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Matching Automatic Gain Control Across Devices in Bimodal Cochlear Implant Users

Veugen, Lidwien C. E.; Chalupper, Josef; Snik, Ad F. M.; Opstal, A. John van; Mens, Lucas H. M.

doi: 10.1097/AUD.0000000000000260
Research Articles
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Objectives: The purpose of this study was to improve bimodal benefit in listeners using a cochlear implant (CI) and a hearing aid (HA) in contralateral ears, by matching the time constants and the number of compression channels of the automatic gain control (AGC) of the HA to the CI. Equivalent AGC was hypothesized to support a balanced loudness for dynamically changing signals like speech and improve bimodal benefit for speech understanding in quiet and with noise presented from the side(s) at 90 degree.

Design: Fifteen subjects participated in the study, all using the same Advanced Bionics Harmony CI processor and HA (Phonak Naida S IX UP). In a 3-visit crossover design with 4 weeks between sessions, performance was measured using a HA with a standard AGC (syllabic multichannel compression with 1 ms attack time and 50 ms release time) or an AGC that was adjusted to match that of the CI processor (dual AGC broadband compression, 3 and 240 msec attack time, 80 and 1500 msec release time). In all devices, the AGC was activated above the threshold of 63 dB SPL. The authors balanced loudness across the devices for soft and loud input sounds in 3 frequency bands (0 to 548, 548 to 1000, and >1000 Hz). Speech understanding was tested in free field in quiet and in noise for three spatial speaker configurations, with target speech always presented from the front. Single-talker noise was either presented from the CI side or the HA side, or uncorrelated stationary speech-weighted noise or single-talker noise was presented from both sides. Questionnaires were administered to assess differences in perception between the two bimodal fittings.

Results: Significant bimodal benefit over the CI alone was only found for the AGC-matched HA for the speech tests with single-talker noise. Compared with the standard HA, matched AGC characteristics significantly improved speech understanding in single-talker noise by 1.9 dB when noise was presented from the HA side. AGC matching increased bimodal benefit insignificantly by 0.6 dB when noise was presented from the CI implanted side, or by 0.8 (single-talker noise) and 1.1 dB (stationary noise) in the more complex configurations with two simultaneous maskers from both sides. In questionnaires, subjects rated the AGC-matched HA higher than the standard HA for understanding of one person in quiet and in noise, and for the quality of sounds. Listening to a slightly raised voice, subjects indicated increased listening comfort with matched AGCs. At the end of the study, 9 of 15 subjects preferred to take home the AGC-matched HA, 1 preferred the standard HA and 5 subjects had no preference.

Conclusion: For bimodal listening, the AGC-matched HA outperformed the standard HA in speech understanding in noise tasks using a single competing talker and it was favored in questionnaires and in a subjective preference test. When noise was presented from the HA side, AGC matching resulted in a 1.9 dB SNR additional benefit, even though the HA was at the least favorable SNR side in this speaker configuration. Our results possibly suggest better binaural processing for matched AGCs.

For listeners using a cochlear implant (CI), we hypothesized to find greater bimodal benefit from a contralateral hearing aid (HA) with matched, as opposed to unmatched automatic gain control (AGC). A HA with syllabic compression was compared to a HA with a dual-loop AGC, with fast and slow time constants similar to the CI processor. Bimodal benefit was assessed for speech understanding in quiet and with noise presented from the side(s) at +/-90°. Matching AGCs significantly improved speech understanding with noise from the HA side by 2 dB SNR, it increased listening comfort evidenced by a listening test and was favored in questionnaires and a subjective preference test.

1Department of Biophysics, Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, Nijmegen, The Netherlands; 2Advanced Bionics European Research Centre (AB ERC), Hannover, Germany; and 3Department of Otorhinolaryngology, Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands.

Part of this study has been presented at CIAP 2013 and IHCON 2014, both in Lake Tahoe, CA.

L.V. performed experiments, analyzed data, and wrote the article; J.C. provided hearing aids and experimental materials. L.V., J.C.A.S., and L.M. designed experiments. All five authors provided critical revision of the article.

L.M. is a member of the Advanced Bionics European Clinical Advisory Board. J.C. is an employee of Advanced Bionics GmbH, Germany. The other authors have no conflicts of interest to disclose.

Received October 2, 2014; accepted November 4, 2015.

Address for correspondence: Lucas H. M. Mens, Department of Otorhinolaryngology, Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. E-mail: Lucas.Mens@radboudumc.nl

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