Cochlear-implant (CI) users with single-sided deafness (SSD)—that is, one normal-hearing (NH) ear and one CI ear—can obtain some unmasking benefits when a mixture of target and masking voices is presented to the NH ear and a copy of just the masking voices is presented to the CI ear. NH listeners show similar benefits in a simulation of SSD-CI listening, whereby a mixture of target and masking voices is presented to one ear and a vocoded copy of the masking voices is presented to the opposite ear. However, the magnitude of the benefit for SSD-CI listeners is highly variable across individuals and is on average less than for NH listeners presented with vocoded stimuli. One possible explanation for the limited benefit observed for some SSD-CI users is that temporal and spectral discrepancies between the acoustic and electric ears might interfere with contralateral unmasking. The present study presented vocoder simulations to NH participants to examine the effects of interaural temporal and spectral mismatches on contralateral unmasking.
Speech-reception performance was measured in a competing-talker paradigm for NH listeners presented with vocoder simulations of SSD-CI listening. In the monaural condition, listeners identified target speech masked by two same-gender interferers, presented to the left ear. In the bilateral condition, the same stimuli were presented to the left ear, but the right ear was presented with a noise-vocoded copy of the interfering voices. This paradigm tested whether listeners could integrate the interfering voices across the ears to better hear the monaural target. Three common distortions inherent in CI processing were introduced to the vocoder processing: spectral shifts, temporal delays, and reduced frequency selectivity.
In experiment 1, contralateral unmasking (i.e., the benefit from adding the vocoded maskers to the second ear) was impaired by spectral mismatches of four equivalent rectangular bandwidths or greater. This is equivalent to roughly a 3.6-mm mismatch between the cochlear places stimulated in the electric and acoustic ears, which is on the low end of the average expected mismatch for SSD-CI listeners. In experiment 2, performance was negatively affected by a temporal mismatch of 24 ms or greater, but not for mismatches in the 0 to 12 ms range expected for SSD-CI listeners. Experiment 3 showed an interaction between spectral shift and spectral resolution, with less effect of interaural spectral mismatches when the number of vocoder channels was reduced. Experiment 4 applied interaural spectral and temporal mismatches in combination. Performance was best when both frequency and timing were aligned, but in cases where a mismatch was present in one dimension (either frequency or latency), the addition of mismatch in the second dimension did not further disrupt performance.
These results emphasize the need for interaural alignment—in timing and especially in frequency—to maximize contralateral unmasking for NH listeners presented with vocoder simulations of SSD-CI listening. Improved processing strategies that reduce mismatch between the electric and acoustic ears of SSD-CI listeners might improve their ability to obtain binaural benefits in multitalker environments.
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1National Military Audiology and Speech Pathology Center, Walter Reed National Military Medical Center, Bethesda, Maryland, USA; and 2Neuroscience and Cognitive Science Program, Department of Hearing and Speech Sciences, University of Maryland, College Park, Maryland, USA.
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This study was supported by Grant DM130007 from the Defense Medical Research and Development Program (J.G.W.B.)
Portions of the article were presented as posters at the 2015 Conference on Implantable Auditory Prostheses, Tahoe City, CA, and at the 2015 Midwinter Meeting of the Association for Research in Otolaryngology, Baltimore, MD.
J.M.W., D.S.B., and J.G.W.B. designed the experiments; J.M.W. and J.G.W.B. recruited participants and collected the data; J.M.W. analyzed the data; J.M.W. and J.G.W.B. wrote the article. All authors discussed the results and implications and commented on the manuscript at all stages.
The opinions and assertions presented are the private views of the authors and are not to be construed as official or as necessarily reflecting the views of the Department of Defense. In addition, the identification of specific organization logos, products, or scientific instrumentation does not constitute endorsement, implied endorsement, or preferential treatment on the part of the authors, DoD, or any component agency where inclusion is an integral part of the scientific record.
The authors have no conflicts of interest to disclose.
Received February 11, 2016; accepted August 18, 2016.
Address for correspondence: Jessica M. Wess, National Military Audiology and Speech Pathology Center, Walter Reed National Military Medical Center, 4954 N. Palmer Rd., Bethesda, MD 20889, USA. E-mail: firstname.lastname@example.org