Binaural hearing with a synchronised bilateral cochlear implant system in adult users

Abstract

Most bilateral cochlear implant (CI) users have horizontal sound-source localisation and speech perception in noise that is better than that of unilateral CI users and worse than that of normal-hearing listeners. The discrepancy between bilateral CI users and normal-hearing people is thought to partly represent technical limitations in current CI signal processing, which hinders the use of interaural time and level differences. One such limitation is the independent and unsynchronised signal processing in the two CIs. The Digisonic® SP Binaural (DSPB) CI aims to improve binaural hearing by providing synchronised processing of the acoustical inputs, using a single speech processor and two microphones. Two further studies have been published since the commencement of this research project on the spatial hearing ability of DSPB users. Although both these studies concluded that both horizontal sound-source localisation and speech perception in noise can be accessible though the DSPB CIs, consistently with what is provided by conventional bilateral CIs, these studies have some limitations that may have influenced their conclusions. For example, the way the localisation data of their subjects were analysed does not help to determine whether DSPB CI subjects can localise sounds at better than expected from guessing and whether their localisation ability is based on interaural cues or monaural cues introduced by the head shadow. Spatial benefits for speech perception were also not fully reported by the previous studies. The aim of the studies reported in this thesis was to address the limitations of the previous studies and explore in more depth the spatial benefits of the DSPB CIs.

The spatial benefits experienced by eight DSPB CI subjects were assessed in horizontal sound-source localisation, speech perception in noise and self-reported measures. Their ability was also compared to eight unilateral CI subjects, who were chosen as likely to be representative of the better unilateral CI performers. Results showed that the majority of the postlingually deaf DSPB subjects could localise sounds at a better than chance range, defined by unbiased and biased guessing, which seems similar to previous results with conventional bilateral CI adults. Although the results for unilateral CI subjects indicate that monaural cues may provide some useful information for localisation, such cues were found to provide lower localisation accuracy than binaural cues provided by DSPB implants. Speech perception thresholds were also assessed with the speech and noise spatially co-located and separated. Results showed that, as with unilateral CI subjects, the DSPB subjects were not able to take advantage of separating speech from noise for speech perception. Results from the Speech, Spatial and Qualities of Hearing Scale indicated better self-reported spatial hearing ability for the DSPB than unilateral CIs groups, although it was not statistically significant. It is concluded that the DSPB CIs seem to provide advantages for horizontal localisation over unilateral CIs for the majority of postlingually deaf DSPB adults. There is, however, no evidence that the way the processing is synchronised in the DSPB CIs can offer any advantages over conventional bilateral CIs for localisation.

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ORCID for Daniel Rowan: orcid.org/0000-0002-7190-9997

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