SPIKE-INITIATION SITE IN ELECTRICALLY STIMULATED HEALTHY AND LESIONED COCHLEAE I: PERIPHERAL-TO-CENTRAL ACTIVATION WITH CATHODIC VERSUS ANODIC POLARITY

Cochlear implants (CIs) are active neuroprostheses that directly, electrically stimulate the spiral ganglion neurons (SGN) of the auditory nerve. Modelling studies propose that the anodic and cathodic polarity excite the bipolar SGNs at central and peripheral processes, respectively. Due to better excitability of the peripheral processes, the cathodic polarity should result in lower thresholds (i.e., cathodic-benefit). Thus, clinical CIs typically use biphasic, cathodic-leading pulses. The frequently observed anodic-benefit in human CI listeners is thought to reflect degenerated peripheral processes. We used the guinea pig model to assess the effectiveness of stimulation and to infer on spike-initiation sites for the two polarities. We analysed the contributions of peripheral and central processes by introducing locally restricted chronic degenerations (~ 500μm; n=13). We stimulated via a species adjusted CI (6 contacts, 700µm spacing) using monophasic and biphasic pulses (monopolar mode) of alternating (leading-phase) polarity. Electrically-evoked compound action potential recordings were analysed separately for the two polarities. The results were compared to those of 20 control ears. We confirmed the cathodic-benefit (lower threshold, larger amplitude, wider dynamic range, and steeper slope) for cochleae with healthy SGN. Longer latencies (50–70μs) to cathodic than anodic pulses confirmed peripheral and central spike-initiation sites, respectively. After soma degeneration, the threshold showed polarity-specific changes, leading to an anodic-benefit. The lack of systematic change in latencies indicated a relocation of the spike-initiation sites to neighbouring SGN. This apical-to-basal change in activation is evaluated in the companion study II.
Supported by MED-EL GmbH, MHH-plus foundation and Deutsche Forschungsgemeinschaft (Exc 2177).