Theoretical and Experimental Characterization of Cochlear-Implant Stimulation Artifacts in EEG Recordings

Cochlear implants (CIs) restore hearing in individuals with severe sensorineural hearing loss. In recent years, electrically evoked auditory steady-state responses (EASSRs) to amplitude modulated (AM) signals have been studied as an objective measure. EASSRs can be objectively detected in electroencephalography (EEG) recordings at the modulation frequency using statistical tests. However, the presence of electrical stimulation artifacts from the CI itself hinders the EASSR detection. Whereas previous research has focused on an experimental characterization of these artifacts, this study presents a theoretical analysis of the stimulation signal together with an experimental analysis of the resulting artifacts to characterize their properties, origins and the effects of system nonlinearities. A stimulation signal model is presented and analyzed. The effects of pulse asymmetry and nonlinearity are examined. The theoretical statements are experimentally validated using an experimental setup containing a head phantom. The analysis shows that the stimulation artifact at the modulation frequency is inherent to the stimulation signal, even in the absence of system nonlinearities. Moreover, when the pulse asymmetry is taken into account, second and higher order polynomial nonlinearities are found to contribute negligibly to the spectral component at the modulation frequency. The experimental analyses indicate the proposed signal model is a more accurate model for the stimulation signal and the resulting stimulation artifact at the modulation frequency. The model may form an important step in determining artifact contamination in EEG recordings of EASSRs and other envelope-following responses in CI recipients, enabling improved response detection.