Effects of stimulus configuration on psychophysical operating levels and on speech recognition with cochlear implants

Effects of electrode configuration and pulse duration on operating levels and on speech recognition performance were studied in a group of 14 adult postlingually deaf human subjects with Nucleus cochlear implants. The operating levels (based on detection threshold and maximum comfortable loudness levels) for narrowly spaced bipolar (BP) stimulation were found to be about 11 dB higher on average than those for widely spaced bipolar (BP+6) or monopolar (MP1) stimulation. Operating levels for common ground (CG) stimulation fell between those for BP and BP+6; the difference between BP and CG detection thresholds depended on pulse duration. Variation in detection thresholds and maximum comfortable loudness levels across the electrode array (electrodes 1-15) was larger for BP and CG stimulation than for BP+6 or MP1 stimulation, suggesting narrower spread of activation for the BP and CG configurations despite the higher current levels. Speech recognition performance was tested using experimental processor configurations. Among the experimental electrode configurations tested (BP, CG, and BP+6), the highest speech recognition scores were obtained with the BP+6 configuration in many subjects. Effects of pulse duration on speech recognition were less consistent and usually smaller than the effects of electrode configuration. The results indicate that electrode configuration is an important variable determining speech recognition performance and suggest that restriction of the size of neural population activated by individual channels of the prosthesis is not necessarily advantageous.