Investigation of binaural hearing with bilateral/hybrid-bilateral cochlear implant in simulated room and noise conditions

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For hearing-impaired people speech perception in environments with reverberation is much more difficult, while normal hearing is supported in such situations by binaural processing. The precedence effect or the 'law of the first wavefront' is such a binaural psychoacoustic effect. When a sound is followed by another sound separated by a sufficiently short time delay (below the listener's echo threshold), listeners perceive a single fused auditory image; its perceived spatial location is dominated by the location of the first-arriving sound (the first wave front, Wallach et al. 1949). For bilaterally implanted cochlear implant (CI) users occurs no such fusion of auditory events and there are either two spatially separated perceptions or the localization of direct sound (lead) is disturbed by reflections (lag) (Seeber & Hafter 2008). To explain the improved hearing performance observed in users of combined electric-acoustic stimulation (EAS) with bilateral acoustic low frequency residual hearing, we suspect that perception is supported by the precedence effect. However, the existing delay between acoustic and electrical components of the EAS devices could hamper the utilization of the precedence effect. Core objective of the project is therefore the investigation of binaural interactions in EAS users in combination with "synchronized" acoustic stimulation.Previous experimental setups (BA 2085/3-1) allowed only limited conclusions about speech perception of EAS and conventional CI users in everyday life. In particular, the extent of the deterioration resulting from the lack of the precedence effect in electrical / combined electric-acoustic stimulation was not determined.As part of the continuation of the project BA 2085/3-1, psychoacoustic listening tests shall assess the threshold of interaural level and delay differences in a group of EAS-users. Results will be compared with other groups of subjects (bilateral cochlear implant, CI in single-sided deafness (SSD), and a group consisting of asymmetrical inner ear hearing loss). In preliminary experiments, the delay time between the electric and acoustic components of the devices will be individually determined and compensated.Through the multichannel sound reproduction system (128 speakers) installed in our laboratory, complex sound fields are generated by means of wave field synthesis in order to investigate binaural effects with cochlear and hybrid implants in everyday environments.A better understanding of the EAS effect will lead to the development of optimized fitting strategies for cochlear implant systems, so that improvement in speech understanding in complex listening situations can be achieved.