Models of the electrically stimulated binaural system: A review

Abstract

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.

Physiological models of the lateral superior olive

“…Binaural information processing is critical for efficient acoustic communication both in animals and in humans, especially in a noisy environment [ 11 , 54 ]. How to preserve and use binaural cues in cochlear implant users has been actively investigated (see [ 158 ] for a review). Combining electric hearing models with binaural neuron models [ 58 , 59 ] would be useful not only to predict how bilateral implantation may or may not contribute to the improvement of perceptual outcome, but also to highlight future directions towards the refinement of coding technologies.…”

Coherent Coding of Enhanced Interaural Cues Improves Sound Localization in Noise With Bilateral Cochlear Implants

“…Even stronger than typical hearing aids, CIs corrupt or even discard most binaural information: ILD information is reduced due to the limited dynamic range (DR) available in electric hearing ( Kerber & Seeber, 2012 ), and ITD information is largely disregarded due to the usage of high pulse rate coding strategies and missing time-synchronization of the two independent sound processors. In addition, even if binaural cues are provided with precisely synchronized timing in controlled laboratory settings, the CI user’s sensitivity to binaural cues is reduced due to the electric stimulation itself (e.g., Smith & Delgutte, 2007 ) and other subject-related factors (see, e.g., Dietz, 2016 , for an overview).…”

Corrective binaural processing for bilateral cochlear implant patients

“…It appears from these results that the binaural representation of at least some individuals in this population can be quite complex, idiosyncratic, and non-monotonic. It is unknown at this time what the source or sources of this variability are, although there are a number of likely candidates, including their reliance on ILDs [ 1 ], and the speech coding strategies of the devices [ 3 , 39 ]. Another likely factor involves the interface between individual electrodes of each device and the regions of auditory nerve they each stimulate.…”