SPIKE-INITIATION SITE IN ELECTRICALLY STIMULATED HEALTHY AND LESIONED COCHLEAE II: APICAL-TO-BASAL ACTIVATION FOR DIFFERENT STIMULUS CONFIGURATIONS

Cochlear implants (CIs) are successful neural protheses, restoring hearing by direct electric stimulation of spiral ganglion neurons (SGN). However, in last 20 years, the average word recognition scores of postlingual CI users stabilized at about 60-70% correct, with considerable variability. One aspect CI users struggle with is pitch perception, an important parameter for speech-understanding, especially in noise. So far, attempts to re-align pitch cues did not result in general improvements.
We used an animal model to evaluate the neuronal activation pattern to stimulation via a species-adjusted CI (6 contacts, 700µm spacing) using multiple stimulus configurations. We assessed the CI placement by micro computed-tomography imaging and reconstructed the cochleotopic origin of the responses via recordings from the tonotopically organized inferior colliculus. We analyzed data from 32 animals (17 female). Of these, 11 (5 female) had induced focal SGN-loss in the cochlear base.
The data showed stimulus-polarity specific off-site activation during monopolar stimulation. Response maxima to monophasic pulses differed more than 2 octaves between polarities. Broad activation patterns were observed for symmetric, biphasic stimuli, similar for both polarities. Subtracting their median activity, unveiled place cues, obscured by an apical-to-basal response gradient. SGN lesions led to basal shifts of response maxima for both polarities. The results point to weak and partially masked spectral pitch in biphasic stimulation. These findings fit to the hypothesis of central-anodic and peripheral-cathodic activation presented in the companion study I and provide potential explanation for difficulties in pitch perception in CI users.
Funding: MHH-plus Foundation, Deutsche Forschungsgemeinschaft (Exc 2177).