Covarying Amplitude Modulation and Pulse Rate Enhances Pitch Discrimination in Cochlear Implant Users

Pitch plays a fundamental role in prosody, lexical tone, and music perception, yet cochlear implant (CI) users exhibit limited temporal pitch sensitivity, particularly at higher pulse rates (e.g., 300 Hz). This study proposes a covarying pitch-encoding method, where the amplitude-modulation (AM) frequency and the pulse rate vary together in predetermined integer ratios to reinforce temporal periodicity cues. A single-channel psychophysical pitch discrimination experiment was conducted at the most apical electrode in 17 ears from 14 Cochlear CI users around reference frequencies of 50 and 300 Hz. The aims were to examine the effects of the integer ratio on pitch discrimination using the covarying method and to compare the performance of the covarying method with two conventional pitch encoding methods--pulse rate only and AM only. An additional consonant recognition task evaluated speech recognition ability. Results showed that at 50 Hz neither integer ratio nor pitch encoding method significantly affected pitch discrimination thresholds ({approx}30%). At 300 Hz, thresholds were overall higher than at 50 Hz, but the covarying method produced lower thresholds (41.8%) than the pulse rate (52.1%) and AM frequency methods (52.4%), and the covarying-pulse-rate difference was statistically significant. These results suggest that covarying stimulation can modestly enhance pitch discrimination at higher frequencies relative to conventional methods. Regression analyses revealed that temporal pitch discrimination in this psychophysical task at 50 Hz deteriorated with longer CI experience, whereas consonant recognition in a more ecologically relevant speech task improved, suggesting distinct neural adaptation mechanisms.