Binaural interaction in electrically and acoustically evoked auditory brainstem responses in six bimodal listeners

Introduction: ‘Bimodal hearing’ refers to the condition in which electrical hearing via a cochlear implant is combined with residual hearing in the non-implanted ear. Several studies demonstrate improvements in speech perception in noise and sound localization. Mechanisms underlying these benefits are still unclear. The objective of this study is to gain insight into binaural processing in bimodal listeners. Methods: Three bimodal listeners participated in this study. All were implanted with Nucleus devices, of which one with a Hybrid L24. Binaural hearing is investigated using behavioral tests and auditory brainstem responses (ABR). ABRs are recorded in three stimulus conditions: monaural left, monaural right and binaural stimulation. The implanted ear is stimulated electrically, whereas the non-implanted ear is stimulated with 500 Hz tone-bursts. A binaural interaction component is derived from the ABR (ABR-BIC) by subtracting the binaural response from the sum of both monaural responses. The presence of an ABR-BIC is linked to performance on speech in noise, sound localization and Speech Spatial Qualities of Hearing Scale (SSQ). Results: A reliable ABR-BIC could only be recorded in the Hybrid patient. Residual hearing in the non-implanted ear was remarkably better in this subject compared to the other participating subjects. Conclusion: These preliminary data suggest that binaural integration of electrical and acoustical signals can occur. However, measuring ABR-BIC in bimodal patients is hampered by the low signal-to-noise ratio of 500 Hz tone-burst ABR. In bimodal listeners with moderate hearing loss in the 500 Hz cochlear region, recording ABR-BICs is extremely challenging.