Characterization of electric-acoustic interaction in cochlear implant users with ipsilateral residual hearing

This thesis details the interaction in hearing caused by combined electric and acoustic stimulation (EAS) in cochlear implant (CI) users with ipsilateral residual hearing. As part of this thesis, methods were designed and implemented to characterize simultaneous masking in electric-acoustic hearing using direct electric stimulation applied via the CI. Moreover, the telemetry capabilities of the CIs were utilized to measure and characterize electric-acoustic interaction objectively. As part of this thesis, a sound coding strategy for EAS users was proposed that virtually extends the CI electrode array to provide a substitute for auditory stimulation. This strategy was used to investigate the effects of electric-acoustic interaction on speech perception.
This work demonstrated that electric-acoustic interaction is a perceivable phenomenon for current CI users with ipsilateral residual hearing. It was shown that acoustic and electric detection thresholds can be elevated in the presence of the respective other stimulation modality. Furthermore, an asymmetry in the spread of masking across stimulation modalities was revealed. This may indicate differences in the underlying mechanism or the origin of these interactions. The intracochlear measurements of cochlear potentials have confirmed that electric-acoustic interactions are measurable via current CIs and demonstrated that interaction in electric-acoustic hearing occurs at the level of the auditory periphery. The results of the evaluation of the proposed sound coding strategy, demonstrated electric-acoustic interaction at speech perception level in EAS. Moreover, the proposed strategy significantly improved the speech perception of EAS users that relied on electric stimulation alone.