Title: Transmission and Processing of Binaural Cues with Bilateral Cochlear Implants

Speaker: Paul Mayo

Abstract: Bilateral inputs provide major benefits across sensory systems, which is why 10-20% of the approximately one million cochlear-implant (CI) users worldwide have bilateral CIs (BI-CIs). Although BI-CI listeners experience many quality-of-life improvements after surgery, their spatial-hearing abilities remain poorer than those of normal-hearing (NH) listeners. This difference likely reflects several interacting limitations, but the exact mechanisms and potential solutions remain unclear. Making CIs more effective in noise could alleviate a significant health, social, and quality of life burden for CI listeners and healthcare providers. The overarching goal of this dissertation is to broaden the field’s understanding of binaural processing under electrical stimulation and identify potential strategies for improving spatial hearing in BI-CI listeners. Study 1 quantified the binaural cues created by BI-CIs programmed with different stimulation strategies for various types of acoustic stimuli and was designed to identify potential processing-related distortions to binaural cues. Results for Study 1 showed that spectral-peak picking stimulation strategies commonly used in BI-CIs introduce processing-related distortions to cues necessary for spatial hearing, whereas constantly stimulating strategies better preserve these cues. Study 2 used observer weighting analyses to determine how BI-CI listeners process and integrate binaural information to perform spatial-hearing tasks. Results revealed that, when conveyed using bilaterally synchronized direct stimulation equipment, interaural time differences (ITDs) encoded via pulse timing in electrical stimulation improve performance on lateralization and localization tasks (beyond that achievable with interaural level differences alone). Lastly, Study 3 proposed and evaluated methods to overcome the known binaural rate limitation in BI-CI listeners using multi-electrode stimulation, emphasizing clinical implementation. Study 3 found that for multi-electrode stimulation, shortening the inter-electrode pulse interval (the time between pulses) results in ITD lateralization responses equivalent to those of single-electrode stimulation. Together, these studies identify multiple avenues for potentially improving spatial hearing in BI-CI listeners. They expand the field’s understanding of binaural processing with electrical stimulation and can inform the development of future signal processing, sound coding, and device-hardware changes that may improve the encoding of spatial information with BI-CIs.