Effect of frequency mismatch and band partitioning on vocal tract length perception in vocoder simulations of cochlear implant processing

Abstract

The vocal tract length (VTL) of a speaker is an important voice cue that aids speech intelligibility in multi-talker situations. However, cochlear implant (CI) users demonstrate poor VTL sensitivity. This may be partially caused by the mismatch between frequencies received by the implant and those corresponding to places of stimulation along the cochlea. This mismatch can distort formant spacing, where VTL cues are encoded. In this study, the effects of frequency mismatch and band partitioning on VTL sensitivity were investigated in normal hearing listeners with vocoder simulations of CI processing. The hypotheses were that VTL sensitivity may be reduced by increased frequency mismatch and insufficient spectral resolution in how the frequency range is partitioned, specifically where formants lie. Moreover, optimal band partitioning might mitigate the detrimental effects of frequency mismatch on VTL sensitivity. Results showed that VTL sensitivity decreased with increased frequency mismatch and reduced spectral resolution near the low frequencies of the band partitioning map. Band partitioning was independent of mismatch, indicating that if a given partitioning is suboptimal, a better partitioning might improve VTL sensitivity despite the degree of mismatch. These findings suggest that customizing the frequency partitioning map may enhance VTL perception in individual CI users.

Does good perception of vocal characteristics relate to better speech-on-speech intelligibility for cochlear implant users?

“…Since sensitivity to both F0 and VTL cues was found to be related to overall SoS performance, the question then arises of whether improving one measure would necessarily lead to an improvement in the other. In a previous study, VTL JNDs were found to depend on the frequency-to-electrode allocation mapping in vocoder simulations of CI processing (El Boghdady et al, 2018). It remains to be seen whether implant parameters, such as the frequency-to-electrode allocation mapping or the coding strategy, could help improve SoS performance in addition to JNDs.…”

Effect of Channel Interaction on Vocal Cue Perception in Cochlear Implant Users

“…This raises the question of whether CIs could be fitted with the goal of mitigating the effect of decreased spectrotemporal resolution that may arise from channel interaction. Several studies (e.g., Di Nardo et al., 2011 ; El Boghdady et al., 2018 ; Fitzgerald et al., 2013 ; Fu & Shannon, 1999 ; Grasmeder et al., 2014 ; Leigh et al., 2004 ; McKay & Henshall, 2002 ; Omran et al., 2011 ) have proposed that optimizing the frequency-to-electrode allocation map could have the potential to address the limited spectral resolution in the implant. More specifically, using vocoder simulations, El Boghdady et al.…”

“…Several studies (e.g., Di Nardo et al, 2011;El Boghdady et al, 2018;Fitzgerald et al, 2013;Fu & Shannon, 1999;Grasmeder et al, 2014;Leigh et al, 2004;McKay & Henshall, 2002;Omran et al, 2011) have proposed that optimizing the frequency-to-electrode allocation map could have the potential to address the limited spectral resolution in the implant. More specifically, using vocoder simulations, El Boghdady et al (2018) have shown that the frequency-to-electrode allocation map could have a direct influence on VTL JNDs and that the frequency mapping, if optimally fitted, could help reduce the detrimental effects of channel interaction and frequency mismatch in the cochlea on VTL JNDs. These studies help to pave the way for investigating whether the CI parameters (such as the frequency allocation map) or signal processing could be optimized in a way to improve both SoS perception and the sensitivity to voice cues.…”

“…Following the paradigm used in a number of previous studies ( El Boghdady et al., 2018 , 2019 , 2020 ; Gaudrain & Başkent, 2015 , 2018 ), JNDs in this experiment were measured along three voice vectors, as indicated by the red crosses in Figure 1 , using a two-down one-up adaptive procedure. This adaptive procedure results in 70.7% correct responses on the psychometric function ( Levitt, 1971 ).…”

Effect of Channel Interaction on Vocal Cue Perception in Cochlear Implant Users

“…This raises the question of whether CIs could be fitted with the goal of mitigating the effect of decreased spectrotemporal resolution that may arise from channel interaction. Several studies (e.g., Di Nardo, Scorpecci, Giannantonio, Cianfrone, & Paludetti, 2011;El Boghdady et al, 2018;Fitzgerald, Sagi, Morbiwala, Tan, & Svirsky, 2013;Fu & Shannon, 1999;Grasmeder, Verschuur, & Batty, 2014;Leigh, Henshall, & McKay, 2004;McKay & Henshall, 2002Omran, Lai, & Dillier, 2011) have proposed that optimizing the frequency-to-electrode allocation map could have the potential to address the limited spectral resolution in the implant. More specifically, using vocoder simulations, El Boghdady et al (2018) have shown that the frequency-to-electrode allocation map could have a direct influence on VTL JNDs, and that the frequency mapping, if optimally fitted, could help reduce the detrimental effects of channel interaction and frequency mismatch in the cochlea on VTL JNDs.…”