Electrophysiological Spread of Excitation and Pitch Perception for Dual and Single Electrodes Using the Nucleus Freedom Cochlear Implant

Abstract

The first objective of the study was to determine whether there were any consistent differences in the electrophysiological spread of excitation (SOE) function, as measured using the electrically evoked compound action potential (ECAP), between dual and single electrode stimulation with the Nucleus Freedom cochlear implant system. Dual electrode stimulation is produced by electrically coupling two adjacent single electrodes. The second objective was to determine whether there were any relationships between the SOE functions and psychophysically measured pitch ranking of dual and single electrodes. Nine adult cochlear implant subjects participated in the study. ECAPs for dual and single electrode stimulation were measured using the forward masking paradigm, as also used in the Neural Response Telemetry (NRT) software with the Nucleus implant. Research software was used to generate the dual and single electrode stimuli and record the ECAPs. Spread of excitations (SOEs) were measured on a dual electrode and the two adjacent single electrodes, at three positions on the array: apical, mid, and basal. Compared were the ECAP amplitudes at the peak of the SOE functions, the widths of the scaled SOE functions at the 75% point, and the electrode positions at the peak of the SOE function and at the 75%, 50%, and 25% points on apical and basal sides of the scaled functions. Pitch ranking was measured for the same sets of dual and single electrodes. A two-alternative forced choice procedure was used, with the electrodes in each set paired with each other as AB and BA pairs. The subject indicated which of the two stimuli had the higher pitch. Dual electrode SOEs could be successfully obtained using the same methods as used to measure single electrode SOEs. The shapes of the dual and single electrodes SOEs were similar. There was a trend of a higher ECAP amplitude for the dual electrode at the peak of the SOE function, but this was only significant for two comparisons at the apical and basal positions. There were no significant differences in the SOE widths between dual and single electrodes. The electrodes at the peak of the SOE function and on the apical and basal sides of the function at the 75% position were, in most cases, tonotopically ordered. At the 50% and 25% positions, there were fewer significant differences between the dual and single electrodes The pitch ranking results showed that in 74% of cases, the single and dual electrodes at each position were successfully ranked in the expected tonotopic order. There were no statistically significant correlations between the pitch ranking results and the ordering of electrodes on the SOE functions. Dual electrode stimulation produced similar SOE functions as single electrode stimulation. A tonotopic ordering of electrodes at the peak of the SOE and on the sides of the functions was found, but this was not statistically related to the pitch ranking results. Electrophysiological spread of neural excitation (SOE) and pitch perception using dual and single electrodes was investigated in nine subjects using the Nucleus Freedom cochlear implant. Dual electrodes are produced by electrically coupling two adjacent single electrodes. The dual and single electrodes SOEs were similar in shape. Higher electrophysiological response amplitudes were generally found for the dual electrodes. There were no differences in SOE widths between dual and single electrodes. In three quarters of cases, dual and single electrodes were successfully pitch ranked in the expected tonotopic order. No significant relationships between pitch ranking and the SOE functions were found.