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Improved Neural Coding of ITD with Bilateral Cochlear Implants by Introducing Short Inter-pulse Intervals

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Abstract

Bilateral cochlear implant (CI) users have poor perceptual sensitivity to interaural time differences (ITDs), which limits their ability to localize sounds and understand speech in noisy environments. This is especially true for high-rate (> 300 pps) periodic pulse trains, which are used as carriers in CI processors. Here, we investigate a novel stimulation strategy in which extra pulses are added to high-rate periodic pulse trains to introduce short inter-pulse intervals (SIPIs). We hypothesized that SIPIs can improve neural ITD sensitivity similarly to the effect observed by randomly jittering IPIs (Hancock et al., J. Neurophysiol. 108:714–28, 2012). To test this hypothesis, we measured ITD sensitivity of single units in the inferior colliculus (IC) of unanesthetized rabbits with bilateral CIs. Introducing SIPIs into high-rate pulse trains significantly increased firing rates for ~ 60 % of IC neurons, and the extra spikes tended to be synchronized to the SIPIs. The additional firings produced by SIPIs uncovered latent ITD sensitivity that was comparable to that observed with low-rate pulse trains. In some neurons, high spontaneous firing rates masked the ITD sensitivity introduced by SIPIs. ITD sensitivity in these neurons could be revealed by emphasizing stimulus-synchronized spikes with a coincidence detection analysis. Overall, these results with SIPIs are consistent with the effects observed previously with jittered pulse trains, with the added benefit of retaining control over the timing and number of SIPIs. A novel CI processing strategy could incorporate SIPIs by inserting them at selected times to high-rate pulse train carriers. Such a strategy could potentially improve ITD perception without degrading speech intelligibility and thereby improve outcomes for bilateral CI users.

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Acknowledgements

We thank Camille Shaw and Melissa McKinnon for technical assistance, Michael Kaplan for advice regarding cochlear implantation, and Cochlear Ltd. for providing the cochlear implants.

Funding

This work was supported by NIH grants R01 DC00575 (BD), P30 DC005209, F31 DC014873 (BDB) and the Hearing Health Foundation (Emerging Research Grant, YC).

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Authors and Affiliations

  1. Eaton-Peabody Laboratories, Massachusetts Eye & Ear, 243 Charles St, Boston, MA, 02114, USA

    Brian D. Buechel, Kenneth E. Hancock, Yoojin Chung & Bertrand Delgutte

  2. Program in Speech and Hearing Bioscience and Technology, Harvard Medical School, Boston, MA, 02115, USA

    Brian D. Buechel

  3. Department of Otolaryngology, Harvard Medical School, Boston, MA, 02115, USA

    Kenneth E. Hancock, Yoojin Chung & Bertrand Delgutte

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  1. Brian D. Buechel
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  2. Kenneth E. Hancock
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  4. Bertrand Delgutte
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Correspondence to Bertrand Delgutte.

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Buechel, B.D., Hancock, K.E., Chung, Y. et al. Improved Neural Coding of ITD with Bilateral Cochlear Implants by Introducing Short Inter-pulse Intervals. JARO 19, 681–702 (2018). https://doi.org/10.1007/s10162-018-00693-0

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