Morphology of the basilar papilla of the bobtail lizard Tiliqua rugosa

Conditions Underlying the Appearance of Spontaneous Otoacoustic Emissions in Mammals

Across the wide range of land vertebrate species, spontaneous otoacoustic emissions (SOAE) are common, but not always found. The reasons for the differences between species of the various groups in their emission patterns are often not well understood, particularly within mammals. This review examines the question as to what determines in mammals whether SOAE are emitted or not, and suggests that the coupling between hair-cell regions diminishes when the space constant of frequency distribution becomes larger. The reduced coupling is assumed to result in a greater likelihood of SOAE being emitted.

Modeling the characteristics of spontaneous otoacoustic emissions in lizards

Lizard auditory papillae have proven to be an attractive object for modelling the production of spontaneous otoacoustic emissions (SOAE). Here we use an established model (Vilfan and Duke, 2008) and extend it by exploring the effect of varying the number of oscillating elements, the strength of the parameters that describe the coupling between oscillators, the strength of the oscillators, and additive noise. The most remarkable result is that the actual number of oscillating elements hardly influences the spectral pattern, explaining why spectra from very different papillar dimensions are similar. Furthermore, the spacing between spectral peaks primarily depends on the reactive coupling between the oscillator elements. This is consistent with observed differences between lizard species with respect to tectorial covering of hair cells and SOAE peak spacings. Thus, the model provides a basic understanding of the variation in SOAE properties across lizard species.

Travelling waves and tonotopicity in the inner ear: a historical and comparative perspective

In the 1940s, Georg von Békésy discovered that in the inner ear of cadavers of various vertebrates, structures responded to sound with a displacement wave that travels in a basal-to-apical direction. This historical review examines this concept and sketches its rôle and significance in the development of the research field of cochlear mechanics. It also illustrates that this concept and that of tonotopicity necessarily correlate, in that travelling waves are consequences of the existence of an ordered, longitudinal array of receptor cells tuned to systematically changing frequencies along the auditory organ.

Spontaneous otoacoustic emissions in teiid lizards

SOAE from the last major lizard family not yet systematically investigated, the teiids, were collected from the genera Callopistes, Tupinambis and Cnemidophorus. Although their papillae show characteristics of the family Teiidae, the papillae differ both in their size and in the arrangement of uni- and bi-directional hair-cell areas. Among these three genera, Callopistes showed few (2 or 3) SOAE peaks, whereas the other two genera showed more (up to 6 per ear). In the absence of knowledge of the tonotopic maps, however, it was not possible to clearly relate the spectral patterns to the differences in papillar anatomy, suggesting that the determinants of these patterns may be more subtle than anticipated.

Comparative Auditory Neuroscience: Understanding the Evolution and Function of Ears

Comparative auditory studies make it possible both to understand the origins of modern ears and the factors underlying the similarities and differences in their performance. After all lineages of land vertebrates had independently evolved tympanic middle ears in the early Mesozoic era, the subsequent tens of millions of years led to the hearing organ of lizards, birds, and mammals becoming larger and their upper frequency limits higher. In extant species, lizard papillae remained relatively small (<2 mm), but avian papillae attained a maximum length of 11 mm, with the highest frequencies in both groups near 12 kHz. Hearing-organ sizes in modern mammals vary more than tenfold, up to >70 mm (made possible by coiling), as do their upper frequency limits (from 12 to >200 kHz). The auditory organs of the three amniote groups differ characteristically in their cellular structure, but their hearing sensitivity and frequency selectivity within their respective hearing ranges hardly differ. In the immediate primate ancestors of humans, the cochlea became larger and lowered its upper frequency limit. Modern humans show an unusual trend in frequency selectivity as a function of frequency. It is conceivable that the frequency selectivity patterns in humans were influenced in their evolution by the development of speech.

The effects of air pressure on spontaneous otoacoustic emissions of lizards

Small changes of air pressure outside the eardrum of five lizard species led to changes in frequency, level, and peak width of spontaneous otoacoustic emissions (SOAE). In contrast to humans, these changes generally occurred at very small pressures (<20 mbar). As in humans, SOAE amplitudes were generally reduced. Changes of SOAE frequency were both positive and negative, while in humans, they are mostly positive. In addition, in lizards, these effects often showed obvious hysteresis and non-repeatability. The correlation between peak width and height was negative in two species (comparable to humans) and positive in one species. In two other species, no correlation was found. Consequently, a simple oscillator model that explained the negative correlation in humans could not be generally applied to lizards. This presumably reflects the fact that in lizards, the spontaneous otoacoustic emission of sound from the ear consists of a combination of stable oscillations (as in humans), unstable narrow-band oscillations, and broad-band emissions, evident as "plateaus" in emission spectra.

A model for the relation between stimulus frequency and spontaneous otoacoustic emissions in lizard papillae

Spontaneous otoacoustic emissions (SOAEs) and stimulus frequency otoacoustic emissions (SFOAEs) have been described from lizard ears. Although there are several models for these systems, none has modeled the characteristics of both of these types of otoacoustic emissions based upon their being derived from hair cells as active oscillators. Data from the ears of two lizard species, one lacking a tectorial membrane and one with a chain of tectorial sallets, as described by Bergevin et al. ["Coupled, active oscillators and lizard otoacoustic emissions," AIP Conf. Proc. 1403, 453 (2008)], are modeled as an array of coupled self-sustained oscillators. The model, originally developed by Vilfan and Duke ["Frequency clustering in spontaneous otoacoustic emissions from a lizard's ear," Biophys. J. 95, 4622-4630 (2008)], well describes both the amplitude and phase characteristics of SFOAEs and the relation between SFOAEs and SOAEs.

Evidence for an auditory fovea in the New Zealand kiwi (Apteryx mantelli)

Kiwi are rare and strictly protected birds of iconic status in New Zealand. Yet, perhaps due to their unusual, nocturnal lifestyle, surprisingly little is known about their behaviour or physiology. In the present study, we exploited known correlations between morphology and physiology in the avian inner ear and brainstem to predict the frequency range of best hearing in the North Island brown kiwi. The mechanosensitive hair bundles of the sensory hair cells in the basilar papilla showed the typical change from tall bundles with few stereovilli to short bundles with many stereovilli along the apical-to-basal tonotopic axis. In contrast to most birds, however, the change was considerably less in the basal half of the epithelium. Dendritic lengths in the brainstem nucleus laminaris also showed the typical change along the tonotopic axis. However, as in the basilar papilla, the change was much less pronounced in the presumed high-frequency regions. Together, these morphological data suggest a fovea-like overrepresentation of a narrow high-frequency band in kiwi. Based on known correlations of hair-cell microanatomy and physiological responses in other birds, a specific prediction for the frequency representation along the basilar papilla of the kiwi was derived. The predicted overrepresentation of approximately 4-6 kHz matches potentially salient frequency bands of kiwi vocalisations and may thus be an adaptation to a nocturnal lifestyle in which auditory communication plays a dominant role.

Tectorial membrane morphological variation: effects upon stimulus frequency otoacoustic emissions

The tectorial membrane (TM) is widely believed to play an important role in determining the ear's ability to detect and resolve incoming acoustic information. While it is still unclear precisely what that role is, the TM has been hypothesized to help overcome viscous forces and thereby sharpen mechanical tuning of the sensory cells. Lizards present a unique opportunity to further study the role of the TM given the diverse inner-ear morphological differences across species. Furthermore, stimulus-frequency otoacoustic emissions (SFOAEs), sounds emitted by the ear in response to a tone, noninvasively probe the frequency selectivity of the ear. We report estimates of auditory tuning derived from SFOAEs for 12 different species of lizards with widely varying TM morphology. Despite gross anatomical differences across the species examined herein, low-level SFOAEs were readily measurable in all ears tested, even in non-TM species whose basilar papilla contained as few as 50-60 hair cells. Our measurements generally support theoretical predictions: longer delays/sharper tuning features are found in species with a TM relative to those without. However, SFOAEs from at least one non-TM species (Anolis) with long delays suggest there are likely additional micromechanical factors at play that can directly affect tuning. Additionally, in the one species examined with a continuous TM (Aspidoscelis) where cell-to-cell coupling is presumably relatively stronger, delays were intermediate. This observation appears consistent with recent reports that suggest the TM may play a more complex macromechanical role in the mammalian cochlea via longitudinal energy distribution (and thereby affect tuning). Although significant differences exist between reptilian and mammalian auditory biophysics, understanding lizard OAE generation mechanisms yields significant insight into fundamental principles at work in all vertebrate ears.

Interactions between hair cells shape spontaneous otoacoustic emissions in a model of the tokay gecko's cochlea

BACKGROUND

The hearing of tetrapods including humans is enhanced by an active process that amplifies the mechanical inputs associated with sound, sharpens frequency selectivity, and compresses the range of responsiveness. The most striking manifestation of the active process is spontaneous otoacoustic emission, the unprovoked emergence of sound from an ear. Hair cells, the sensory receptors of the inner ear, are known to provide the energy for such emissions; it is unclear, though, how ensembles of such cells collude to power observable emissions.

METHODOLOGY AND PRINCIPAL FINDINGS

We have measured and modeled spontaneous otoacoustic emissions from the ear of the tokay gecko, a convenient experimental subject that produces robust emissions. Using a van der Pol formulation to represent each cluster of hair cells within a tonotopic array, we have examined the factors that influence the cooperative interaction between oscillators.

CONCLUSIONS AND SIGNIFICANCE

A model that includes viscous interactions between adjacent hair cells fails to produce emissions similar to those observed experimentally. In contrast, elastic coupling yields realistic results, especially if the oscillators near the ends of the array are weakened so as to minimize boundary effects. Introducing stochastic irregularity in the strength of oscillators stabilizes peaks in the spectrum of modeled emissions, further increasing the similarity to the responses of actual ears. Finally, and again in agreement with experimental findings, the inclusion of a pure-tone external stimulus repels the spectral peaks of spontaneous emissions. Our results suggest that elastic coupling between oscillators of slightly differing strength explains several properties of the spontaneous otoacoustic emissions in the gecko.

Exceptional high-frequency hearing and matched vocalizations in Australian pygopod geckos

We describe exceptional high-frequency hearing and vocalizations in a genus of pygopod lizards (Delma) that is endemic to Australia. Pygopods are a legless subfamily of geckos and share their highly specialized hearing organ. Hearing and vocalizations of amniote vertebrates were previously thought to differ clearly in their frequency ranges according to their systematic grouping. The upper frequency limit would thus be lowest in chelonians and increasingly higher in crocodilians, lizards, birds and mammals. We report data from four Delma species (D. desmosa, D. fraseri, D. haroldi, D. pax) from the Pilbara region of Western Australia that were studied using recordings of auditory-nerve compound action potentials (CAP) under remote field conditions. Hearing limits and vocalization energy of Delma species extended to frequencies far above those reported for any other lizard group, 14 kHz and &gt;20 kHz, respectively. Their remarkable high-frequency hearing derives from the basilar papilla, and forward masking of CAP responses suggests a unique division of labor between groups of sensory cells within the hearing organ. These data also indicate that rather than having only strictly group-specific frequency ranges, amniote vertebrate hearing is strongly influenced by species-specific physical and ecological constraints.

Lizard auditory papillae: an evolutionary kaleidoscope

The evolutionary processes that modified the structure and function of lizard auditory papillae during the separation of the familial lineages during the Jurassic have resulted in a remarkable variety of family-typical papillae. These papillae vary structurally in their size, in the patterns of the distribution of hair-cell types, in the presence or absence of sub-papillae and in the configurations of the tectorial membranes. Functional differences, however, are much smaller than the structural variations might lead one to expect. To some extent, differences in innervation patterns and tectorial configurations compensate for 10-fold differences in papillar length. Nonetheless, although lizards with tiny papillae are able to maintain frequency-selective and relatively sensitive hearing, the best selectivity and most sensitive hearing is found in the largest and most complex papillae. Fundamental considerations of the tonotopic organisation of papillae leads to a likely scheme mapping the evolution of the hearing organs found in modern lizard families.

Efferent innervation to the auditory basilar papilla of scincid lizards

Hair cells of the inner ear of vertebrates are innervated by afferent neurons that transmit sensory information to the brain as well as efferent neurons that receive feedback from the brainstem. The function of the efferent feedback system is poorly understood and may have changed during evolution when different tetrapod groups acquired sensitivity to airborne sound and extended their hearing ranges to higher frequencies. Lizards show a unique subdivision of their basilar papilla (homologous to the mammalian organ of Corti) into a low-frequency (<1 kHz) and a high-frequency (approximately 1-5 kHz) region. The high-frequency region was reported to have lost its efferent innervation, suggesting it was insignificant or even functionally detrimental at higher frequencies. We re-examined the innervation to the basilar papilla of five species of Australian scincid lizards, by using immunohistochemistry. Anti-choline acetyltransferase (ChAT) was used as an efferent marker. Co-localization with anti-synaptic vesicle protein 2 confirmed the synaptic identity of label. Cholinergic terminals were observed along the whole length of the basilar papilla, including the regions that had previously been described as devoid of efferent innervation. However, there was a clear decrease in terminal density from apical, low-frequency to basal, high-frequency locations. Our findings suggest that efferent innervation is a general feature of the hair cells in the basilar papilla of lizards, irrespective of tonotopic location. This re-enforces the notion that efferent feedback control of hair cells is a fundamental and important property of all vertebrate hearing organs.

Spontaneous otoacoustic emissions in lizards: a comparison of the skink-like lizard families Cordylidae and Gerrhosauridae

Lizard families can be grouped into larger units comprising those families that are closely related and whose auditory papillae are morphologically very similar. Based on the few species studied at that time [Manley, G.A., 1997. Diversity in hearing-organ structure and the characteristics of spontaneous otoacoustic emissions in lizards. In: Lewis, E.R., Long, G.R., Lyon, R.F., Narins, P.M., Steele, C.R. (Eds.), Diversity in Auditory Mechanics. World Scientific Publishing Co., Singapore, pp. 32-38], it was suggested that SOAE spectral patterns are strongly influenced by papillar anatomy. However, in two family groups, only one single species has been studied and we have no data on the regularity of pattern within related lizard families. Within the group of skink-like lizards, whose papillae all have salletal tectorial structures, the only detailed SOAE studies so far were on the skink genus Tiliqua. To ascertain the similarity of SOAE in species from families related to the skinks, we have studied one species each from two families that are closely related to skinks, the Cordylidae (Girdle-tailed lizards) and the Gerrhosauridae (plated lizards). Gerrhosaurus and Cordylus have a similar number and amplitudes of SOAE to Tiliqua (Skinkidae). The maximal frequency shifts of SOAE under the influence of external tones is also similar to that of Tiliqua. However, the maximal suppression and maximal facilitation are smaller. In general, the patterns displayed by the SOAE of lizards of these two new families are recognizably similar to the skink Tiliqua, suggesting that the anatomy of the papilla and the tectorial structures do play an important role in determining how SOAE are manifested in papillae that possess tectorial sallets.

Frequency clustering in spontaneous otoacoustic emissions from a lizard's ear

Spontaneous otoacoustic emissions (SOAEs) are indicators of an active process in the inner ear that enhances the sensitivity and frequency selectivity of hearing. They are particularly regular and robust in certain lizards, so these animals are good model organisms for studying how SOAEs are generated. We show that the published properties of SOAEs in the bobtail lizard are wholly consistent with a mathematical model in which active oscillators, with exponentially varying characteristic frequencies, are coupled together in a chain by visco-elastic elements. Physically, each oscillator corresponds to a small group of hair cells, covered by a tectorial sallet, so our theoretical analysis directly links SOAEs to the micromechanics of active hair bundles.

The effect of hair bundle shape on hair bundle hydrodynamics of non-mammalian inner ear hair cells for the full frequency range

The effect of the size and the shape of the hair bundle of a hair cell in the inner ear of non-mammals on its motion for the full range of frequencies is determined thereby extending the results of a previous analysis of hair bundle motion for high and low frequencies [Hear Res. 141 (2000) 39-50]. A hemispheroid is used to represent the hair bundle because it can represent a full range of shapes, from thin, pencil-like shapes to wide, flat, disk-like shapes. Boundary element methods are used to approximate the solution for the hydrodynamics. For physiologically relevant parameters, an excellent match is obtained between the model's predictions and measurements of hair bundle motion in the free-standing region of the basilar papilla of the alligator lizard [Aranyosi, Measuring sound-induced motions of the alligator lizard cochlea. Massachusetts Institute of Technology, PhD Thesis, 2002]. Neither in the model's predictions nor in experimental measurements is sharp tuning observed. The model predicted the low frequency region of neural tuning curves for the alligator lizard and bobtail lizard, but could not predict the sharp tuning or the high frequency region. An element that represents an active mechanism is added to the hair bundle model to predict neural tuning curves, which are sharply tuned, and an excellent match is obtained for all the characteristics of neural tuning curves for the alligator lizard, and for the low and high frequency regions for the bobtail lizard. The model does not predict well the sharp tuning of the shorter hair bundles of the bobtail lizard, possibly because it does not represent tectorial sallets.

In vivo evidence for a cochlear amplifier in the hair-cell bundle of lizards

Vertebrate sensory hair cells achieve high sensitivity and frequency selectivity by adding self-generated mechanical energy to low-level signals. This allows them to detect signals that are smaller than thermal molecular motion and to achieve significant resonance amplitudes and frequency selectivity despite the viscosity of the surrounding fluid. In nonmammals, a great deal of in vitro evidence indicates that the active process responsible for this amplification is intimately associated with the hair cells' transduction channels in the stereovillar bundle. Here, we provide in vivo evidence of hair-cell bundle involvement in active processes. Electrical stimulation of the inner ear of a lizard at frequencies typical for this hearing organ induced low-level otoacoustic emissions that could be modulated by low-frequency sound. The unique modulation pattern permitted the tracing of the active process involved to the stereovillar bundles of the sensory hair cells. This supports the notion that, in nonmammals, the cochlear amplifier in the hair cells is driven by a bundle motor system.

The effect of hair bundle shape on hair bundle hydrodynamics of inner ear hair cells at low and high frequencies

The relationship between size and shape of the hair bundle of a hair cell in the inner ear and its sensitivity at asymptotically high and low frequencies was determined, thereby extending the results of an analysis of hair bundle hydrodynamics in two dimensions (Freeman and Weiss, 1990. Hydrodynamic analysis of a two-dimensional model for micromechanical resonance of free-standing hair bundles. Hear. Res. 48, 37-68) to three dimensions. A hemispheroid was used to represent the hair bundle. The hemispheroid had a number of advantages: it could represent shapes that range from thin, pencil-like shapes, to wide, flat, disk-like shapes. Also analytic methods could be used in the high frequency range to obtain an exact solution to the equations of motion. In the low frequency range, where an approximate solution was found using boundary element methods, the sensitivity of the responses of hair cells was mainly proportional to the cube of the heights of their hair bundles, and at high frequencies, the sensitivity of the hair cells was mainly proportional to the inverse of their heights. An excellent match was obtained between measurements of sensitivity curves in the basillar papilla of the alligator and bobtail lizards and the model's predictions. These results also suggest why hair bundles of hair cells in vestibular organs which are sensitive to low frequencies have ranges of heights that are an order of magnitude larger than the range of heights of hair bundles of hair cells found in auditory organs.

Quantitative anatomical basis for a model of micromechanical frequency tuning in the Tokay gecko, Gekko gecko

The basilar papilla of the Tokay gecko was studied with standard light- and scanning electron microscopy methods. Several parameters thought to be of particular importance for the mechanical response properties of the system were quantitatively measured, separately for the three different hair-cell areas that are typical for this lizard family. In the basal third, papillar structure was very uniform. The apical two-thirds are subdivided into two hair-cell areas running parallel to each other along the papilla and covered by very different types of tectorial material. Both of those areas showed prominent gradients in hair-cell bundle morphology, i.e., in the height of the stereovillar bundles and the number of stereovilli per bundle, as well as in hair cell density and the size of their respective tectorial covering. Based on the direction of the observed anatomical gradients, a 'reverse' tonotopic organization is suggested, with the highest frequencies represented at the apical end.

A model of frequency tuning in the basilar papilla of the Tokay gecko, Gekko gecko

This paper uses the quantitative details of the anatomy of the auditory papilla in the Tokay gecko Gekko gecko (as described in the companion paper) to make a quantitative model predicting the tonotopic organization of two of the three papillar areas. Assuming that hair-cell bundle stiffness is similar to that of other species, a model of resonance frequencies for the apical areas of the papilla was constructed, taking into account factors such as the number of hair cells per resonant unit, their bundle dimensions, the volume of the tectorial mass, etc. The model predicts that the apical pre- and postaxial areas, although anatomically adjacent, respond to different frequency ranges, a phenomenon not yet reported from any vertebrate. The model predicts that together, these areas respond best to frequencies between 1.1 and 5.3 kHz, close to the range found physiologically [Eatock et al. (1981) J. Comp. Physiol. 142, 203-218] (0.8 to 5 kHz) for the high-frequency range for this species. Only physiological experiments tracing responses to specific papillar nerve fibres can confirm or refute these interesting predictions of the model. The model also indicates that, compared to free-standing hair-cell bundles, the semi-isolated tectorial structures called sallets not only lower the range of characteristic frequencies but also increase the frequency selectivity of the attached hair cells.

Spontaneous otoacoustic emissions in the bobtail lizard. II: Interactions with external tones

The response of spontaneous otoacoustic emissions to the presentation of external tones was studied in the Australian bobtail lizard. Three basic types of effects were observed: suppression (a reduction in the emission's amplitude), facilitation (an increase in the emission's amplitude) and frequency shifting. The suppressive effect was highly frequency selective. Iso-suppression tuning curves resembled the rate-threshold tuning curves of the high-frequency population of VIIIth nerve fibres in this species. The frequency with the lowest threshold for suppression corresponded, on average, to the emission's own frequency and did not show any systematic deviation from it. Facilitation of between 2 and 10 dB occurred, but only in response to frequencies within certain narrow ranges, and at sound pressure levels below those that suppressed. The most commonly-observed facilitation range lay between 0.2 and 0.6 octaves above the emission's own frequency and coincided in frequency with a characteristic notch in the iso-suppression tuning curve. In the same narrow frequency range, the input/output functions of amplitude suppression always showed a pronounced increase in slope. The emissions moved their own frequency away from that of an external tone. The observed shifts were comparatively large (up to -330 Hz) and were more pronounced in the downward direction.

Spontaneous otoacoustic emissions in the bobtail lizard. I: General characteristics

Spontaneous otoacoustic emissions in the external ear canal of the bobtail lizard were identified on the basis of their consistent presence, their temperature- and hypoxia-dependence, and their suppressibility by external tones. They were found in 86% of ears investigated, and each ear generated on average 10 emissions. Their sound-pressure levels lay between -10 and 9.3 dB SPL, and their centre frequencies between 0.93 and 4.61 kHz at 30 degrees C body temperature. Previous studies have shown that these frequencies are processed in the basal basilar-papillar segment by hair-cell areas that are strictly bidirectionally oriented and are covered by tectorial sallets. In contrast, no spontaneous otoacoustic emissions were found in the frequency range known to be processed by the apical, low-frequency segment of the basilar papilla. The mean frequency distance between emissions varied systematically across the frequency range in a way consistent with the hypothesis that they are generated by anatomically-defined groups of hair cells and their tectorial sallets. The 3dB-bandwidth of the emissions depended on their amplitude above the noise, but was at least 9 Hz.

Observations of the sensing and the tectorial membrane in bullfrog amphibian papilla: their possible functional roles

Three dimensional reconstructions of the amphibian papilla were performed with light microscopic observations, mainly for the sensing membrane (SM). In horizontal sections of the papilla, the anteromedial end of the SM, which makes contact with the massive anterior portion of the tectorial membrane (TM), is several times thicker than the posterolateral end close to the column of the innervating nerves. This gradient of thickness is observed in all the sections from the dorsal portion attached to the TM to the ventral floor of the papilla. The SM connects to the TM in a topological manner; the anteromedial portion of the TM relates to the anterior end of the SM and the anterolateral and the middle portions of the TM correspond to the sites shifting posteriorly on the SM. The morphology of the SM and its manner of connection to the TM suggest that the SM plays important roles in the occurrence of frequency selectivity and of tonotopic organization of the amphibian papilla.