Spontaneous otoacoustic emissions from free-standing stereovillar bundles of ten species of lizard with small papillae

Otoacoustic Emissions in Non-Mammals

Otoacoustic emissions (OAE) that were sound-induced, current-induced, or spontaneous have been measured in non-mammalian land vertebrates, including in amphibians, reptiles, and birds. There are no forms of emissions known from mammals that have not also been observed in non-mammals. In each group and species, the emission frequencies clearly lie in the range known to be processed by the hair cells of the respective hearing organs. With some notable exceptions, the patterns underlying the measured spectra, input-output functions, suppression threshold curves, etc., show strong similarities to OAE measured in mammals. These profound similarities are presumably traceable to the fact that emissions are produced by active hair-cell mechanisms that are themselves dependent upon comparable nonlinear cellular processes. The differences observed—for example, in the width of spontaneous emission peaks and delay times in interactions between peaks—should provide insights into how hair-cell activity is coupled within the organ and thus partially routed out into the middle ear.

Evidence supporting synchrony between two active ears due to interaural coupling

Motivated by recent developments suggesting that interaural coupling in non-mammals allows for the two active ears to effectively synchronize, this report describes otoacoustic measurements made in the oral cavity of lizards. As expected from that model, spontaneous otoacoustic emissions (SOAEs) were readily measurable in the mouth, which is contiguous with the interaural airspace. Additionally, finite element model calculations were made to simulate the interaural acoustics based upon SOAE-related tympanic membrane vibrational data. Taken together, these data support the notion of two active ears synchronizing by virtue of acoustic coupling and have potential implications for sound localization at low-levels.

Suppression tuning of spontaneous otoacoustic emissions in the barn owl (Tyto alba)

Spontaneous otoacoustic emissions (SOAEs) have been observed in a variety of different vertebrates, including humans and barn owls (Tyto alba). The underlying mechanisms producing the SOAEs and the meaning of their characteristics regarding the frequency selectivity of an individual and species are, however, still under debate. In the present study, we measured SOAE spectra in lightly anesthetized barn owls and suppressed their amplitudes by presenting pure tones at different frequencies and sound levels. Suppression effects were quantified by deriving suppression tuning curves (STCs) with a criterion of 2 dB suppression. SOAEs were found in 100% of ears (n = 14), with an average of 12.7 SOAEs per ear. Across the whole SOAE frequency range of 3.4-10.2 kHz, the distances between neighboring SOAEs were relatively uniform, with a median distance of 430 Hz. The majority (87.6%) of SOAEs were recorded at frequencies that fall within the barn owl's auditory fovea (5-10 kHz). The STCs were V-shaped and sharply tuned, similar to STCs from humans and other species. Between 5 and 10 kHz, the median Q_10dB value of STC was 4.87 and was thus lower than that of owl single-unit neural data. There was no evidence for secondary STC side lobes, as seen in humans. The best thresholds of the STCs varied from 7.0 to 57.5 dB SPL and correlated with SOAE level, such that smaller SOAEs tended to require a higher sound level to be suppressed. While similar, the frequency-threshold curves of auditory-nerve fibers and STCs of SOAEs differ in some respects in their tuning characteristics indicating that SOAE suppression tuning in the barn owl may not directly reflect neural tuning in primary auditory nerve fibers.

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.

An active oscillator model describes the statistics of spontaneous otoacoustic emissions

Even in the absence of external stimulation, the cochleas of most humans emit very faint sounds below the threshold of hearing, sounds that are known as spontaneous otoacoustic emissions. They are a signature of the active amplification mechanism in the cochlea. Emissions occur at frequencies that are unique for an individual and change little over time. The statistics of a population of ears exhibit characteristic features such as a preferred relative frequency distance between emissions (interemission intervals). We propose a simplified cochlea model comprising an array of active nonlinear oscillators coupled both hydrodynamically and viscoelastically. The oscillators are subject to a weak spatial disorder that lends individuality to the simulated cochlea. Our model captures basic statistical features of the emissions: distributions of 1), emission frequencies; 2), number of emissions per ear; and 3), interemission intervals. In addition, the model reproduces systematic changes of the interemission intervals with frequency. We show that the mechanism for the preferred interemission interval in our model is the occurrence of synchronized clusters of oscillators.

Integrating the active process of hair cells with cochlear function

Uniquely among human senses, hearing is not simply a passive response to stimulation. Our auditory system is instead enhanced by an active process in cochlear hair cells that amplifies acoustic signals several hundred-fold, sharpens frequency selectivity and broadens the ear's dynamic range. Active motility of the mechanoreceptive hair bundles underlies the active process in amphibians and some reptiles; in mammals, this mechanism operates in conjunction with prestin-based somatic motility. Both individual hair bundles and the cochlea as a whole operate near a dynamical instability, the Hopf bifurcation, which accounts for the cardinal features of the active process.

The physics of hearing: fluid mechanics and the active process of the inner ear

Most sounds of interest consist of complex, time-dependent admixtures of tones of diverse frequencies and variable amplitudes. To detect and process these signals, the ear employs a highly nonlinear, adaptive, real-time spectral analyzer: the cochlea. Sound excites vibration of the eardrum and the three miniscule bones of the middle ear, the last of which acts as a piston to initiate oscillatory pressure changes within the liquid-filled chambers of the cochlea. The basilar membrane, an elastic band spiraling along the cochlea between two of these chambers, responds to these pressures by conducting a largely independent traveling wave for each frequency component of the input. Because the basilar membrane is graded in mass and stiffness along its length, however, each traveling wave grows in magnitude and decreases in wavelength until it peaks at a specific, frequency-dependent position: low frequencies propagate to the cochlear apex, whereas high frequencies culminate at the base. The oscillations of the basilar membrane deflect hair bundles, the mechanically sensitive organelles of the ear's sensory receptors, the hair cells. As mechanically sensitive ion channels open and close, each hair cell responds with an electrical signal that is chemically transmitted to an afferent nerve fiber and thence into the brain. In addition to transducing mechanical inputs, hair cells amplify them by two means. Channel gating endows a hair bundle with negative stiffness, an instability that interacts with the motor protein myosin-1c to produce a mechanical amplifier and oscillator. Acting through the piezoelectric membrane protein prestin, electrical responses also cause outer hair cells to elongate and shorten, thus pumping energy into the basilar membrane's movements. The two forms of motility constitute an active process that amplifies mechanical inputs, sharpens frequency discrimination, and confers a compressive nonlinearity on responsiveness. These features arise because the active process operates near a Hopf bifurcation, the generic properties of which explain several key features of hearing. Moreover, when the gain of the active process rises sufficiently in ultraquiet circumstances, the system traverses the bifurcation and even a normal ear actually emits sound. The remarkable properties of hearing thus stem from the propagation of traveling waves on a nonlinear and excitable medium.

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.

Comparison of otoacoustic emissions within gecko subfamilies: morphological implications for auditory function in lizards

Otoacoustic emissions (OAEs) are sounds emitted by the ear and provide a non-invasive probe into mechanisms underlying peripheral auditory transduction. This study focuses upon a comparison of emission properties in two phylogenetically similar pairs of gecko: Gekko gecko and Hemidactylus turcicus and Eublepharis macularius and Coleonyx variegatus. Each pair consists of two closely related species within the same subfamily, with quantitatively known morphological properties at the level of the auditory sensory organ (basilar papilla) in the inner ear. Essentially, the comparison boils down to an issue of size: how does overall body size, as well as the inner-ear dimensions (e.g., papilla length and number of hair cells), affect peripheral auditory function as inferred from OAEs? Estimates of frequency selectivity derived from stimulus-frequency emissions (emissions evoked by a single low-level tone) indicate that tuning is broader in the species with fewer hair cells/shorter papilla. Furthermore, emissions extend outwards to higher frequencies (for similar body temperatures) in the species with the smaller body size/narrower interaural spacing. This observation suggests the smaller species have relatively improved high-frequency sensitivity, possibly related to vocalizations and/or aiding azimuthal sound localization. For one species (Eublepharis), emissions were also examined in both juveniles and adults. Qualitatively similar emission properties in both suggests that inner-ear function is adult like soon after hatching and that external body size (e.g., middle-ear dimensions and interaural spacing) has a relatively small impact upon emission properties within a species.

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.

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.

Mechanical amplification by hair cells in the semicircular canals

Sensory hair cells are the essential mechanotransducers of the inner ear, responsible not only for the transduction of sound and motion stimuli but also, remarkably, for nanomechanical amplification of sensory stimuli. Here we show that semicircular canal hair cells generate a mechanical nonlinearity in vivo that increases sensitivity to angular motion by amplification at low stimulus strengths. Sensitivity at high stimulus strengths is linear and shows no evidence of amplification. Results suggest that the mechanical work done by hair cells contributes approximately 97 zJ/cell of amplification per stimulus cycle, improving sensitivity to angular velocity stimuli below approximately 5 degrees /s (0.3-Hz sinusoidal motion). We further show that mechanical amplification can be inhibited by the brain via activation of efferent synaptic contacts on hair cells. The experimental model was the oyster toadfish, Opsanus tau. Physiological manifestation of mechanical amplification and efferent control in a teleost vestibular organ suggests the active motor process in sensory hair cells is ancestral. The biophysical basis of the motor(s) remains hypothetical, but a key discriminating question may involve how changes in somatic electrical impedance evoked by efferent synaptic action alter function of the motor(s).

Vigilance against predators induced by eavesdropping on heterospecific alarm calls in a non-vocal lizard Oplurus cuvieri cuvieri (Reptilia: Iguania)

Prey animals can reduce their risk of predation by detecting potential predators before encounters occur. Some animals gain information about nearby predators by eavesdropping on heterospecific alarm calls. Despite having well-developed ears, most lizards do not use vocal information for intraspecific communication, and few studies have shown practical use of the ears in wild lizards. Here, we show that the Madagascan spiny-tailed iguana (Oplurus cuvieri cuvieri) obtains auditory signals for predator detection. The Madagascan spiny-tailed iguana and the Madagascar paradise flycatcher (Terpsiphone mutata) are syntopic inhabitants of the Ampijoroa dry deciduous forest of Madagascar. The iguana and the flycatcher have neither a predator-prey relationship nor resource competition, but they have shared predators such as raptors and snakes. Using playback experiments, we demonstrated that the iguana discriminates mobbing alarm calls of the flycatcher from its songs and then enhances its vigilance behaviour. Our results demonstrate the occurrence of an asymmetrical ecological relationship between the Madagascan spiny-tailed iguana and the paradise flycatcher through eavesdropping on information about the presence of predators. This implies that indirect interspecific interactions through information recognition may be more common than generally thought in an animal community.

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.

Effects of low-frequency biasing on spontaneous otoacoustic emissions: frequency modulation

It was previously reported that low-frequency biasing of cochlear structures can suppress and modulate the amplitudes of spontaneous otoacoustic emissions (SOAEs) in humans [Bian, L. and Watts, K. L. (2008). "Effects of low-frequency biasing on spontaneous otoacoustic emissions: Amplitude modulation," J. Acoust. Soc. Am. 123, 887-898]. In addition to amplitude modulation, the bias tone produced an upward shift of the SOAE frequency and a frequency modulation. These frequency effects usually occurred prior to significant modifications of SOAE amplitudes and were dependent on the relative strength of the bias tone and a particular SOAE. The overall SOAE frequency shifts were usually less than 2%. A quasistatic modulation pattern showed that biasing in either positive or negative pressure direction increased SOAE frequency. The instantaneous SOAE frequency revealed a "W-shaped" modulation pattern within one biasing cycle. The SOAE frequency was maximal at the biasing extremes and minimized at the zero crossings of the bias tone. The temporal modulation of SOAE frequency occurred with a short delay. These static and dynamic effects indicate that modifications of the mechanical properties of the cochlear transducer could underlie the frequency shift and modulation. These biasing effects are consistent with the suppression and modulation of SOAE amplitude due to shifting of the cochlear transducer operating point.

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.

What Do Sex, Twins, Spotted Hyenas, ADHD, and Sexual Orientation Have in Common?

The otoacoustic emissions (OAEs) measured in a collection of special populations of humans and certain nonhuman species suggest that OAEs may provide a window into some processes of human prenatal development and sexual differentiation. For reasons that are unclear, OAEs appear to be highly sensitive to events occurring during prenatal development that seem to be related to the degree of exposure to androgens a fetus receives. The (largely circumstantial) evidence for a relationship between androgen exposure and OAE strength comes from a series of studies of twins, children with attention-deficit/hyperactivity disorder, people of differing sexual orientations, and spotted hyenas, among others. Some conclusions are bolstered by parallel studies using auditory evoked potentials (AEPs). OAEs and AEPs are simple, objective, noninvasive measures that appear to have potential as tools of value to researchers working on a wide variety of basic and applied topics beyond audition.

Otoacoustic emissions from insect ears: evidence of active hearing?

Sensitive hearing organs often employ nonlinear mechanical sound processing which generates distortion-product otoacoustic emissions (DPOAE). Such emissions are also recordable from tympanal organs of insects. In vertebrates (including humans), otoacoustic emissions are considered by-products of active sound amplification through specialized sensory receptor cells in the inner ear. Force generated by these cells primarily augments the displacement amplitude of the basilar membrane and thus increases auditory sensitivity. As in vertebrates, the emissions from insect ears are based on nonlinear mechanical properties of the sense organ. Apparently, to achieve maximum sensitivity, convergent evolutionary principles have been realized in the micromechanics of these hearing organs-although vertebrates and insects possess quite different types of receptor cells in their ears. Just as in vertebrates, otoacoustic emissions from insects ears are vulnerable and depend on an intact metabolism, but so far in tympanal organs, it is not clear if auditory nonlinearity is achieved by active motility of the sensory neurons or if passive cellular characteristics cause the nonlinear behavior. In the antennal ears of flies and mosquitoes, however, active vibrations of the flagellum have been demonstrated. Our review concentrates on experiments studying the tympanal organs of grasshoppers and moths; we show that their otoacoustic emissions are produced in a frequency-specific way and can be modified by electrical stimulation of the sensory cells. Even the simple ears of notodontid moths produce distinct emissions, although they have just one auditory neuron. At present it is still uncertain, both in vertebrates and in insects, if the nonlinear amplification so essential for sensitive sound processing is primarily due to motility of the somata of specialized sensory cells or to active movement of their (stereo-)cilia. We anticipate that further experiments with the relatively simple ears of insects will help answer these questions.

Mechanics of the exceptional anuran ear

The anuran ear is frequently used for studying fundamental properties of vertebrate auditory systems. This is due to its unique anatomical features, most prominently the lack of a basilar membrane and the presence of two dedicated acoustic end organs, the basilar papilla and the amphibian papilla. Our current anatomical and functional knowledge implies that three distinct regions can be identified within these two organs. The basilar papilla functions as a single auditory filter. The low-frequency portion of the amphibian papilla is an electrically tuned, tonotopically organized auditory end organ. The high-frequency portion of the amphibian papilla is mechanically tuned and tonotopically organized, and it emits spontaneous otoacoustic emissions. This high-frequency portion of the amphibian papilla shows a remarkable, functional resemblance to the mammalian cochlea.

What have lizard ears taught us about auditory physiology?

The structure of the basilar papilla of the inner ear of lizards is the most diverse among all vertebrates. Research on a variety of lizard ears, animals that are remarkably robust under laboratory conditions, has provided the field of auditory research with valuable information, particularly on the minimum structural requirements for sensitive, selective hearing and on the importance of the tectorial membrane and active processes in this regard. Despite the absence of a tuned basilar membrane, lizard ears produce highly frequency selective hearing through micromechanical tuning of small, resonant hair-cell-tectorial units or of free-standing hair bundles. These units are driven by an active process that also underlies spontaneous and other otoacoustic emissions. Lizard ears provided the first in vivo evidence that the active process is calcium-sensitive and lies within the stereovillar bundles of the hair cells.