Redundant Information Is Sometimes More Beneficial Than Spatial Information to Understand Speech in Noise

Objectives: To establish a framework to unambiguously define and relate the different spatial effects in speech understanding: head shadow, redundancy, squelch, spatial release from masking (SRM), and so on. Next, to investigate the contribution of interaural time and level differences to these spatial effects in speech understanding and how this is influenced by the type of masking noise. Design: In our framework, SRM is uniquely characterized as a linear combination of head shadow, binaural redundancy, and binaural squelch. The latter two terms are combined into one binaural term, which we define as binaural contrast: a benefit of interaural differences. In this way, SRM is a simple sum of a monaural and a binaural term. We used the framework to quantify these spatial effects in 10 listeners with normal hearing. The participants performed speech intelligibility tasks in different spatial setups. We used head-related transfer functions to manipulate the presence of interaural time and level differences. We used three spectrally matched masker types: stationary speech-weighted noise, a competing talker, and speech-weighted noise that was modulated with the broadband temporal envelope of the competing talker. Results: We found that (1) binaural contrast was increased by interaural time differences, but reduced by interaural level differences, irrespective of masker type, and (2) large redundancy (the benefit of having identical information in two ears) could reduce binaural contrast and thus also reduce SRM. Conclusions: Our framework yielded new insights in binaural processing in speech intelligibility. First, interaural level differences disturb speech intelligibility in realistic listening conditions. Therefore, to optimize speech intelligibility in hearing aids, it is more beneficial to improve monaural signal-to-noise ratios rather than to preserve interaural level differences. Second, although redundancy is mostly ignored when considering spatial hearing, it might explain reduced SRM in some cases. ACKNOWLEDGMENTS: This research is funded by the Research Foundation—Flanders (SB PhD fellow at FWO), project 1S45817N; this research is jointly funded by Cochlear Ltd. and Flanders Innovation & Entrepreneurship (formerly IWT), project 150432; this project has also received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 637424, ERC starting Grant to Tom Francart). We thank our participants for their patience and enthusiasm during our experiment. Benjamin Dieudonné and Tom Francart designed experiments, analyzed data, and wrote the article. Benjamin Dieudonné performed experiments. The authors have no conflicts of interest to disclose. Address for correspondence: Tom Francart, Experimental Oto-rhino-laryngology, Department of Neurosciences, KU Leuven, University of Leuven, Herestraat 49 bus 721, 3000 Leuven, Belgium. E-mail: tom.francart@med.kuleuven.be, benjamin.dieudonne@med.kuleuven.be Received February 15, 2018; accepted August 6, 2018. Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.

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