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

A point-wise artifact rejection method for estimating transient-evoked otoacoustic emissions and their group delay

Due to its low intensity, measurement of transient-evoked otoacoustic emission (TEOAE) requires repeated stimulation. When any acoustic artifact occurs, an entire click interval is typically abandoned. Here, a point-wise artifact rejection strategy is proposed, and it partially preserves the data when artifacts occur in an interval. At the noisiest setting (-46 dB signal-to-noise ratio) the proposed strategy retains four times more data and thereby reduces the root mean square signal estimation error by over 60%. Consequently, the group delay can be calculated more accurately. These findings might facilitate TEOAE measurement at home or in other noisy environments in the future.

Vocalization by extant nonavian reptiles: A synthetic overview of phonation and the vocal apparatus

Among amniote vertebrates, nonavian reptiles (chelonians, crocodilians, and lepidosaurs) are regarded as using vocal signals rarely (compared to birds and mammals). In all three reptilian clades, however, certain taxa emit distress calls and advertisement calls using modifications of regions of the upper respiratory tract. There is no central tendency in either acoustic mechanisms or the structure of the vocal apparatus, and many taxa that vocalize emit only relatively simple sounds. Available evidence indicates multiple origins of true vocal abilities within these lineages. Reptiles thus provide opportunities for studying the early evolutionary stages of vocalization. The early literature on the diversity of form of the laryngotracheal apparatus of reptiles boded well for the study of form-function relationships, but this potential was not extensively explored. Emphasis shifted away from anatomy, however, and centered instead on acoustic analysis of the sounds that are produced. New investigative techniques have provided novel ways of studying the form-function aspects of the structures involved in phonation and have brought anatomical investigation to the forefront again. In this review we summarize what is known about hearing in reptiles in order to contextualize the vocal signals they generate and the sound-producing mechanisms responsible for them. The diversity of form of the sound producing apparatus and the increasing evidence that reptiles are more dependent upon vocalization as a communication medium than previously thought indicates that they have a significant role to play in the understanding of the evolution of vocalization in amniotes.

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.

Gekko japonicus genome reveals evolution of adhesive toe pads and tail regeneration

Reptiles are the most morphologically and physiologically diverse tetrapods, and have undergone 300 million years of adaptive evolution. Within the reptilian tetrapods, geckos possess several interesting features, including the ability to regenerate autotomized tails and to climb on smooth surfaces. Here we sequence the genome of Gekko japonicus (Schlegel's Japanese Gecko) and investigate genetic elements related to its physiology. We obtain a draft G. japonicus genome sequence of 2.55 Gb and annotated 22,487 genes. Comparative genomic analysis reveals specific gene family expansions or reductions that are associated with the formation of adhesive setae, nocturnal vision and tail regeneration, as well as the diversification of olfactory sensation. The obtained genomic data provide robust genetic evidence of adaptive evolution in reptiles.

Geckos are small, agile reptiles with nocturnal habits. Here, the authors sequence the genome of the Schlegel's Japanese Gecko and reveal gene family expansions and reductions associated with formation of adhesive setae, nocturnal vision, tail regeneration, and diversification of olfactory sensation.

A resonance approach to cochlear mechanics

BACKGROUND

How does the cochlea analyse sound into its component frequencies? In the 1850s Helmholtz thought it occurred by resonance, whereas a century later Békésy's work indicated a travelling wave. The latter answer seemed to settle the question, but with the discovery in 1978 that the cochlea emits sound, the mechanics of the cochlea was back on the drawing board. Recent studies have raised questions about whether the travelling wave, as currently understood, is adequate to explain observations.

APPROACH

Applying basic resonance principles, this paper revisits the question. A graded bank of harmonic oscillators with cochlear-like frequencies and quality factors is simultaneously excited, and it is found that resonance gives rise to similar frequency responses, group delays, and travelling wave velocities as observed by experiment. The overall effect of the group delay gradient is to produce a decelerating wave of peak displacement moving from base to apex at characteristic travelling wave speeds. The extensive literature on chains of coupled oscillators is considered, and the occurrence of travelling waves, pseudowaves, phase plateaus, and forced resonance in such systems is noted.

CONCLUSION AND SIGNIFICANCE

This alternative approach to cochlear mechanics shows that a travelling wave can simply arise as an apparently moving amplitude peak which passes along a bank of resonators without carrying energy. This highlights the possible role of the fast pressure wave and indicates how phase delays and group delays of a set of driven harmonic oscillators can generate an apparent travelling wave. It is possible to view the cochlea as a chain of globally forced coupled oscillators, and this model incorporates fundamental aspects of both the resonance and travelling wave theories.

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.

Probing cochlear tuning and tonotopy in the tiger using otoacoustic emissions

Otoacoustic emissions (sound emitted from the ear) allow cochlear function to be probed noninvasively. The emissions evoked by pure tones, known as stimulus-frequency emissions (SFOAEs), have been shown to provide reliable estimates of peripheral frequency tuning in a variety of mammalian and non-mammalian species. Here, we apply the same methodology to explore peripheral auditory function in the largest member of the cat family, the tiger (Panthera tigris). We measured SFOAEs in 9 unique ears of 5 anesthetized tigers. The tigers, housed at the Henry Doorly Zoo (Omaha, NE), were of both sexes and ranged in age from 3 to 10 years. SFOAE phase-gradient delays are significantly longer in tigers--by approximately a factor of two above 2 kHz and even more at lower frequencies--than in domestic cats (Felis catus), a species commonly used in auditory studies. Based on correlations between tuning and delay established in other species, our results imply that cochlear tuning in the tiger is significantly sharper than in domestic cat and appears comparable to that of humans. Furthermore, the SFOAE data indicate that tigers have a larger tonotopic mapping constant (mm/octave) than domestic cats. A larger mapping constant in tiger is consistent both with auditory brainstem response thresholds (that suggest a lower upper frequency limit of hearing for the tiger than domestic cat) and with measurements of basilar-membrane length (about 1.5 times longer in the tiger than domestic cat).

Interrelationships between spontaneous and low-level stimulus-frequency otoacoustic emissions in humans

It has been proposed that OAEs be classified not on the basis of the stimuli used to evoke them, but on the mechanisms that produce them (Shera and Guinan, 1999). One branch of this taxonomy focuses on a coherent reflection model and explicitly describes interrelationships between spontaneous emissions (SOAEs) and stimulus-frequency emissions (SFOAEs). The present study empirically examines SOAEs and SFOAEs from individual ears within the context of model predictions, using a low stimulus level (20 dB SPL) to evoke SFOAEs. Emissions were recorded from ears of normal-hearing young adults, both with and without prominent SOAE activity. When spontaneous activity was observed, SFOAEs demonstrated a localized increase about the SOAE peaks. The converse was not necessarily true though, i.e., robust SFOAEs could be measured where no SOAE peaks were observed. There was no significant difference in SFOAE phase-gradient delays between those with and without observable SOAE activity. However, delays were larger for a 20 dB SPL stimulus level than those previously reported for 40 dB SPL. The total amount of SFOAE phase accumulation occurring between adjacent SOAE peaks tended to cluster about an integral number of cycles. Overall, the present data appear congruous with predictions stemming from the coherent reflection model and support the notion that such comparisons ideally are made with emissions evoked using relatively lower stimulus levels.