The Effects of Dynamic-range Automatic Gain Control on Sentence Intelligibility With a Speech Masker in Simulated Cochlear Implant Listening

Objectives: “Channel-linked” and “multi-band” front-end automatic gain control (AGC) were examined as alternatives to single-band, channel-unlinked AGC in simulated bilateral cochlear implant (CI) processing. In channel-linked AGC, the same gain control signal was applied to the input signals to both of the two CIs (“channels”). In multi-band AGC, gain control acted independently on each of a number of narrow frequency regions per channel. Design: Speech intelligibility performance was measured with a single target (to the left, at –15 or –30º) and a single, symmetrically-opposed masker (to the right) at a signal-to-noise ratio (SNR) of –2 decibels. Binaural sentence intelligibility was measured as a function of whether channel linking was present and of the number of AGC bands. Analysis of variance was performed to assess condition effects on percent correct across the two spatial arrangements, both at a high and a low AGC threshold. Acoustic analysis was conducted to compare postcompressed better-ear SNR, interaural differences, and monaural within-band envelope levels across processing conditions. Results: Analyses of variance indicated significant main effects of both channel linking and number of bands at low threshold, and of channel linking at high threshold. These improvements were accompanied by several acoustic changes. Linked AGC produced a more favorable better-ear SNR and better preserved broadband interaural level difference statistics, but did not reduce dynamic range as much as unlinked AGC. Multi-band AGC sometimes improved better-ear SNR statistics and always improved broadband interaural level difference statistics whenever the AGC channels were unlinked. Multi-band AGC produced output envelope levels that were higher than single-band AGC. Conclusions: These results favor strategies that incorporate channel-linked AGC and multi-band AGC for bilateral CIs. Linked AGC aids speech intelligibility in spatially separated speech, but reduces the degree to which dynamic range is compressed. Combining multi-band and channel-linked AGC offsets the potential impact of diminished dynamic range with linked AGC without sacrificing the intelligibility gains observed with linked AGC. ACKNOWLEDGMENTS: The authors thank Bernhard U. Seeber for providing the automatic gain control algorithm on which our implementation was based, and Joshua S. Stohl for his helpful feedback on this article. The authors also thank Katrina Killian and Lauren Dubyne for their work in coordinating data collection. This work was supported by NIDCD (Brown R-01 DC008329) and University of Pittsburgh T-32 Training Grant (T32-DC011499). The authors have no conflicts of interest or finance to disclose. Address for correspondence: Nathaniel J. Spencer, Department of Communication Sciences and Disorders, University of Pittsburgh, 5056 Forbes Tower, 3600 Forbes at Atwood, Pittsburgh, PA 15213, USA. E-mail: nathaniel.spencer.1.ctr@us.af.mil Received March 24, 2016; accepted July 16, 2018. Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.

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