Bimodal processing with AEP recordings, part II: evidence from the auditory brainstem response

Introduction: Continuous advances in cochlear implant (CI) technology have led to a relaxation of the implantation criteria. Hence, an increasing amount of CI-users combine electrical and acoustical hearing in opposite ears. To date, it is unclear how the central auditory system processes binaurally presented electrical and acoustical signals. This study explores the utility of the auditory brainstem response (ABR) to assess brainstem processing of bimodal stimulation. Methods: Ten CI-users with substantial contralateral residual hearing volunteered in the study. The non-implanted ear was stimulated acoustically, whereas the implanted ear was stimulated acoustically. Before starting the ABR acquisition, the electrical and acoustical stimulation was pitch and loudness balanced. Subsequently, ABRs were recorded to monaural acoustical, monaural electrical, and bimodal stimulation. Results: Electrical ABRs were found in all subjects in both the monaural and bimodal condition. Acoustical ABRs were obtained in three subjects. The presence of monaural acoustical ABRs depends both on the degree and configuration of the contralateral residual hearing. The bimodal ABR of each of these three subjects showed a different, but reproducible, morphology. Conclusion: Although acoustical ABRs were not always present, these preliminary data show that it is feasible to obtain brainstem responses to bimodal stimulation