from 1120 to 2080 Hz at 50 dB SPL. DPOAE f2-sweep phase gradient was also obtained to allow comparisons with the literature. All three assays were performed across with- and no-CAS conditions. Study sample: Twenty-two young, normal-hearing adults. Results: CAS did not produce a statistically significant change in the tuning metric in any of the OAE methods used, despite producing significant reductions in the OAE magnitude. Conclusion: It is unknown whether this insensitivity to CAS is due to an insensitivity of these three measures to cochlear mechanical tuning. The results suggest that any changes in tuning induced by CAS that may occur are small and difficult to detect using the OAE measurement paradigms used here.

Obtaining reliable phase-gradient delays from otoacoustic emission data

Reflection-source otoacoustic emission phase-gradient delays are widely used to obtain noninvasive estimates of cochlear function and properties, such as the sharpness of mechanical tuning and its variation along the length of the cochlear partition. Although different data-processing strategies are known to yield different delay estimates and trends, their relative reliability has not been established. This paper uses in silico experiments to evaluate six methods for extracting delay trends from reflection-source otoacoustic emissions (OAEs). The six methods include both previously published procedures (e.g., phase smoothing, energy-weighting, data exclusion based on signal-to-noise ratio) and novel strategies (e.g., peak-picking, all-pass factorization). Although some of the methods perform well (e.g., peak-picking), others introduce substantial bias (e.g., phase smoothing) and are not recommended. In addition, since standing waves caused by multiple internal reflection can complicate the interpretation and compromise the application of OAE delays, this paper develops and evaluates two promising signal-processing strategies, the first based on time-frequency filtering using the continuous wavelet transform and the second on cepstral analysis, for separating the direct emission from its subsequent reflections. Altogether, the results help to resolve previous disagreements about the frequency dependence of human OAE delays and the sharpness of cochlear tuning while providing useful analysis methods for future studies.

Suppression tuning of distortion product otoacoustic emissions in humans: results from cochlear mechanics simulation

Is human hearing more sharply tuned than other mammals? This has been a heavily debated subject in the field of cochlear mechanics. The debate continues partially due to lack of non-invasive methods to estimate human cochlear tuning accurately. Recently, Gorga et al. (2011, JASA) derived tuning curves from suppression of distortion product (DP) otoacoustic emissions (OAEs) in normal-hearing human ears. Frequencies of the primary tones were varied from 0.5 to 8 kHz. Sharpness of tuning was analyzed in terms of the Q-value of equivalent rectangular bandwith, which ranged from 4 to 10 at the lowest stimulus level tested. The Q-values were similar to that of psychoacoustic tuning but lower than inferred from latencies of stimulus-frequency OAEs (Shera et al. 2002, PNAS). In the present work, we simulate DPOAE suppression based on a computer model of cochlear mechanics (Liu and Neely, 2010, JASA). The simulated DPOAE suppression tuning curves (STCs) resemble those obtained in experiments but discrepancies remain. At high frequencies, the simulated DPOAE STCs are not as sharply tuned as the magnitude response of traveling waves in the model. Confounding factors and interpretation of results will be discussed. (Supported by Taiwan's NSC and NTHU)

Stimulus characteristics which lessen the impact of threshold fine structure on estimates of hearing status

When hearing thresholds are measured with high-frequency resolution there is a pseudo-periodic variation in thresholds across frequency of up to 15–20dB. This variation is called threshold fine structure (previously referred to as threshold microstructure). Consequently, estimates of auditory status based on threshold measures can depend greatly on the specific frequency evaluated. The impact of threshold fine structure on the prediction of auditory status was examined by measuring detection thresholds of pure tones (providing an indication of threshold fine structure) and comparing them with thresholds obtained using linear sweeps, sinusoidally frequency modulated tones, and narrow-band noise. Spontaneous otoacoustic emissions (SOAEs) were also obtained to confirm the established relationship between threshold fine structure and SOAEs. Thresholds obtained using linear sweeps and narrow-band noise provided stable threshold estimates indicating that such threshold estimates were less influenced by threshold fine structure. Consequently, thresholds obtained with these stimuli may provide estimates of cochlear status less dependent of the exact frequency being evaluated, permitting better prediction of performance on other psychoacoustic measures (such as cochlear tuning and loudness perception) and the properties of their more objective measures (such as otoacoustic emissions).

Frequency selectivity in Old-World monkeys corroborates sharp cochlear tuning in humans

Frequency selectivity in the inner ear is fundamental to hearing and is traditionally thought to be similar across mammals. Although direct measurements are not possible in humans, estimates of frequency tuning based on noninvasive recordings of sound evoked from the cochlea (otoacoustic emissions) have suggested substantially sharper tuning in humans but remain controversial. We report measurements of frequency tuning in macaque monkeys, Old-World primates phylogenetically closer to humans than the laboratory animals often taken as models of human hearing (e.g., cats, guinea pigs, chinchillas). We find that measurements of tuning obtained directly from individual auditory-nerve fibers and indirectly using otoacoustic emissions both indicate that at characteristic frequencies above about 500 Hz, peripheral frequency selectivity in macaques is significantly sharper than in these common laboratory animals, matching that inferred for humans above 4-5 kHz. Compared with the macaque, the human otoacoustic estimates thus appear neither prohibitively sharp nor exceptional. Our results validate the use of otoacoustic emissions for noninvasive measurement of cochlear tuning and corroborate the finding of sharp tuning in humans. The results have important implications for understanding the mechanical and neural coding of sound in the human cochlea, and thus for developing strategies to compensate for the degradation of tuning in the hearing-impaired.

Human cochlear tuning estimates from stimulus-frequency otoacoustic emissions

Two objective measures of human cochlear tuning, using stimulus-frequency otoacoustic emissions (SFOAE), have been proposed. One measure used SFOAE phase-gradient delay and the other two-tone suppression (2TS) tuning curves. Here, it is hypothesized that the two measures lead to different frequency functions in the same listener. Two experiments were conducted in ten young adult normal-hearing listeners in three frequency bands (1-2 kHz, 3-4 kHz and 5-6 kHz). Experiment 1 recorded SFOAE latency as a function of stimulus frequency, and experiment 2 recorded 2TS iso-input tuning curves. In both cases, the output was converted into a sharpness-of-tuning factor based on the equivalent rectangular bandwidth. In both experiments, sharpness-of-tuning curves were shown to be frequency dependent, yielding sharper relative tuning with increasing frequency. Only a weak frequency dependence of the sharpness-of-tuning curves was observed for experiment 2, consistent with objective and behavioural estimates from the literature. Most importantly, the absolute difference between the two tuning estimates was very large and statistically significant. It is argued that the 2TS estimates of cochlear tuning likely represents the underlying properties of the suppression mechanism, and not necessarily cochlear tuning. Thus the phase-gradient delay estimate is the most likely one to reflect cochlear tuning.

Otoacoustic emission delays as a probe to measure cochlear tuning: Comparative validations.

The ear is not only sensitive to sound but selective as well: Tonotopic tuning of the inner ear provides a means to resolve incoming spectral information. Measurements of frequency selectivity have traditionally relied upon either subjective psychophysical or objective (but invasive) physiological approaches. Sounds emitted from a healthy ear, known as otoacoustic emissions (OAEs), have been proposed to both objectively and non‐invasively estimate peripheral auditory tuning. Despite diverse inner‐ear morphological variation across animals, OAEs are a universal feature and correlate well to an animal’s range of active hearing. Recent studies focusing on emission delays in response to a single stimulus frequency (SFOAEs), conducted systematically across species in a variety of classes (mammals, aves, reptiles, and amphibians), support predictions relating emissions and tuning. Longer SFOAE delays presumably reflect the sharper tuning associated with resonant build‐up time of the underlying auditory filters. Differences in tuning estimated from OAEs appear generally congruous with known anatomical and functional considerations: Larger sensory organs (i.e., more “filters”) with smaller ranges of audition exhibit sharper tuning. Comparisons made both broadly (inter‐class) and within phylogenetically‐matched groups (intra‐family) indicate that SFOAE delays in humans are longer than any other species so far examined, suggestive of exceptionally sharper tuning.

Filter characteristics derived from auditory-nerve fiber responses following noise-induced hearing loss.

Sensorineural hearing loss (SNHL) produces a loss of frequency selectivity and elevated thresholds in the auditory periphery. Broadened cochlear tuning significantly affects magnitude and phase responses of auditory filters and thus can change cochlear-response timing. The present study compared auditory-filter characteristics in normal-hearing and noise-exposed anesthetized chinchillas. Impulse responses of auditory filters were derived from responses of auditory-nerve fibers to broadband noise using spike-triggered averaging techniques. Frequency modulations (or frequency glides) in impulse responses were quantified by computing instantaneous frequencies from impulse-response zero crossings. Group delays for individual auditory filters were computed from phase-frequency responses. Cochlear traveling-wave delays were estimated from shuffled cross-correlograms. Preliminary results suggest that auditory-filter best frequency shifts apically following SNHL. Latencies of impulse responses and traveling-wave delays were much shorter following noise exposure. For normal-hearing fibers, frequency glides in impulse responses increased as a function of time for high characteristic-frequency (CF) fibers and decreased for low-CF fibers, consistent with previous studies. SNHL significantly affected frequency glides of noise-exposed fibers in a manner consistent with the loss of tonotopicity. These characterizations of impaired auditory filters have important implications for SNHL effects on temporal and spatiotemporal coding of complex sounds, such as speech. [Work supported by NIH Grant No. R01DC009838.]

Feed-Forward and Feed-Backward Amplification Model from Cochlear Cytoarchitecture: An Interspecies Comparison

The high sensitivity and wide bandwidth of mammalian hearing are thought to derive from an active process involving the somatic and hair-bundle motility of the thousands of outer hair cells uniquely found in mammalian cochleae. To better understand this, a biophysical three-dimensional cochlear fluid model was developed for gerbil, chinchilla, cat, and human, featuring an active “push-pull” cochlear amplifier mechanism based on the cytoarchitecture of the organ of Corti and using the time-averaged Lagrangian method. Cochlear responses are simulated and compared with in vivo physiological measurements for the basilar membrane (BM) velocity, VBM, frequency tuning of the BM vibration, and Q10 values representing the sharpness of the cochlear tuning curves. The VBM simulation results for gerbil and chinchilla are consistent with in vivo cochlea measurements. Simulated mechanical tuning curves based on maintaining a constant VBM value agree with neural-tuning threshold measurements better than those based on a constant displacement value, which implies that the inner hair cells are more sensitive to VBM than to BM displacement. The Q10 values of the VBM tuning curve agree well with those of cochlear neurons across species, and appear to be related in part to the width of the basilar membrane.

Understanding the mathematics of hearing using electronic circuits

The human cochlea is a fascinating transduction organ that illustrates the ingenious way in which engineering problems are solved in nature. A healthy cochlea has a dynamic range in the order of~120\,dB; that is, the difference between the roar of the engines of a Boeing~747 and the faintest whisper. We discuss the recent assertion that the cochlea is governed by the dynamics of a Hopf bifurcation. In our cochlea model we discretise the basilar membrane into resonant sections with logarithmically decreasing characteristic frequencies. We show that the observed active behaviour of the cochlea can be modelled as a change in the quality factor of the individual resonant sections in a discretised model, and that this has dynamics which embody the Hopf bifurcation. References S. Camalet, T. Duke, F. Julicher, and J. Prost, Auditory Sensitivity Provided by Self-Tuned Critical Oscillations of Hair Cells. Proceedings of the National Academy of Sciences of the United States of America 97, 2000, pp. 3183--3188. V. M. Eguiluz, M. Ospeck, Y. Choe, A. J. Hudspeth, and M. O. Magnasco, Essential Nonlinearities in Hearing. Physical Review Letters 84, 2000, 5232--5235. E. Fragniere, Analogue VLSI Emulation of the Cochlea, Doctoral Thesis, Departement d'electricite, EPFL, Lausanne, 1998. http://library.epfl.ch/en/theses/?nr=1796 T. J. Hamilton, C. Jin, A. van Schaik, and J. Tapson, An Active 2-D Silicon Cochlea. Biomedical Circuits and Systems, IEEE Transactions on 2, 2008, 30--43. T. J. Hamilton, J. Tapson, C. Jin, and A. van Schaik, Analogue VLSI implementations of two dimensional, nonlinear, active cochlea models, Biomedical Circuits and Systems Conference, 2008. IEEE, 2008, pp. 153--156. A. Kern, J.-J. van der Vyver, and R. Stoop, Towards a biomorphic silicon Hopf cochlea, Proceedings of the 10th IEEE conference on nonlinear dynamics of electronic systems, 2, 2002. A. Kern, and R. Stoop, Essential Role of Couplings between Hearing Nonlinearities, Physical Review Letters 91 (12) 2003, 128101. M. O. Magnasco, A Wave Traveling over a Hopf Instability Shapes the Cochlear Tuning Curve, Physical Review Letters 90 (5) 2003, 058101. C. J. Plack, The Sense of Hearing, Lawrence Erlbaum Associates, Inc., New Jersey, 2005. M. A. Ruggero, Responses to sound of the basilar membrane of the mammalian cochlea. Current Opinion in Neurobiology 2, 1992, 449--456. S. H. Strogatz, Nonlinear Dynamics and Chaos, Westview Press, Cambridge, 1994. J. Tapson, T. J. Hamilton, C. Jin, and A. van Schaik, Self-Tuned Regenerative Amplification and the Hopf Bifurcation, IEEE International Symposium on Circuits and Systems (ISCAS), Seattle, USA, 2008. A. van Schaik, and E. Fragniere, Pseudo-voltage domain implementation of a 2-dimensional silicon cochlea, International Symposium on Circuits and Systems (ISCAS), Sydney, Australia, 2001, pp. 185--188 vol. 2 doi:10.1109/ISCAS.2001921277

Otoacoustic Estimation of Cochlear Tuning: Validation in the Chinchilla

We analyze published auditory-nerve and otoacoustic measurements in chinchilla to test a network of hypothesized relationships between cochlear tuning, cochlear traveling-wave delay, and stimulus-frequency otoacoustic emissions (SFOAEs). We find that the physiological data generally corroborate the network of relationships, including predictions from filter theory and the coherent-reflection model of OAE generation, at locations throughout the cochlea. The results support the use of otoacoustic emissions as noninvasive probes of cochlear tuning. Developing this application, we find that tuning ratios-defined as the ratio of tuning sharpness to SFOAE phase-gradient delay in periods-have a nearly species-invariant form in cat, guinea pig, and chinchilla. Analysis of the tuning ratios identifies a species-dependent parameter that locates a transition between "apical-like" and "basal-like" behavior involving multiple aspects of cochlear physiology. Approximate invariance of the tuning ratio allows determination of cochlear tuning from SFOAE delays. We quantify the procedure and show that otoacoustic estimates of chinchilla cochlear tuning match direct measures obtained from the auditory nerve. By assuming that invariance of the tuning ratio extends to humans, we derive new otoacoustic estimates of human cochlear tuning that remain mutually consistent with independent behavioral measurements obtained using different rationales, methodologies, and analysis procedures. The results confirm that at any given characteristic frequency (CF) human cochlear tuning appears sharper than that in the other animals studied, but varies similarly with CF. We show, however, that the exceptionality of human tuning can be exaggerated by the ways in which species are conventionally compared, which take no account of evident differences between the base and apex of the cochlea. Finally, our estimates of human tuning suggest that the spatial spread of excitation of a pure tone along the human basilar membrane is comparable to that in other common laboratory animals.

Coherent reflection without traveling waves: On the origin of long-latency otoacoustic emissions in lizards

Lizard ears produce otoacoustic emissions with characteristics often strikingly reminiscent of those measured in mammals. The similarity of their emissions is surprising, given that lizards and mammals manifest major differences in aspects of inner ear morphology and function believed to be relevant to emission generation. For example, lizards such as the gecko evidently lack traveling waves along their basilar membrane. Despite the absence of traveling waves, the phase-gradient delays of gecko stimulus-frequency otoacoustic emissions (SFOAEs) are comparable to those measured in many mammals. This paper describes a model of emission generation inspired by the gecko inner ear. The model consists of an array of coupled harmonic oscillators whose effective damping manifests a small degree of irregularity. Model delays increase with the assumed sharpness of tuning, reflecting the build-up time associated with mechanical resonance. When tuning bandwidths are chosen to match those of gecko auditory-nerve fibers, the model reproduces the major features of gecko SFOAEs, including their spectral structure and the magnitude and frequency dependence of their phase-gradient delays. The same model with appropriately modified parameters reproduces the features of SFOAEs in alligator lizards. Analysis of the model demonstrates that the basic mechanisms operating in the model are similar to those of the coherent-reflection model developed to describe mammalian emissions. These results support the notion that SFOAE delays provide a noninvasive measure of the sharpness of cochlear tuning.

Transient evoked otoacoustic emission latency and estimates of cochlear tuning in preterm neonates

The latency of transient evoked otoacoustic emissions has been evaluated in a sample of 58 ears from 34 preterm neonates, to understand if the estimates of cochlear tuning based on the otoacoustic emission latency show signs of developmental changes. A previous study on the same otoacoustic emissions analyzed here [Tognola et al. (2005). "Cochlear maturation and otoacoustic emissions in preterm infants: A time-frequency approach," Hear. Res., 199, 71-80] reported indeed a significant change in the otoacoustic emission latency with postconception age. This last result, which would imply a significant decrease of tuning, was partially biased by the presence of spontaneous emissions. In this study, the same neonate data are reanalyzed using a novel time-frequency algorithm, less sensitive to spontaneous emissions. Asymmetry between right and left ears has been found, with the left ears showing no significant change, whereas in the right ears and in the 1.5-2.5 kHz frequency range only, a slow decrease of latency with postconception age (0.1-0.2 ms/week) was observed. The correspondent tuning estimates based on latency decrease by 0.4-0.5/week. Significant differences between neonate and adult latency were confirmed, which could be either cochlear or middle ear in nature. These findings are compared to previous studies on distortion product suppression tuning curves in preterm neonates.