Objective assessment of optimal group delays in cochlear implants

The human auditory periphery is a complex mechano-electrical system that transduces sound waves into nerve action potentials. In this sensory modality sound conduction to and frequency analysis in the cochlea produce frequency-dependent signal propagation delays. A cochlear implant (CI) is a neural prosthesis that replaces the peripheral auditory system partially by stimulating the auditory nerve electrically. This modality is in turn accompanied by artificial signal transmission delays. This study deals with the question how well the timing of neural excitation in these two modalities fit one another. For this purpose, we investigated wave V latencies of auditory brainstem responses evoked either acoustically (ABR) or electrically via the CI (EABR). The sum of delays consisting of CI signal processing and EABR wave V latencies allowed an estimation of the entire CI-channel-specific delay. We compared these values with ABR wave V latencies measured in normal hearing listeners in different frequency bands. As EABR wave V latencies were shorter than those evoked acoustically, appropriate values for delay elements (FIR group delays) in the CI system were determined and compared with the already implemented group delays. Optimized interaural stimulation timing in unilateral deaf subjects provided with a CI reduces the need for central auditory compensation and can improve speech recognition.