Does the Pulse Shape of Cochlear Implant Stimulations Influence the Strength and Spread of Neuronal Responses in the Guinea Pig Primary Auditory Cortex?

Background: A vast research area is still working on improving the coding strategies of cochlear implants (CI). The stimulation mode, pulse shape and grounding schemes can exert either moderate or drastic consequences on the response strength, spread of excitation, electrode discriminability and nerve excitability. Several strategies are currently used to implement sound loudness such as increasing the pulse amplitude, the pulse duration or the pulse rate. In this study, responses from auditory cortex neurons to stimulations delivered through a CI for which several parameters were modified such as the pulse amplitude, the pulse duration and the pulse shape were investigated.

Materials and Methods: Experiments were performed in urethane anesthetized guinea pigs. An acute stimulation setup comprising dedicated 6-stimulation electrode array and a 16-recording cortical electrode plot was used to estimate the tonotopic gradient of the primary auditory cortex (prior CI implantation) and it`s reponse to electrical stimulation after the CI implantation. Different electrical pattern were then used by varying the shape of a monopolar biphasic stimulation, such as increasing in pulse amplitude or duration, pseudomonophasic or ramped waveforms.

Results: These data suggest that equivalent cortical activation can be achieved by coding sound loudness either with pulse amplitude or with pulse duration. Analyzing the effect of asymmetrical pulses indicates that evoked activities and spatial activation tend to decrease as the ratio increased. Preliminary results on cortical activation by ramped pulses will be presented.