Pulse-rate effects on single- and multi-channel envelope processing and consonant identification in cochlear implant users
Mahan Azadpour1, Jonathan D Neukam1, Nicole Hope Capach1
1New York University Grossman School of Medicine, Department of Otolaryngology, 550 First Avenue, New York, NY, 10016, USA.
Abstract:
Cochlear implants (CIs) stimulate auditory neurons with electric pulse trains whose amplitudes are modulated using envelopes extracted from different frequency bands, conveying essential speech cues. This study investigated how the rate of stimulation pulses affects envelope processing and speech perception in ten post-lingually deaf CI users. Amplitude modulation (AM) detection and consonant identification were examined using acoustic stimuli processed by multi-channel and single-channel strategies, with pulse rates ranging from 125 to 2000 pulses per second (pps) for multi-channel stimulation and from 125 to 4000pps for single-channel stimulation. The single-channel conditions encoded the overall broad-band speech envelope by a single electrode and were used to isolate within-electrode temporal modulation coding. For AM detection, the modulation frequency was 25 Hz ensuring sufficient modulation sampling across all carrier rates tested. Even at 125pps, the 25-Hz modulation was represented by five pulses per modulation cycle. Results revealed a significant decline in both AM detection and consonant identification at 125pps, suggesting that poorer speech recognition at low rates may be partly attributable to limitations in temporal modulation processing rather than solely to a restricted range of representable envelope frequencies. Consonant recognition performance remained stable across clinically relevant pulse rates (250-2000pps), though significant listener- and feature-specific effects were observed. Notably, while AM detection and consonant recognition were not significantly correlated within this small cohort, significant correlations were found between single-channel and multi-channel performance in both tasks. These findings support an important role for within-electrode temporal coding in CI speech perception and highlight the clinical relevance of individual variability in pulse rate effects.
