Current Steering and Current Focusing in Cochlear Implants: Comparison of Monopolar, Tripolar, and Virtual Channel Electrode Configurations

Abstract

Objectives: 

To compare the effects of Monopole (Mono), Tripole (Tri), and “Virtual channel” (Vchan) electrode configurations on spectral resolution and speech perception in a crossover design.

Design: 

Nine experienced adults who received an Advanced Bionics CII/90K cochlear implant participated in a crossover design using three experimental strategies for 2 wk each. Three strategies were compared: (1) Mono; (2) Tri with current partly returning to adjacent electrodes and partly (25 or 75%) to the extracochlear reference; and (3) a monopolar “Vchan” strategy creating seven intermediate channels between two contacts. Each strategy was a variant of the standard “HiRes” processing strategy using 14 channels and 1105 pulses/sec/channel, and a pulse duration of 32 μsec/phase. Spectral resolution was measured using broadband noise with a sinusoidally rippled spectral envelope with peaks evenly spaced on a logarithmic frequency scale. Speech perception was measured for monosyllables in quiet and in steady-state and fluctuating noises. Subjective comments on music experience and preferences in everyday use were assessed through questionnaires.

Results: 

Thresholds and most comfortable levels with Mono and Vchan were both significantly lower than levels with Tri. Spectral resolution was significantly higher with Tri than with Mono; spectral resolution with Vchan did not differ significantly from the other configurations. Moderate but significant correlations between word recognition and spectral resolution were found in speech in quiet and fluctuating noise. For speech in quiet, word recognition was best with Mono and worst with Vchan; Tri did not significantly differ from the other configurations. Pooled across the noise conditions, word recognition was best with Tri and worst with Vchan (Mono did not significantly differ from the other configurations). These differences were small and insufficient to result in a clear increase in performance across subjects if the result from the best configuration per subject was compared with the result from Mono. Across all subjects, music appreciation and satisfaction in everyday use did not clearly differ between configurations.

Conclusions: 

(1) Although spectral resolution was improved with the tripolar configuration, differences in speech performance were too small in this limited group of subjects to justify clinical introduction. (2) Overall spectral resolution remained extremely poor compared with normal hearing; it remains to be seen whether further manipulations of the electrical field will be more effective.

In Brief

Compared to Monopolar stimulation currently used in cochlear implants, Tripolar stimulation has been shown to increase spatial selectivity in animal studies. The “Virtual channels” approach attempts to increase spectral resolution by providing a high number of effectively Monopolar channels. In this study, we compared the effects of Monopole, Tripole, and “Virtual channel” electrode configurations on spectral resolution and on speech perception using a crossover design in nine adults who received an Advanced Bionics CII/90K cochlear implant. “Hybrid” Tripolar configurations had to be used with current partly flowing to the extracochlear reference to enable sufficient loudness growth. Although spectral resolution improved slightly with the Tripolar configuration, the differences were small in this limited group of subjects. More efficient stimulation schemes are necessary for future testing of Tripolar stimulation benefits.