Abstract
The stimulation rate in cochlear implant (CI) sound coding, or the “carrier” rate in pulses per second (pps), is known to influence pitch perception, as well as loudness perception and sound quality. Our main objective was to investigate the effects of reduced carrier rate on the loudness and pitch of coded speech samples. We describe two experiments with 16 Nucleus® CI users, where we controlled modulation characteristics and carrier rate using Spectral and Temporal Enhanced Processing (STEP), a novel experimental multichannel sound coder. We used a fixed set of threshold and comfortable stimulation levels for each subject, obtained from clinical MAPs. In the first experiment, we determined equivalence for voice pitch ranking and voice gender categorization between the Advanced Combination Encoder (ACE), a widely used clinical strategy in Nucleus® recipients, and STEP for fundamental frequencies (F0) 120–250 Hz. In the second experiment, loudness was determined as a function of the input amplitude of speech samples for carrier rates of 1000, 500, and 250 pps per channel. Then, using equally loud sound coder programs, we evaluated the effect of carrier rate on voice pitch perception. Although nearly all subjects could categorize voice gender significantly above chance, pitch ranking varied across subjects. Overall, carrier rate did not substantially affect voice pitch ranking or voice gender categorization: as long as the carrier rate was at least twice the fundamental frequency, or when stimulation pulses for the lowest, 250 pps carrier were aligned to F0 peaks. These results indicate that carrier rates as low as 250 pps per channel are sufficient to support functional voice pitch perception for those CI users sensitive to temporal pitch cues; at least when temporal modulations and pulse timings in the coder output are well controlled by novel strategies such as STEP.
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ACKNOWLEDGEMENTS
The authors would like to thank Dr. Boška Munivrana Dervišbegović from the SUVAG Polyclinic and Danijel Nejašmić for the assistance in data collection from the CI subjects, and Tonći Kozina for the technical and programming assistance.
Funding
This study was supported by the Investigator-Initiated Research (IIR) Grants #2098 and #853 from Cochlear© awarded to DK. Besides, DK was also partially supported by the EU structural grant for Croatia (code: HR.3.2.01–0320) and Split-Dalmatia County.
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The study was approved by the ethical committee of the SUVAG Polyclinic (# 510–08/17–20). Either CI subject or one of their parents gave written informed consent, depending on the subjects’ age.
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Kovačić, D., James, C.J. Stimulation Rate and Voice Pitch Perception in Cochlear Implants. JARO 23, 665–680 (2022). https://doi.org/10.1007/s10162-022-00854-2
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DOI: https://doi.org/10.1007/s10162-022-00854-2