Sound localization with microsecond precision in mammals: what is it we do not understand?

Oligodendrocyte arrangement, identification and morphology in the developing superior olivary complex

Oligodendrocytes provide myelination, metabolic and developmental support for neurons and circuits. Within the auditory superior olivary complex (SOC), relevant for sound localization and spectro-temporal integration, oligodendrocytes are fundamental for fast neuronal communication and accurate timing of sound signals. Despite their important role in function and development, an assessment of their developmental arrangement and morphology is missing for the SOC. Here, immunofluorescence labeling and single cell electroporation was used to quantify their distribution, identification and morphology between postnatal day (P) 5 and ~ P54 in the SOC of Mongolian gerbils (Meriones unguiculatus). Oligodendrocytes show developmental, region-specific accumulations, redistributions and density profiles. Their identification by Olig2 and SOX10 appears age specific, while myelinating oligodendrocytes are detected by co-labeling with S100 irrespective of age. Comparison of oligodendrocyte density and identification between mature gerbil and Etruscan shrew (Suncus etruscus), revealed species-specific differences. Morphologically, the number of myelinating processes decreased, while process length, diameter and coverage area of oligodendrocytes increased during development. Together, oligodendrocyte developmental alterations occur at moments of SOC circuit refinement supporting functions beyond myelination.

Lower interaural coherence in off-signal bands impairs binaural detection

Differences in interaural phase configuration between a target and a masker can lead to substantial binaural unmasking. This effect is decreased for masking noises with an interaural time difference (ITD). Adding a second noise with an opposing ITD in most cases further reduces binaural unmasking. Thus far, modeling of these detection thresholds required both a mechanism for internal ITD compensation and an increased filter bandwidth. An alternative explanation for the reduction is that unmasking is impaired by the lower interaural coherence in off-frequency regions caused by the second masker [Marquardt and McAlpine (2009). J. Acoust. Soc. Am. 126(6), EL177-EL182]. Based on this hypothesis, the current work proposes a quantitative multi-channel model using monaurally derived peripheral filter bandwidths and an across-channel incoherence interference mechanism. This mechanism differs from wider filters since it has no effect when the masker coherence is constant across frequency bands. Combined with a monaural energy discrimination pathway, the model predicts the differences between a single delayed noise and two opposingly delayed noises as well as four other data sets. It helps resolve the inconsistency that simulating some data requires wide filters while others require narrow filters.

A framework for testing and comparing binaural models

Auditory research has a rich history of combining experimental evidence with computational simulations of auditory processing in order to deepen our theoretical understanding of how sound is processed in the ears and in the brain. Despite significant progress in the amount of detail and breadth covered by auditory models, for many components of the auditory pathway there are still different model approaches that are often not equivalent but rather in conflict with each other. Similarly, some experimental studies yield conflicting results which has led to controversies. This can be best resolved by a systematic comparison of multiple experimental data sets and model approaches. Binaural processing is a prominent example of how the development of quantitative theories can advance our understanding of the phenomena, but there remain several unresolved questions for which competing model approaches exist. This article discusses a number of current unresolved or disputed issues in binaural modelling, as well as some of the significant challenges in comparing binaural models with each other and with the experimental data. We introduce an auditory model framework, which we believe can become a useful infrastructure for resolving some of the current controversies. It operates models over the same paradigms that are used experimentally. The core of the proposed framework is an interface that connects three components irrespective of their underlying programming language: The experiment software, an auditory pathway model, and task-dependent decision stages called artificial observers that provide the same output format as the test subject.

Models of the electrically stimulated binaural system: A review

In an increasing number of countries, the standard treatment for deaf individuals is moving toward the implantation of two cochlear implants. Today's device technology and fitting procedure, however, appears as if the two implants would serve two independent ears and brains. Many experimental studies have demonstrated that after careful matching and balancing of left and right stimulation in controlled laboratory studies most patients have almost normal sensitivity to interaural level differences and some sensitivity to interaural time differences (ITDs). Mechanisms underlying the limited ITD sensitivity are still poorly understood and many different aspects may contribute. Recent pioneering computational approaches identified some of the functional implications the electric input imposes on the neural brainstem circuits. Simultaneously these studies have raised new questions and certainly demonstrated that further refinement of the model stages is necessary. They join the experimental study's conclusions that binaural device technology, binaural fitting, specific speech coding strategies, and binaural signal processing algorithms are obviously missing components to maximize the benefit of bilateral implantation. Within this review, the existing models of the electrically stimulated binaural system are explained, compared, and discussed from a viewpoint of a "CI device with auditory system" and from that of neurophysiological research.