Extent of lateralization at large interaural time differences in simulated electric hearing and bilateral cochlear implant users.

Normal-hearing (NH) listeners are able to localize sound sources with extraordinary accuracy through interaural cues, most importantly interaural time differences (ITDs) in the temporal fine structure. Bilateral cochlear implant (CI) users are also able to localize sound sources, yet generally at lower accuracy than NH listeners. The gap in performance can in part be attributed to current CI systems not faithfully transmitting interaural cues, especially ITDs. With the introduction of binaurally linked CI systems, the presentation of ITD cues for bilateral CI users is foreseeable. The current study therefore investigated extent-of-lateralization percepts elicited in bilateral CI listeners when presented with single-electrode pulse-trains carrying controlled ITD cues. The results were compared against NH listeners listening to broadband stimuli as well as simulations of CI listening. Broadband stimuli in NH listeners were perceived as fully lateralized within the natural ITD range. Using simulated as well as real CI stimuli, however, only a fraction of the full extent of lateralization range was covered by natural ITDs. The maximum extent of lateralization was reached at ITDs as large as twice the natural limit. The results suggest that ITD-enhancement might be a viable option for improving localization abilities with future binaural CI systems.

Comparing Binaural Pre-processing Strategies I: Instrumental Evaluation.

In a collaborative research project, several monaural and binaural noise reduction algorithms have been comprehensively evaluated. In this article, eight selected noise reduction algorithms were assessed using instrumental measures, with a focus on the instrumental evaluation of speech intelligibility. Four distinct, reverberant scenarios were created to reflect everyday listening situations: a stationary speech-shaped noise, a multitalker babble noise, a single interfering talker, and a realistic cafeteria noise. Three instrumental measures were employed to assess predicted speech intelligibility and predicted sound quality: the intelligibility-weighted signal-to-noise ratio, the short-time objective intelligibility measure, and the perceptual evaluation of speech quality. The results show substantial improvements in predicted speech intelligibility as well as sound quality for the proposed algorithms. The evaluated coherence-based noise reduction algorithm was able to provide improvements in predicted audio signal quality. For the tested single-channel noise reduction algorithm, improvements in intelligibility-weighted signal-to-noise ratio were observed in all but the nonstationary cafeteria ambient noise scenario. Binaural minimum variance distortionless response beamforming algorithms performed particularly well in all noise scenarios.

Comparing Binaural Pre-processing Strategies II: Speech Intelligibility of Bilateral Cochlear Implant Users.

Several binaural audio signal enhancement algorithms were evaluated with respect to their potential to improve speech intelligibility in noise for users of bilateral cochlear implants (CIs). 50% speech reception thresholds (SRT50) were assessed using an adaptive procedure in three distinct, realistic noise scenarios. All scenarios were highly nonstationary, complex, and included a significant amount of reverberation. Other aspects, such as the perfectly frontal target position, were idealized laboratory settings, allowing the algorithms to perform better than in corresponding real-world conditions. Eight bilaterally implanted CI users, wearing devices from three manufacturers, participated in the study. In all noise conditions, a substantial improvement in SRT50 compared to the unprocessed signal was observed for most of the algorithms tested, with the largest improvements generally provided by binaural minimum variance distortionless response (MVDR) beamforming algorithms. The largest overall improvement in speech intelligibility was achieved by an adaptive binaural MVDR in a spatially separated, single competing talker noise scenario. A no-pre-processing condition and adaptive differential microphones without a binaural link served as the two baseline conditions. SRT50 improvements provided by the binaural MVDR beamformers surpassed the performance of the adaptive differential microphones in most cases. Speech intelligibility improvements predicted by instrumental measures were shown to account for some but not all aspects of the perceptually obtained SRT50 improvements measured in bilaterally implanted CI users.

A Binaural Steering Beamformer System for Enhancing a Moving Speech Source.

In many daily life communication situations, several sound sources are simultaneously active. While normal-hearing listeners can easily distinguish the target sound source from interfering sound sources-as long as target and interferers are spatially or spectrally separated-and concentrate on the target, hearing-impaired listeners and cochlear implant users have difficulties in making such a distinction. In this article, we propose a binaural approach composed of a spatial filter controlled by a direction-of-arrival estimator to track and enhance a moving target sound. This approach was implemented on a real-time signal processing platform enabling experiments with test subjects in situ. To evaluate the proposed method, a data set of sound signals with a single moving sound source in an anechoic diffuse noise environment was generated using virtual acoustics. The proposed steering method was compared with a fixed (nonsteering) method that enhances sound from the frontal direction in an objective evaluation and subjective experiments using this database. In both cases, the obtained results indicated a significant improvement in speech intelligibility and quality compared with the unprocessed signal. Furthermore, the proposed method outperformed the nonsteering method.

Selective filtering of particles by the extracellular matrix: an electrostatic bandpass.

The transport of microscopic particles such as growth factors, proteins, or drugs through the extracellular matrix (ECM) is based on diffusion, a ubiquitous mechanism in nature. The ECM shapes the local distribution of the transported macromolecules and at the same time constitutes an important barrier toward infectious agents. To fulfill these competing tasks, the hydrogels have to employ highly selective filtering mechanisms. Yet, the underlying microscopic principles are still an enigma in cell biology and drug delivery. Here, we show that the extracellular matrix presents an effective electrostatic bandpass, suppressing the diffusive motion of both positively and negatively charged objects. This mechanism allows uncharged particles to easily diffuse through the matrix, while charged particles are effectively trapped. However, by tuning the strength of this physical interaction of the particles with the biopolymer matrix, the microscopic mobility of formerly trapped particles can be rescued on demand. Moreover, we identify heparan sulfate chains to be one important key factor for the barrier function of the extracellular matrix. We propose that localized charge patches in the ECM are responsible for its highly unspecific but strongly selective filtering effect. Such localized interactions could also account for the observed tunability and selectivity of many other important permeability barriers that are established by biopolymer-based hydrogels, e.g., the mucus layer of endothelial cells or the hydrogel in the nuclear core complex.