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A Binaural Cochlear Implant Sound Coding Strategy Inspired by the Contralateral Medial Olivocochlear Reflex

Lopez-Poveda, Enrique A.; Eustaquio-Martín, Almudena; Stohl, Joshua S.; Wolford, Robert D.; Schatzer, Reinhold; Wilson, Blake S.

doi: 10.1097/AUD.0000000000000273
e-Research Articles
Open Access

Objectives: In natural hearing, cochlear mechanical compression is dynamically adjusted via the efferent medial olivocochlear reflex (MOCR). These adjustments probably help understanding speech in noisy environments and are not available to the users of current cochlear implants (CIs). The aims of the present study are to: (1) present a binaural CI sound processing strategy inspired by the control of cochlear compression provided by the contralateral MOCR in natural hearing; and (2) assess the benefits of the new strategy for understanding speech presented in competition with steady noise with a speech-like spectrum in various spatial configurations of the speech and noise sources.

Design: Pairs of CI sound processors (one per ear) were constructed to mimic or not mimic the effects of the contralateral MOCR on compression. For the nonmimicking condition (standard strategy or STD), the two processors in a pair functioned similarly to standard clinical processors (i.e., with fixed back-end compression and independently of each other). When configured to mimic the effects of the MOCR (MOC strategy), the two processors communicated with each other and the amount of back-end compression in a given frequency channel of each processor in the pair decreased/increased dynamically (so that output levels dropped/increased) with increases/decreases in the output energy from the corresponding frequency channel in the contralateral processor. Speech reception thresholds in speech-shaped noise were measured for 3 bilateral CI users and 2 single-sided deaf unilateral CI users. Thresholds were compared for the STD and MOC strategies in unilateral and bilateral listening conditions and for three spatial configurations of the speech and noise sources in simulated free-field conditions: speech and noise sources colocated in front of the listener, speech on the left ear with noise in front of the listener, and speech on the left ear with noise on the right ear. In both bilateral and unilateral listening, the electrical stimulus delivered to the test ear(s) was always calculated as if the listeners were wearing bilateral processors.

Results: In both unilateral and bilateral listening conditions, mean speech reception thresholds were comparable with the two strategies for colocated speech and noise sources, but were at least 2 dB lower (better) with the MOC than with the STD strategy for spatially separated speech and noise sources. In unilateral listening conditions, mean thresholds improved with increasing the spatial separation between the speech and noise sources regardless of the strategy but the improvement was significantly greater with the MOC strategy. In bilateral listening conditions, thresholds improved significantly with increasing the speech-noise spatial separation only with the MOC strategy.

Conclusions: The MOC strategy (1) significantly improved the intelligibility of speech presented in competition with a spatially separated noise source, both in unilateral and bilateral listening conditions; (2) produced significant spatial release from masking in bilateral listening conditions, something that did not occur with fixed compression; and (3) enhanced spatial release from masking in unilateral listening conditions. The MOC strategy as implemented here, or a modified version of it, may be usefully applied in CIs and in hearing aids.

In natural hearing, cochlear mechanical compression is dynamically adjusted via the medial olivocochlear efferent reflex (MOCR). These adjustments likely help understanding speech in noisy environments and are not available to the users of cochlear implants (CIs). The present study presents a bilateral CI sound processing strategy that reinstates some of the effects of the contralateral MOCR to CI users using frequency-specific, contralaterally controlled dynamic compression. The new strategy facilitates understanding speech in competition with speech-shaped noise in the bilateral and unilateral listening conditions tested, and produces significant spatial release from masking. The strategy may be usefully applied in hearing prostheses.

1Instituto de Neurociencias de Castilla y León, 2Instituto de Investigación Biomédica de Salamanca, 3Departamento de Cirugía, Facultad de Medicina, Universidad de Salamanca, Salamanca, Spain; 4USA Laboratory, MED-EL GmbH, Innsbruck, Austria; 5Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina, USA; 6Institute of Mechatronics, University of Innsbruck, Innsbruck, Austria; 7Department of Surgery, Duke University Medical Center, Durham, North Carolina, USA; 8Department of Biomedical Engineering, Duke University, Durham, North Carolina, USA.

This research was funded by European Regional Development Funds, by the Spanish Ministry of Economy and Competitiveness (Grants BFU2009-07909 and BFU2012-39544-C02) to E.A.L.-P. and by MED-EL GmbH.

E.A.L.-P. conceived the processor and the study. A.E.-M. and J.S.S. implemented the sound processors and testing tools. R.D.W. collected the data. E.A.L.-P., A.E.-M., and J.S.S. analyzed the data. E.A.L.-P. and J.S.S. wrote the manuscript. All authors designed the testing conditions and revised the manuscript.

Portions of this article were presented at the 167th Meeting of the Acoustical Society of America on May 5–9, 2014, in Providence, Rhode Island, and at the Bernstein Sparks Workshop, 13th International Conference on Cochlear Implants on June 18–21, 2014, in Munich, Germany.

To protect the intellectual property, the University of Salamanca has filed applications to the Spanish Patent Office (WO2013/164511A1) and to the European Patent Office (WO2015/169649A1) related to the present research.

The authors have no conflicts of interest to disclose.

Received February 13, 2015; accepted December 15, 2015.

Address for correspondence: Enrique A. Lopez-Poveda, Instituto de Neurociencias de Castilla y León, Universidad de Salamanca, Calle Pintor Fernando Gallego 1, 37007 Salamanca, Spain. E-mail: ealopezpoveda@usal.es

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