The Effect of Pulse Shape on Pitch Sensitivity of Cochlear Implant Users

Current cochlear implant (CI) devices use cathodic-leading symmetric biphasic (BP) electrical pulses to convey acoustic information to profoundly deaf people. However, research indicates that asymmetric charge-balanced pseudo-monophasic pulses may reduce spread of excitation as compared to BP pulses, resulting in improved spectral resolution (e.g., Frijns et al., 1996). In addition, anodic-centered quadra-phasic pulses appear more effective in stimulating auditory nerve fibers with lower most comfortable levels than cathodic-centered quadra-phasic pulses (e.g., Macherey et al., 2017), while the polarity effect on threshold level may depend on auditory neural health (e.g., Rattay, 1999). To date, the effect of pulse shape on pitch sensitivity is largely unknown, except that anodic-centered quadra-phasic pulses have been reported to yield higher percent correct scores for discrimination of 20- and 35-Hz amplitude modulation frequencies than cathodic-centered quadra-phasic pulses (Undurraga et al., 2021).This research examined if anodic-centered triphasic (A-TP) pulses could enhance CI users’ place and temporal pitch sensitivity as compared to cathodic-centered triphasic (C-TP) and BP pulses. Eight post-lingually deaf adult CI users with a total of 12 implanted ears participated in this study. Experiment 1 measured virtual channel ranking (VCR) thresholds using BP, A-TP, and C-TP pulses on apical, middle, and basal electrodes at a low pulse rate (99 pulses per second; pps), which may enhance the polarity effect on auditory nerve stimulation (Undurraga et al., 2021). The VCR thresholds were significantly higher (i.e., worse) on basal electrodes than on middle electrodes. However, the effect of pulse shape on VCR thresholds was not significant, possibly due to the interference of a fixed 99-Hz temporal pitch.Experiment 2 used a 1000-pps pulse rate to measure VCR thresholds as well as amplitude modulation frequency ranking (AMFR) thresholds with a 100-Hz base AM frequency. Both the VCR and AMFR thresholds were significantly higher (i.e., worse) on basal electrodes than on apical and middle electrodes. Moreover, the VCR thresholds with A-TP pulses were significantly lower (i.e., better) than those with C-TP pulses, and were nearly significantly better than those with BP pulses. In contrast, no significant effect of pulse shape was found on AMFR thresholds. The results revealed a polarity effect on supra-threshold place-pitch perception with CIs and suggest that A-TP pulses may more selectively stimulate auditory nerve fibers and potentially enable CI users to more accurately discriminate place pitches.Research was supported in part by a Discovery Grant from the American Hearing Research Foundation.