Frequency selectivity of contralateral residual acoustic hearing in bimodal cochlear implant users, and limitations on the ability to match the pitch of electric and acoustic stimuli.

OBJECTIVE: To assess the reliability of across-ear, acoustic-electric pitch/timbre comparisons for determining effective characteristic frequencies of cochlear implant electrodes. STUDY SAMPLE: Nine CI users with contralateral residual acoustic hearing.
DESIGN: Absolute acoustic thresholds in the unimplanted ear were measured and frequency selectivity was assessed via psychophysical tuning curves. An adjustment method was used to match the percepts elicited by pulse trains on individual electrodes with various acoustic signals (pure tones, narrow-band noises, and bandpass filtered pulse trains). The starting frequency of the acoustic signal was roved and matches were obtained at different loudness levels.
RESULTS: Acoustic frequency selectivity varied widely. Two subjects showed clear evidence of frequency selectivity extending above 500 Hz. Only these subjects produced consistent pitch matches over repeated measurements. For other subjects, the acoustic frequency eventually selected tended to correlate with the initially presented frequency. There was limited evidence of level effects and these were inconsistent across subjects and electrodes.
CONCLUSIONS: Across-modality pitch/timbre matching appears unlikely to provide a generally applicable method for determining the effective characteristic frequencies of cochlear implant electrodes. Frequency selectivity above 500 Hz may be necessary for consistent pitch/timbre matches.