Biology subjects

Bergevin, C.

Publications and source records attributed to Bergevin, C..

5 recordsLinked to original sources

bioRxiv2025

Synchronization of two active ears via binaural coupling

The two ears of many non-mammalian vertebrates are acoustically coupled through an interaural cavity. Sounds can hit both the external and internal surfaces of the tympanic membranes, providing mechanical directional sensitivity for sound localization. This coupling may also give rise to binaural synchronization. In other words, each inner ear could influence the function of the other. Previous work in lizards demonstrates that binaural coupling affects the spontaneous otoacoustic emissions (SOAEs) measured at each external auditory meatus, with properties indicative of binaural synchronization. However, it is unclear how binaural coupling and, consequently, synchronization contribute to SOAE generation, which is typically modelled as being localized to an individual ear. We simultaneously measured SOAEs at both ears of green anole lizards (Anolis carolinensis) and found robust relationships between them, including evidence that binaural synchronization could not be attributed to one ear uniformly driving the other. Instead, we observed frequency-dependent phase-locking between the two ears, primarily at frequencies where SOAE peaks occurred in both ears. While some pairs of ears were more strongly synchronized than others, we consistently found that binaural coupling could have greater effects on the resulting emissions than the coupling between generators within the same ear. We propose a framework for active hearing that incorporates binaural coupling, accounting for its effects on SOAE generation and sound localization in the green anole.

neuroscience↗

bioRxiv2025

Spontaneous otocoherence of the active ear

Spontaneous otoacoustic emission (SOAE) provides compelling evidence of active force generation inside the inner ear, although there is significant debate about the underlying generation mechanism(s). SOAE is commonly characterized by peaks in a spectral domain representation (as derived from a discrete Fourier transform), occurring at idiosyncratic frequencies unique to a given ear. Such is typically computed as an averaged magnitude spectrum that discards phase information. Here, we explore the hypothesis that SOAE phase, readily extracted from pre-existing recordings, contains complementary information. We propose several measures to use this information to quantify otocoherence (a form of autocoherence referring to a phenomenon of the ear), primarily by measuring the consistency in SOAE phase accumulation over a particular timescale. We present results based on recordings from different species with disparate inner ear morphologies (humans, barn owls, lizards). For regions of SOAE activity we extract time constants representing the timescale over which otocoherence is maintained. We demonstrate that these vary significantly across species and (for the barn owl and Tokay gecko where this data is available) appear to correlate with measures of auditory nerve fiber tuning. Additionally, we adapted the method to identify regions of weak SOAE activity among fluctuations in the noise floor. These methods can readily be employed to re-analyze SOAE waveforms previously collected from a variety of species, making them of broader comparative utility to reveal information about SOAE generation and thereby active cochlear mechanics. Significance StatementWe examined spontaneous otoacoustic emission, a widely accepted hallmark of active auditory biomechanics, and describe a novel approach to extract measures of its self-coherence. This was explored comparatively across different species of disparate inner ear morphology to provide insight into how active amplification operates in the presence of noise. Our approach elucidates the notion that what has been traditionally referred to as "spontaneous activity" is not unstructured noise, but fluctuations that reveal internal system dynamics. The methodology described here is straightforward to employ and can be applied to a variety of data types, both within and beyond auditory neuroscience.

neuroscience↗

bioRxiv2024

Otoacoustic emissions predict cochlear-nerve but not behavioral frequency tuning in an avian vocal-communication specialist

Frequency analysis by the cochlea forms a key foundation for all subsequent auditory processing. Stimulus-frequency otoacoustic emissions (SFOAEs) are a potentially powerful alternative to traditional behavioral experiments for estimating cochlear tuning without invasive testing, as is necessary in humans. Which methods accurately predict cochlear tuning remains controversial due to only a single animal study comparing SFOAE-based, behavioral, and cochlear frequency tuning in the same species. The budgerigar (Melopsittacus undulatus) is a parakeet species with human-like behavioral sensitivity to many sounds and the capacity to mimic speech. Intriguingly, previous studies of critical bands, psychophysical tuning curves, and critical ratios in budgerigars show that behavioral tuning sharpness increases dramatically with increasing frequency from 1-3.5 kHz, doubling once per octave with peak tuning sharpness from 3.5-4 kHz. The pattern contrasts with slower monotonic growth of behavioral tuning sharpness with increasing frequency in other animals, including most avian species, suggesting a possible auditory specialization in budgerigars. We measured SFOAE-based and cochlear-afferent tuning in budgerigars, for comparison to previously reported behavioral results. SFOAE-based and cochlear-afferent tuning sharpness both increased monotonically and relatively slowly for higher frequencies, in contrast to the behavioral pattern. SFOAE-based tuning in budgerigars accurately predicted cochlear frequency tuning, and both measures aligned with typical patterns of cochlear tuning in other species. Divergent behavioral tuning in budgerigars is unlikely attributable to the periphery and could reflect specializations for central processing of masked signals. Our findings highlight the value of SFOAEs for estimating cochlear tuning and caution against direct inference of peripheral tuning from behavioral critical bands, psychophysical tuning curves, and critical ratios.

neuroscience↗

bioRxiv2024

Auditory Cellular Cooperativity Probed Via Spontaneous Otoacoustic Emissions

As a sound pressure detector that uses energy to boost both its sensitivity and selectivity, the inner ear is an active non-equilibrium system. The collective processes of the inner ear giving rise to this exquisite functionality remain poorly understood. One manifestation of the active ear across the animal kingdom is the presence of spontaneous otoacoustic emission (SOAE), idiosyncratic arrays of spectral peaks that can be measured using a sensitive microphone in the ear canal.1 Current SOAE models attempt to explain how multiple peaks arise, and generally assume a spatially-distributed tonotopic system. However, the nature of the generators, their coupling, and the role of noise (e.g., Brownian motion) are hotly debated, especially given the inner ear morphological diversity across vertebrates. One means of probing these facets of emission generation is studying fluctuations in SOAE peak properties, which produce amplitude (AM) and frequency modulations (FM). These properties are likely related to the presence of noise affecting active cellular generation elements, and the coupling between generators. To better biophysically constrain models, this study characterizes the fluctuations in filtered SOAE peak waveforms, focusing on interrelations within and across peaks. A systematic approach is taken, examining three species that exhibit disparate inner ear morphologies: humans, barn owls, and green anole lizards. To varying degrees across all three groups, SOAE peaks have intra-(IrP) and interpeak (IPP) correlations indicative of interactions between generative elements. Activity from anole lizards, whose auditory sensory organ is relatively much smaller than that of humans or barn owls, showed a much higher incidence of IPP correlations. Taken together, we propose that these data are indicative of SOAE cellular generators acting cooperatively, allowing the ear to function as an optimized detector. Significance StatementThe inner ear is a complex biomechanical system whose function is not well understood. To further elucidate the role of coupling in emission generation, this study systematically compares fluctuations in sound emitted spontaneously from the ear (spontaneous otoacoustic emission, SOAE) across three vertebrates. Ultimately these data serve to illustrate that the inner ear is a non-equilibrium, active system whose cellular elements work cooperatively. A clearer understanding of SOAE generation and how it manifests across the animal kingdom will significantly advance our understanding of both normal and impaired auditory function.

biophysics↗

bioRxiv2019

Overtone focusing in biphonic Tuvan throat singing

Khoomei is a unique singing style originating from the Central Asian republic of Tuva. Singers produce two pitches simultaneously: a booming low-frequency rumble alongside a hovering high-pitched whistle-like tone. The biomechanics of this biphonation are not well-understood. Here, we use sound analysis, dynamic magnetic resonance imaging, and vocal tract modeling to demonstrate how biphonation is achieved by modulating vocal tract morphology. Tuvan singers show remarkable control in shaping their vocal tract to narrowly focus the harmonics (or overtones) emanating from their vocal cords. The biphonic sound is a combination of the fundamental pitch and a focused filter state, which is at the higher pitch (1-2 kHz) and formed by merging two formants, thereby greatly enhancing sound-production in a very narrow frequency range. Most importantly, we demonstrate that this biphonation is a phenomenon arising from linear filtering rather than a nonlinear source.

biophysics↗