Spread of excitation varies for different electrical pulse shapes and stimulation modes in cochlear implants

In Cochlear Implants (CI) Bipolar (BP) electrical stimulation has been suggested as a method to reduce the spread of current along the cochlea. However, behavioral measurements in BP mode have shown either similar or worse performance than in Monopolar (MP) mode. This could be explained by a bimodal excitation pattern, with two main excitation peaks at the sites of the stimulating electrodes. We measured the Spread of Excitation (SOE) by means of the Electrically Evoked Compound Action Potential (ECAP), obtained using the forward-masked paradigm. The aim was to measure the bimodality of the excitation and to determine whether it could be reduced by using asymmetric pulses. Three types of maskers shapes were used: Symmetric (SYM), Pseudomonophasic (PS), and Symmetric with a long Inter-Phase Gap (SYM-IPG) pulses. Maskers were presented in BP + 9 (wide), BP + 3 (narrow) and MP (only SYM) mode on fixed electrodes. The SOE obtained with the MP masker showed a main excitation peak close to the masker electrode. Wide SYM maskers produced bimodal excitation patterns showing two peaks close to the electrodes of the masker channel, whereas SYM-IPG maskers showed a single main peak near the electrode for which the masker's second phase (responsible for most of the masking) was anodic. Narrow SYM maskers showed complex and wider excitation patterns than asymmetric stimuli consistent with the overlap of the patterns produced by each channel's electrodes. The masking produced by narrow SYM-IPG and PS stimuli was more pronounced close to the masker electrode for which the effective phase was anodic. These results showed that the anodic polarity is the most effective one in BP mode and that the bimodal patterns produced by SYM maskers could be partially reduced by using asymmetric pulses.