| Cochlear implants are neural prosthetic devices that restore partial hearing in many, but not all, hearing impaired individuals. In a cochlear implant device, sound is processed by an external speech processor, encoded as a data stream, and transmitted via a radio-frequency link across the skin to a subcutaneously-implanted receiver/stimulator located near the external ear. The signals are decoded, converted to current pulses and delivered into the cochlea by means of a surgically-implanted, multiple-contact, electrode array to stimulate surviving auditory nerve fibers in a tonotopic manner. Stimuli are typically delivered in a monopolar-coupled manner relative to a remote return electrode. Specific knowledge of how currents flow within and out of the implanted cochlea are important for understanding how present devices recruit surviving auditory fibers, as well as improving the design and clinical application of future devices. Few studies have addressed this problem to date, so our specific knowledge is limited. Consequently, the goal of this dissertation was to better understand the routes taken by the stimulus current as it leaves the cochlea in individual cochlear implant subjects. This study assumes that a better understanding of the injected current flow patterns would lead to improved control over stimulus current, which may result in the reduction of extracochlear stimulation and better-targeted stimulation of the auditory nerve. Because current flow cannot be directly measured in cochlear implant users, this study uses surface artifact potentials to test predictions about how current may flow within and outside the cochlea. These surface potentials represent the far field of the stimulation delivered by the device, and are recorded non-invasively on the scalp, neck, and face of cochlear implant subjects during the active stimulation by the device. Results from the study indicate that differences exist in the primary current flow pathways for stimulation of apical and basal electrode contacts. This observation is counter to long held assumptions about current flow within the cochlea. Analytical head models and inverse dipole source localization methods have been developed to interpret these results further. Knowledge gained from this study may eventually lead to higher levels of performance for all cochlear implant users. |
