Characterization and Modelling of the Electrode-Nerve Interface for Electro-Acoustic Stimulation in Cochlear Implant Users

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Everyday more people with residual hearing receive cochlear implants (CIs) to obtain electroacoustic stimulation (EAS), i.e. combined auditory stimulation delivered electrically from CI electrodes and acoustically through a hearing aid (HA). For EAS users low frequency acoustic hearing produces almost no recognition for speech in noise, but the performance is significantly enhanced when combined with the electric hearing delivered by the CI. However, it is still unknown how the acoustic and electric stimulation modalities interact with each other. Clinical experience has found that some EAS users do not use the HA at all and rely solely on electric stimulation. Moreover, it has been observed that EAS users require more time to achieve their maximum performance with the CI alone than regular CI users. These limitations may have influence in defining the implantation criteria of EAS. Possible explanations for these limitations are interactions between acoustic and electric stimulation. These interactions become more and more important for people with large residual hearing because the overlap between electric and acoustic stimulation is larger. For example, it has been shown that the range of hearing loss for which EAS devices are prescribed is getting closer to the range of age related hearing loss. Age related hearing loss, as EAS, is characterized by lost of audibility in the high frequencies. Given that the population in Europe is becoming older with increased hearing loss related problems new treatments are required to rehabilitate people with age related hearing loss. Cognitive decline and hearing loss seem to be related and new treatments such as EAS are very promising to improve quality of life of the elderly population. This work is about characterizing the interaction between acoustic and electric stimulation. The characterization will consist of psychophysical, electrophysiological, imaging and backward telemetry measures in EAS users. Psychophysically, the acoustic and electric interaction will be measured by means of simultaneous or forward masking experiments. Cone beam computer tomography (CBCT) for each study participant will be used to assess the insertion depth and electrode positions. Electrical field imaging (EFI) from the backward telemetry of the implant will be used to characterize the electrode neuron interface, and finally electrocochleography (eChochG) measures will be used to assess the overlap between acoustic and electric stimulation at the neural level. The outcomes of this project are datasets, fitting procedures and a model characterizing the interaction between acoustic and electric stimulation of each EAS user. These outcomes have application to optimize the configuration of EAS devices and to assess whether the indication for EAS implantation can be potentially extended to people with age related hearing loss.