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Channel Interaction and Current Level Affect Across-Electrode Integration of Interaural Time Differences in Bilateral Cochlear-Implant Listeners

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Abstract

Sensitivity to interaural time differences (ITDs) is important for sound localization. Normal-hearing listeners benefit from across-frequency processing, as seen with improved ITD thresholds when consistent ITD cues are presented over a range of frequency channels compared with when ITD information is only presented in a single frequency channel. This study aimed to clarify whether cochlear-implant (CI) listeners can make use of similar processing when being stimulated with multiple interaural electrode pairs transmitting consistent ITD information. ITD thresholds for unmodulated, 100-pulse-per-second pulse trains were measured in seven bilateral CI listeners using research interfaces. Consistent ITDs were presented at either one or two electrode pairs at different current levels, allowing for comparisons at either constant level per component electrode or equal overall loudness. Different tonotopic distances between the pairs were tested in order to clarify the potential influence of channel interaction. Comparison of ITD thresholds between double pairs and the respective single pairs revealed systematic effects of tonotopic separation and current level. At constant levels, performance with double-pair stimulation improved compared with single-pair stimulation but only for large tonotopic separation. Comparisons at equal overall loudness revealed no benefit from presenting ITD information at two electrode pairs for any tonotopic spacing. Irrespective of electrode-pair configuration, ITD sensitivity improved with increasing current level. Hence, the improved ITD sensitivity for double pairs found for a large tonotopic separation and constant current levels seems to be due to increased loudness. The overall data suggest that CI listeners can benefit from combining consistent ITD information across multiple electrodes, provided sufficient stimulus levels and that stimulating electrode pairs are widely spaced.

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Acknowledgments

We thank all our listeners for their patience and their participation in this study. We are grateful to Stefan-Marcel Pok for help in recruiting listeners, Michael Mihocic for help in collecting the data, and Peter Schleich and Peter Nopp for fruitful discussions. We thank the Institute of Ion Physics and Applied Physics, Leopold-Franzens-University of Innsbruck, Austria for providing the equipment for direct electric stimulation. We thank the associate editor Ruth Litovsky and two anonymous reviewers for helpful comments on an earlier version of this article. This study was supported by MED-EL Corporation.

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Authors and Affiliations

  1. Acoustics Research Institute, Austrian Academy of Sciences, Wohllebengasse 12-14, 1040, Vienna, Austria

    Katharina Egger, Piotr Majdak & Bernhard Laback

Authors
  1. Katharina Egger
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  2. Piotr Majdak
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  3. Bernhard Laback
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Corresponding author

Correspondence to Piotr Majdak.

Appendix

Appendix

For the interested reader, Figure 7 presents the individual listeners’ ITD thresholds for single pairs (open symbols) and double pairs (filled symbols), plotted for large, small, and zero tonotopic separations of the double pairs. Thresholds are shown for the three level conditions LHI (circles), LMI (triangles), and LLO (squares). Colors differentiate between the single pairs B (red), C (green), and D (blue). Note that, for CI52 and CI61, only conditions with small and zero tonotopic separations were tested.

FIG. 7
figure 7

Individual listeners’ ITD thresholds for single pairs (open symbols) and double pairs (filled symbols), plotted for large, small, and zero tonotopic separations of the double pairs. Thresholds are shown for the three level conditions LHI (circles), LMI (triangles), and LLO (squares). The colors differentiate between the single pairs B (red), C (green), and D (blue). For CI52 and CI61, only conditions with small and zero tonotopic separations were tested.

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Egger, K., Majdak, P. & Laback, B. Channel Interaction and Current Level Affect Across-Electrode Integration of Interaural Time Differences in Bilateral Cochlear-Implant Listeners. JARO 17, 55–67 (2016). https://doi.org/10.1007/s10162-015-0542-8

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  • DOI: https://doi.org/10.1007/s10162-015-0542-8

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