Speech perception with mono- and quadrupolar electrode configurations: a crossover study.

OBJECTIVE: To study the effect of two multipolar electrode configurations on speech perception, pitch perception, and the intracochlear electrical field. STUDY
DESIGN: Crossover design; within subject.
SETTING: Tertiary referral center. PATIENTS: Eight experienced adult cochlear implant users. INTERVENTION: Each subject used each of three experimental processors for 3 weeks. The following processors were compared that differed only in electrode configuration: 1) monopolar; 2) hybrid quadrupolar, in which half of the current returned to the extracochlear reference electrode and half to two electrodes immediately to the left and right of the active electrode; and 3) flat tripolar +2, which directed all the current to four reference electrodes (two on each side), separated from the active electrode by two inactive electrodes. All the processors used the standard Advanced Bionics HiRes speech-processing strategy, 12 channels, 1,220 pulses per second per channel, and with a pulse width of 33 (micros/phase).
RESULTS: The monopolar processors had the largest stimulation efficiency and the smallest dynamic range in linear current units. The reverse was true of flat tripolar +2 processor, whereas the hybrid quadrupolar processor fell in between. Insufficient loudness growth prevented the use of the flat tripolar +2 processor in three subjects. Word recognition did not differ between the clinically used 16-channel monopolar processor and the experimental monopolar processor, regardless of the differences in the number of channels, pulse rate, and duration of experience. Word recognition with the flat tripolar +2 processor was significantly poorer than with the monopolar and hybrid quadrupolar processors; monopolar and quadrupolar processors did not differ. There was no significant interaction between processor type and competing noise type (stationary or fluctuating), but performance at the higher level of fluctuating noise was best with the hybrid quadrupolar processor in almost all the subjects. Pitch scaling showed ceiling performance in five subjects and differed between processors in the two other subjects with imperfect tonotopy. Intracochlear current spread was considerable with the monopolar configuration; it was reduced with the hybrid quadrupolar configuration and virtually absent beyond the active electrodes with the tripolar configuration.
CONCLUSION: More confined configurations reduced the longitudinal width of the electrical field, which was expected to enhance channel separation, but no improvement in word recognition was found. More research is needed to test confined configurations that have enhanced efficiency and to evaluate the fundamental effects of configuration on channel discriminability.