Dietmar J. Hecker

Lack of brain-derived neurotrophic factor hampers inner hair cell synapse physiology, but protects against noise-induced hearing loss

The precision of sound information transmitted to the brain depends on the transfer characteristics of the inner hair cell (IHC) ribbon synapse and its multiple contacting auditory fibers. We found that brain derived neurotrophic factor (BDNF) differentially influences IHC characteristics in the intact and injured cochlea. Using conditional knock-out mice (BDNFPax2 KO) we found that resting membrane potentials, membrane capacitance and resting linear leak conductance of adult BDNFPax2 KO IHCs showed a normal maturation. Likewise, in BDNFPax2 KO membrane capacitance (ΔCm) as a function of inward calcium current (ICa) follows the linear relationship typical for normal adult IHCs. In contrast the maximal ΔCm, but not the maximal size of the calcium current, was significantly reduced by 45% in basal but not in apical cochlear turns in BDNFPax2 KO IHCs. Maximal ΔCm correlated with a loss of IHC ribbons in these cochlear turns and a reduced activity of the auditory nerve (auditory brainstem response wave I). Remarkably, a noise-induced loss of IHC ribbons, followed by reduced activity of the auditory nerve and reduced centrally generated wave II and III observed in control mice, was prevented in equally noise-exposed BDNFPax2 KO mice. Data suggest that BDNF expressed in the cochlea is essential for maintenance of adult IHC transmitter release sites and that BDNF upholds opposing afferents in high-frequency turns and scales them down following noise exposure.

Cell-type-specific tuning of Cav1.3 Ca2+-channels by a C-terminal automodulatory domain

Cav1.3 L-type Ca2+-channel function is regulated by a C-terminal automodulatory domain (CTM). It affects channel binding of calmodulin and thereby tunes channel activity by interfering with Ca2+- and voltage-dependent gating. Alternative splicing generates short C-terminal channel variants lacking the CTM resulting in enhanced Ca2+-dependent inactivation and stronger voltage-sensitivity upon heterologous expression. However, the role of this modulatory domain for channel function in its native environment is unkown. To determine its functional significance in vivo, we interrupted the CTM with a hemagglutinin tag in mutant mice (Cav1.3DCRDHA/HA). Using these mice we provide biochemical evidence for the existence of long (CTM-containing) and short (CTM-deficient) Cav1.3 α1-subunits in brain. The long (HA-labeled) Cav1.3 isoform was present in all ribbon synapses of cochlear inner hair cells. CTM-elimination impaired Ca2+-dependent inactivation of Ca2+-currents in hair cells but increased it in chromaffin cells, resulting in hyperpolarized resting potentials and reduced pacemaking. CTM disruption did not affect hearing thresholds. We show that the modulatory function of the CTM is affected by its native environment in different cells and thus occurs in a cell-type specific manner in vivo. It stabilizes gating properties of Cav1.3 channels required for normal electrical excitability.

Involvement of the anterior semicircular canal in posttraumatic benign paroxysmal positioning vertigo.

Objective: To study the involvement of the different semicircular canals in posttraumatic benign paroxysmal positioning vertigo (BPPV) with special reference to the anterior canal (AC). Study Design: Retrospective review. Setting: Tertiary referral center. Patients: Seventy-four BPPV patients. Interventions: Neurotologic assessment with video-oculography; treatment of BPPV with the canalith repositioning procedure appropriate for the affected semicircular canal. Main Outcome Measures: Number of patients with AC, posterior canal (PC), horizontal canal (HC), and multiple-canal involvement in posttraumatic versus idiopathic BPPV. Results: 85.1% of patients were classified as idiopathic BPPV, whereas 14.9% had a history of posttraumatic BPPV. The prevalence of AC BPPV was significantly higher in the posttraumatic group (27.3%) compared with that in the idiopathic group (3.2%; Fishers exact test: p = 0.021). Multiple-canal (combined) BPPV was observed more frequently after head trauma (27.3%) compared with the idiopathic form of the disorder (1.6%; p = 0.009). In particular, the risk for combined AC/PC BPPV was greater in posttraumatic than idiopathic cases (odds ratio, 13.78; 95% confidence interval, 1.13-167.8). No significant differences were observed for the involvement of the PC and HC between the two groups. Two cases of combined AC/PC BPPV are presented with particular respect to the underlying trauma mechanism. Conclusion: Head trauma is a risk factor for AC and combined BPPV, in particular AC/PC BPPV. Involvement of the AC should especially be considered in patients who experienced head trauma resulting in a nonupright position of the body.

α2δ2 Controls the Function and Trans-Synaptic Coupling of Cav1.3 Channels in Mouse Inner Hair Cells and Is Essential for Normal Hearing

The auxiliary subunit α2δ2 modulates the abundance and function of voltage-gated calcium channels. Here we show that α2δ2 mRNA is expressed in neonatal and mature hair cells. A functional α2δ2-null mouse, the ducky mouse (du), showed elevated auditory brainstem response click and frequency-dependent hearing thresholds. Otoacoustic emissions were not impaired pointing to normal outer hair cell function. Peak Ca2+ and Ba2+ currents of mature du/du inner hair cells (IHCs) were reduced by 30–40%, respectively, and gating properties, such as the voltage of half-maximum activation and voltage sensitivity, were altered, indicating that Cav1.3 channels normally coassemble with α2δ2 at IHC presynapses. The reduction of depolarization-evoked exocytosis in du/du IHCs reflected their reduced Ca2+ currents. Ca2+- and voltage-dependent K+ (BK) currents and the expression of the pore-forming BKα protein were normal. Cav1.3 and Cavβ2 protein expression was unchanged in du/du IHCs, forming clusters at presynaptic ribbons. However, the close apposition of presynaptic Cav1.3 clusters with postsynaptic glutamate receptor GluA4 and PSD-95 clusters was significantly impaired in du/du mice. This implies that, in addition to controlling the expression and gating properties of Cav1.3 channels, the largely extracellularly localized α2δ2 subunit moreover plays a so far unknown role in mediating trans-synaptic alignment of presynaptic Ca2+ channels and postsynaptic AMPA receptors. SIGNIFICANCE STATEMENT Inner hair cells possess calcium channels that are essential for transmitting sound information into synaptic transmitter release. Voltage-gated calcium channels can coassemble with auxiliary subunit α2δ isoforms 1–4. We found that hair cells of the mouse express the auxiliary subunit α2δ2, which is needed for normal hearing thresholds. Using a mouse model with a mutant, nonfunctional α2δ2 protein, we showed that the α2δ2 protein is necessary for normal calcium currents and exocytosis in inner hair cells. Unexpectedly, the α2δ2 protein is moreover required for the optimal spatial alignment of presynaptic calcium channels and postsynaptic glutamate receptor proteins across the synaptic cleft. This suggests that α2δ2 plays a novel role in organizing the synapse.

Ultra-Fast Detection of Hearing Thresholds by Single Sweeps of Auditory Brainstem Responses: A New Novelty Detection Paradigm

The evaluation of auditory brainstem responses (ABRs) is accepted to be the most reliable method for the objective diagnosis and quantification of hearing loss in newborns. However, in currently available setups and devices, a large number of sweeps has to be averaged to obtain a meaningful signal at low stimulation levels due to a poor signal-to-noise ratio. Due to the time consuming averaging procedure which requires the state of spontaneous sleep, sedation, or narcosis of the newborns, the evaluation of ABRs can only be used at the last stage of area-wide universal newborn hearing screening programs, increasing their implementation cost significantly and thus making their realization impossible in many cases. Here we propose a new novelty detection paradigm for the fast detection of hearing thresholds using the synchronization stability of auditory brainstem response single sweeps. For this, large-scale correlates of the neural group synchronization at the brainstem level by a stimulation level above the hearing threshold are detected as novel event. This novelty detection paradigm allows for the adaptive inclusion of individual measurement conditions using the spontaneous EEG. For the individual patient, at the challenging stimulation level of 30dB(HL) the synchronization stability allowed the discrimination of stimulated from the non-stimulated condition in a fraction of time of conventional devices used for newborn hearing screening. It is concluded that the proposed method might be used for the ultra-fast detection of hearing thresholds and is thus ideally suited for universal hearing screening programs

A New Method to Analyze Distortion Product Otoacoustic Emissions (DPOAEs) in the High-Frequency Range Up to 18 kHz Using Windowed Periodograms

Distortion product otoacoustic emissions (DPOAEs) are widely used as an objective examination procedure to determine cochlear function. In a clinical routine setting, the amplitude of the DPOAE signal at 2f1-f2 is applied as an indicator for a potential hearing loss up to 8 kHz. Due to their poor signal-to-noise ratio, meatal nodes from standing waves and calibration issues, high-frequency DPOAEs >; 8 kHz have hardly been addressed in experimental and clinical audiology so far. Here, we present a new method of measuring DPOAE signal levels based on optimal maximum likelihood estimation with windowed power spectral density estimation of stochastic signals and filtering theory. Analysis of simulated data showed that the proposed method effectively reduces the disturbing noise floor compared to conventional averaging techniques. Robust DPOAE signals were measured in 20 ears from ten normally hearing young adults (21-27 years) from 0.5 to 18 kHz . Repeated DPOAE recordings in one individual yielded a good to very good test-retest reliability of the proposed method. These observations are discussed in the context of DPOAE signal processing and possible clinical applications of high-frequency DPOAE measurements.

Electromotive Triggering and Single Sweep Analysis of Vestibular Evoked Myogenic Potentials (VEMPs)

Cervical (c) and ocular (o) vestibular evoked myogenic potentials (VEMPs) provide important tools for measuring otolith function. However, two major drawbacks of this method are encountered in clinical practice. First, recording of oVEMPs is compromised by small n10 amplitudes. Second, VEMP analysis is currently based on the averaging technique, resulting in a loss of information compared to single sweep analysis. Here, we: 1) developed a novel electromotive trigger mechanism for evoking VEMPs by bone-conducted vibration to the forehead and 2) established maximum entropy extraction of complex wavelet transforms for calculation of phase synchronization between VEMP single sweeps. Both c- and oVEMPs were recorded for n=10 healthy individuals. The oVEMP n10 amplitude was consistently higher (right: 24.84±9.71 μV; left: 27.40±14.55 μV) than previously described. Stable VEMP signals were reached after a smaller number of head taps (oVEMPs 6; cVEMPs 11) compared to current recommendations. Phase synchronization vectors and phase shift values were successfully determined for simulated and clinically recorded VEMPs, providing information about the impact of noise and phase jitter on the VEMP signal. Thus, the proposed method constitutes an easy-to-use approach for the fast detection and analysis of VEMPs in clinical practice.

Quantifying spatiotemporal properties of vocal fold dynamics based on a multiscale analysis of phonovibrograms.

In order to objectively assess the laryngeal vibratory behavior, endoscopic high-speed cameras capture several thousand frames per second of the vocal folds during phonation. However, judging all inherent clinically relevant features is a challenging task and requires well-founded expert knowledge. In this study, an automated wavelet-based analysis of laryngeal high-speed videos based on phonovibrograms is presented. The phonovibrogram is an image representation of the spatiotemporal pattern of vocal fold vibration and constitutes the basis for a computer-based analysis of laryngeal dynamics. The features extracted from the wavelet transform are shown to be closely related to a basic set of video-based measurements categorized by the European Laryngological Society for a subjective assessment of pathologic voices. The wavelet-based analysis further offers information about irregularity and lateral asymmetry and asynchrony. It is demonstrated in healthy and pathologic subjects as well as for a surgical group that was examined before and after the removal of a vocal fold polyp. The features were found to not only classify glottal closure characteristics but also quantify the impact of pathologies on the vibratory behavior. The interpretability and the discriminative power of the proposed feature set show promising relevance for a computer-assisted diagnosis and classification of voice disorders.

A generalized procedure for analyzing sustained and dynamic vocal fold vibrations from laryngeal high-speed videos using phonovibrograms

OBJECTIVE This work presents a computer-based approach to analyze the two-dimensional vocal fold dynamics of endoscopic high-speed videos, and constitutes an extension and generalization of a previously proposed wavelet-based procedure. While most approaches aim for analyzing sustained phonation conditions, the proposed method allows for a clinically adequate analysis of both dynamic as well as sustained phonation paradigms. MATERIALS AND METHODS The analysis procedure is based on a spatio-temporal visualization technique, the phonovibrogram, that facilitates the documentation of the visible laryngeal dynamics. From the phonovibrogram, a low-dimensional set of features is computed using a principle component analysis strategy that quantifies the type of vibration patterns, irregularity, lateral symmetry and synchronicity, as a function of time. Two different test bench data sets are used to validate the approach: (I) 150 healthy and pathologic subjects examined during sustained phonation. (II) 20 healthy and pathologic subjects that were examined twice: during sustained phonation and a glissando from a low to a higher fundamental frequency. In order to assess the discriminative power of the extracted features, a Support Vector Machine is trained to distinguish between physiologic and pathologic vibrations. The results for sustained phonation sequences are compared to the previous approach. Finally, the classification performance of the stationary analyzing procedure is compared to the transient analysis of the glissando maneuver. RESULTS For the first test bench the proposed procedure outperformed the previous approach (proposed feature set: accuracy: 91.3%, sensitivity: 80%, specificity: 97%, previous approach: accuracy: 89.3%, sensitivity: 76%, specificity: 96%). Comparing the classification performance of the second test bench further corroborates that analyzing transient paradigms provides clear additional diagnostic value (glissando maneuver: accuracy: 90%, sensitivity: 100%, specificity: 80%, sustained phonation: accuracy: 75%, sensitivity: 80%, specificity: 70%). CONCLUSIONS The incorporation of parameters describing the temporal evolvement of vocal fold vibration clearly improves the automatic identification of pathologic vibration patterns. Furthermore, incorporating a dynamic phonation paradigm provides additional valuable information about the underlying laryngeal dynamics that cannot be derived from sustained conditions. The proposed generalized approach provides a better overall classification performance than the previous approach, and hence constitutes a new advantageous tool for an improved clinical diagnosis of voice disorders.

A multiscale product approach for an automatic classification of voice disorders from endoscopic high-speed videos

Direct observation of vocal fold vibration is indispensable for a clinical diagnosis of voice disorders. Among current imaging techniques, high-speed videoendoscopy constitutes a state-of-the-art method capturing several thousand frames per second of the vocal folds during phonation. Recently, a method for extracting descriptive features from phonovibrograms, a two-dimensional image containing the spatio-temporal pattern of vocal fold dynamics, was presented. The derived features are closely related to a clinically established protocol for functional assessment of pathologic voices. The discriminative power of these features for different pathologic findings and configurations has not been assessed yet. In the current study, a collective of 220 subjects is considered for two- and multi-class problems of healthy and pathologic findings. The performance of the proposed feature set is compared to conventional feature reduction routines and was found to clearly outperform these. As such, the proposed procedure shows great potential for diagnostical issues of vocal fold disorders.