Alessandro Altoè

Transmission line cochlear models: Improved accuracy and efficiency

This paper presents an efficient method to compute the numerical solutions of transmission-line (TL) cochlear models, and its application on the model of Verhulst et al. The stability region of the model is extended by adopting a variable step numerical method to solve the system of ordinary differential equations that describes it, and by adopting an adaptive scheme to take in account variations in the system status within each numerical step. The presented method leads to improve simulations numerical accuracy and large computational savings, leading to employ TL models for more extensive simulations than currently possible.

Integrated processing of spatial cues in human auditory cortex

Human sound source localization relies on acoustical cues, most importantly, the interaural differences in time and level (ITD and ILD). For reaching a unified representation of auditory space the auditory nervous system needs to combine the information provided by these two cues. In search for such a unified representation, we conducted a magnetoencephalography (MEG) experiment that took advantage of the location-specific adaptation of the auditory cortical N1 response. In general, the attenuation caused by a preceding adaptor sound to the response elicited by a probe depends on their spatial arrangement: if the two sounds coincide, adaptation is stronger than when the locations differ. Here, we presented adaptor-probe pairs that contained different localization cues, for instance, adaptors with ITD and probes with ILD. We found that the adaptation of the N1 amplitude was location-specific across localization cues. This result can be explained by the existence of auditory cortical neurons that are sensitive to sound source location independent on which cue, ITD or ILD, provides the location information. Such neurons would form a cue-independent, unified representation of auditory space in human auditory cortex.

Human cortical sensitivity to interaural time difference in high-frequency sounds

Human sound source localization relies on various acoustical cues one of the most important being the interaural time difference (ITD). ITD is best detected in the fine structure of low-frequency sounds but it may also contribute to spatial hearing at higher frequencies if extracted from the sound envelope. The human brain mechanisms related to this envelope ITD cue remain unexplored. Here, we tested the sensitivity of the human auditory cortex to envelope ITD in magnetoencephalography (MEG) recordings. We found two types of sensitivity to envelope ITD. First, the amplitude of the auditory cortical N1m response was smaller for zero envelope ITD than for long envelope ITDs corresponding to the sound being in opposite phase in the two ears. Second, the N1m response amplitude showed ITD-specific adaptation for both fine-structure and for envelope ITD. The auditory cortical sensitivity was weaker for envelope ITD in high-frequency sounds than for fine-structure ITD in low-frequency sounds but occurred within a range of ITDs that are encountered in natural conditions. Finally, the participants were briefly tested for their behavioral ability to detect envelope ITD. Interestingly, we found a correlation between the behavioral performance and the neural sensitivity to envelope ITD. In conclusion, our findings show that the human auditory cortex is sensitive to ITD in the envelope of high-frequency sounds and this sensitivity may have behavioral relevance.

Model-based estimation of the frequency tuning of the inner-hair-cell stereocilia from neural tuning curves

This study proposes that the frequency tuning of the inner-hair-cell (IHC) stereocilia in the intact organ of Corti can be derived from the responses of the auditory fibers (AFs) using computational tools. The frequency-dependent relationship between the AF threshold and the amplitude of the stereocilia vibration is estimated using a model of the IHC-mediated mechanical to neural transduction. Depending on the response properties of the considered AF, the amplitude of stereocilia deflection required to drive the simulated AF above threshold is 1.4 to 9.2 dB smaller at low frequencies (≤500 Hz) than at high frequencies (≥4 kHz). The estimated frequency-dependent relationship between ciliary deflection and neural threshold is employed to derive constant-stereocilia-deflection contours from previously published AF recordings from the chinchilla cochlea. This analysis shows that the transduction process partially accounts for the observed differences between the tuning of the basilar membrane and that of the AFs.

Decoupling the level dependence of the basilar membrane gain and phase in nonlinear cochlea models

In animal experiments, the strong dependence on stimulus level of the basilar membrane gain and tuning is not matched by a corresponding change in the phase slope in the resonant region. Linear models, in which the gain dependence on the stimulus level has to be schematized by explicitly changing the tuning parameters of the resonant model, do not easily match this feature of the experimental data. Nonlinear models predict a phase slope that is relatively decoupled from tuning. In addition, delayed-stiffness and feed-forward models also show a significant intrinsic decoupling between gain and tuning, which helps in matching the experimental data.

Computational modeling of the human auditory periphery: auditory-nerve responses, evoked potentials and hearing loss

&NA; Models of the human auditory periphery range from very basic functional descriptions of auditory filtering to detailed computational models of cochlear mechanics, inner‐hair cell (IHC), auditory‐nerve (AN) and brainstem signal processing. It is challenging to include detailed physiological descriptions of cellular components into human auditory models because single‐cell data stems from invasive animal recordings while human reference data only exists in the form of population responses (e.g., otoacoustic emissions, auditory evoked potentials). To embed physiological models within a comprehensive human auditory periphery framework, it is important to capitalize on the success of basic functional models of hearing and render their descriptions more biophysical where possible. At the same time, comprehensive models should capture a variety of key auditory features, rather than fitting their parameters to a single reference dataset. In this study, we review and improve existing models of the IHC‐AN complex by updating their equations and expressing their fitting parameters into biophysical quantities. The quality of the model framework for human auditory processing is evaluated using recorded auditory brainstem response (ABR) and envelope‐following response (EFR) reference data from normal and hearing‐impaired listeners. We present a model with 12 fitting parameters from the cochlea to the brainstem that can be rendered hearing impaired to simulate how cochlear gain loss and synaptopathy affect human population responses. The model description forms a compromise between capturing well‐described single‐unit IHC and AN properties and human population response features. HighlightsAn overview of computational models from cochlea to auditory‐nerve (AN).IHC‐AN model descriptions are made biophysical to reduce model fitting parameters.The presented auditory model captures key aspects of human OAE, ABR and EFRs.Simulated impact of sensorineural hearing loss on human population responses.

Dynamics of cochlear nonlinearity: Automatic gain control or instantaneous damping?

Measurements of basilar-membrane (BM) motion show that the compressive nonlinearity of cochlear mechanical responses is not an instantaneous phenomenon. For this reason, the cochlear amplifier has been thought to incorporate an automatic gain control (AGC) mechanism characterized by a finite reaction time. This paper studies the effect of instantaneous nonlinear damping on the responses of oscillatory systems. The principal results are that (i) instantaneous nonlinear damping produces a noninstantaneous gain control that differs markedly from typical AGC strategies; (ii) the kinetics of compressive nonlinearity implied by the finite reaction time of an AGC system appear inconsistent with the nonlinear dynamics measured on the gerbil basilar membrane; and (iii) conversely, those nonlinear dynamics can be reproduced using an harmonic oscillator with instantaneous nonlinear damping. Furthermore, existing cochlear models that include instantaneous gain-control mechanisms capture the principal kinetics of BM nonlinearity. Thus, an AGC system with finite reaction time appears neither necessary nor sufficient to explain nonlinear gain control in the cochlea.

The effect of the inner-hair-cell mediated transduction on the shape of neural tuning curves

The inner hair cells of the mammalian cochlea transform the vibrations of their stereocilia into releases of neurotransmitter at the ribbon synapses, thereby controlling the activity of the afferent auditory fibers. The mechanical-to-neural transduction is a highly nonlinear process and it introduces differences between the frequency-tuning of the stereocilia and that of the afferent fibers. Using a computational model of the inner hair cell that is based on in vitro data, we estimated that smaller vibrations of the stereocilia are necessary to drive the afferent fibers above threshold at low (≤0.5 kHz) than at high (≥4 kHz) driving frequencies. In the base of the cochlea, the transduction process affects the low-frequency tails of neural tuning curves. In particular, it introduces differences between the frequency-tuning of the stereocilia and that of the auditory fibers resembling those between basilar membrane velocity and auditory fibers tuning curves in the chinchilla base. For units with a characteristic frequency between 1 and 4 kHz, the transduction process yields shallower neural than stereocilia tuning curves as the characteristic frequency decreases. This study proposes that transduction contributes to the progressive broadening of neural tuning curves from the base to the apex.

Temporal dynamics of the generator of stimulated otoacoustic emissions

The early emission work of David Kemp investigated temporal properties of click-evoked otoacoustic emissions (CEOAEs). Suppressor clicks were positioned up to 10 ms before or after the evoking (test) click and reduced the emission amplitude. Intriguingly, suppressors that preceded the test click by 1-2 ms were more effective than simultaneously presented suppressors. This observation seems not to support the hypothesis that emission generation operates on the basis of an instantaneous gain/suppression mechanism, for which simultaneously presented suppressors should be most effective. Kemps’ observations thus left the field with an important OAE generation question that inspired several OAE studies. Kemps’ results can be explained on the basis of (i) a non-instantaneous gain mechanism at the OAE generation site, or (ii), complex temporal interactions of basilar-membrane impulse responses (BM IRs) that operate with an instantaneous nonlinearity, but yield the observed non-instantaneous suppression properties. We investigated these dynamics using a nonlinear transmission-line model of the human cochlea and found that maximal CEOAE suppression can occur for preceding clicks even when the cochlear nonlinearity is kept time-invariant, supporting hypothesis (ii). Additionally, we used the frequency-dependence of CEOAE suppression to quantify human BM IR duration and cochlear filter tuning, yielding QERB estimates of 13.8 F[in kHz]0.22.The early emission work of David Kemp investigated temporal properties of click-evoked otoacoustic emissions (CEOAEs). Suppressor clicks were positioned up to 10 ms before or after the evoking (test) click and reduced the emission amplitude. Intriguingly, suppressors that preceded the test click by 1-2 ms were more effective than simultaneously presented suppressors. This observation seems not to support the hypothesis that emission generation operates on the basis of an instantaneous gain/suppression mechanism, for which simultaneously presented suppressors should be most effective. Kemps’ observations thus left the field with an important OAE generation question that inspired several OAE studies. Kemps’ results can be explained on the basis of (i) a non-instantaneous gain mechanism at the OAE generation site, or (ii), complex temporal interactions of basilar-membrane impulse responses (BM IRs) that operate with an instantaneous nonlinearity, but yield the observed non-instantaneous suppression propertie...

The effects of the activation of the inner-hair-cell basolateral K+ channels on auditory nerve responses

&NA; The basolateral membrane of the mammalian inner hair cell (IHC) expresses large voltage and Ca2+ gated outward K+ currents. To quantify how the voltage‐dependent activation of the K+ channels affects the functionality of the auditory nerve innervating the IHC, this study adopts a model of mechanical‐to‐neural transduction in which the basolateral K+ conductances of the IHC can be made voltage‐dependent or not. The model shows that the voltage‐dependent activation of the K+ channels (i) enhances the phase‐locking properties of the auditory fiber (AF) responses; (ii) enables the auditory nerve to encode a large dynamic range of sound levels; (iii) enables the AF responses to synchronize precisely with the envelope of amplitude modulated stimuli; and (iv), is responsible for the steep offset responses of the AFs. These results suggest that the basolateral K+ channels play a major role in determining the well‐known response properties of the AFs and challenge the classical view that describes the IHC membrane as an electrical low‐pass filter. In contrast to previous models of the IHC‐AF complex, this study ascribes many of the AF response properties to fairly basic mechanisms in the IHC membrane rather than to complex mechanisms in the synapse. HighlightsWe studied the effects of the IHC basolateral K+ currents in a model.Basolateral K+ channels are fundamental for auditory nerve phase‐locking.Basolateral K+ channels allow for steep offset responses of the auditory neurons.Differences between neural adaptation and recovery originate in the IHC membrane.The functional role of the IHC membrane is not just that of a low‐pass filter.