ITD coding in bilateral cochlear implants for listening in reverberant space
* Presenting author
Abstract:
Interaural time differences (ITDs) help normal hearing listeners perceive, locate and understand sounds in noisy and reverberant conditions, but their benefit is limited with bilateral cochlear implants. Monaghan & Seeber (2016) presented a temporal enhancement algorithm that sharpens and deepens modulations at onsets of selected peaks in the signal envelope and showed improved ITD sensitivity with tone-vocoded speech. Based on the short-term direct-to-reverberant ratio the algorithm selects peaks that are dominated by the direct sound rather than reflections, and applies enhancements at times derived from the timings of envelope peaks. However, extensive analysis revealed that envelope peak timings do not robustly convey the ITD of a source in reverberant rooms. We present an updated version of the algorithm that estimates the short-term ITD from the signal’s temporal fine structure and adjusts the enhancement timings to encode that ITD. Furthermore, the new algorithm enhances envelope peaks based on the short-term interaural coherence. Results from the ongoing evaluation using acute testing of CI users show that the new algorithm improves ITD-based lateralization relative to the old algorithm and the unenhanced case. Unlike in previous vocoder testing with normal hearing listeners, in the acute test some CI users showed slightly degraded speech understanding.