Effects of electrode configuration and place of stimulation on speech perception with cochlear prostheses

Recent research and clinical experience with cochlear implants suggest that subjects' speech recognition with monopolar or broad bipolar stimulation might be equal to or better than that obtained with narrow bipolar stimulation or other spatially restricted electrode configurations. Furthermore, subjects often prefer the monopolar configurations. The mechanisms underlying these effects are not clear. Two hypotheses are (a) that broader configurations excite more neurons resulting in a more detailed and robust neural representation of the signal and (b) that broader configurations achieve a better spatial distribution of the excited neurons. In this study we compared the effects of electrode configuration and the effects of longitudinal placement and spacing of the active electrodes on speech recognition in human subjects. We used experimental processor maps consisting of 11 active electrodes in a 22-electrode scala tympani array. Narrow bipolar (BP), wide bipolar (BP + 6), and monopolar (MP2) configurations were tested with various locations of active electrodes. We tested basal, centered, and apical locations (with adjacent active electrodes) and spatially distributed locations (with every other electrode active) with electrode configuration held constant. Ten postlingually deafened adult human subjects with Nucleus prostheses were tested using the SPEAK processing strategy. The effects of electrode configuration and longitudinal place of stimulation on recognition of CNC phonemes and words in quiet and CUNY sentences in noise (+10 dB S/N) were similar. Both independent variables had large effects on speech recognition and there were interactions between these variables. These results suggest that the effects of electrode configuration on speech recognition might be due, in part, to differences among the various configurations in the spatial location of stimulation. Correlations of subjective judgments of sound quality with speech-recognition ability were moderate, suggesting that the mechanisms contributing to subjective quality and speech-recognition ability do not completely overlap.