Effect of Spectral Contrast Enhancement on Speech-on-Speech Intelligibility and Voice Cue Sensitivity in Cochlear Implant Users

Abstract

Objectives: Speech intelligibility in the presence of a competing talker (speech-on-speech; SoS) presents more difficulties for cochlear implant (CI) users compared with normal-hearing listeners. A recent study implied that these difficulties may be related to CI users’ low sensitivity to two fundamental voice cues, namely, the fundamental frequency (F0) and the vocal tract length (VTL) of the speaker. Because of the limited spectral resolution in the implant, important spectral cues carrying F0 and VTL information are expected to be distorted. This study aims to address two questions: (1) whether spectral contrast enhancement (SCE), previously shown to enhance CI users’ speech intelligibility in the presence of steady state background noise, could also improve CI users’ SoS intelligibility, and (2) whether such improvements in SoS from SCE processing are due to enhancements in CI users’ sensitivity to F0 and VTL differences between the competing talkers. Design: The effect of SCE on SoS intelligibility and comprehension was measured in two separate tasks in a sample of 14 CI users with Cochlear devices. In the first task, the CI users were asked to repeat the sentence spoken by the target speaker in the presence of a single competing talker. The competing talker was the same target speaker whose F0 and VTL were parametrically manipulated to obtain the different experimental conditions. SoS intelligibility, in terms of the percentage of correctly repeated words from the target sentence, was assessed using the standard advanced combination encoder (ACE) strategy and SCE for each voice condition. In the second task, SoS comprehension accuracy and response times were measured using the same experimental setup as in the first task, but with a different corpus. In the final task, CI users’ sensitivity to F0 and VTL differences were measured for the ACE and SCE strategies. The benefit in F0 and VTL discrimination from SCE processing was evaluated with respect to the improvement in SoS perception from SCE. Results: While SCE demonstrated the potential of improving SoS intelligibility in CI users, this effect appeared to stem from SCE improving the overall signal to noise ratio in SoS rather than improving the sensitivity to the underlying F0 and VTL differences. A second key finding of this study was that, contrary to what has been observed in a previous study for childlike voice manipulations, F0 and VTL manipulations of a reference female speaker (target speaker) toward male-like voices provided a small but significant release from masking for the CI users tested. Conclusions: The present findings, together with those previously reported in the literature, indicate that SCE could serve as a possible background-noise-reduction strategy in commercial CI speech processors that could enhance speech intelligibility especially in the presence of background talkers that have longer VTLs compared with the target speaker.

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

“…In that study, the authors reasoned that masking voices taken from the lower-left quadrant, as performed in the current study, should be expected to yield a benefit in SoS performance for CI users. This was shown to be the case in another later study by the same authors ( El Boghdady et al., 2020 ).…”

“…The methods for this experiment are largely similar to the ones described in El Boghdady et al. (2019) and identical to those in El Boghdady et al. (2020) .…”

“…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 ).…”

“…Translation from Dutch to German and the evaluation was performed thoroughly by three independent native German speakers: Two of those speakers were also fluent in Dutch, while the third had sufficient knowledge of the language (see El Boghdady et al., 2020 , for a full description of the translation procedure). One sentence pair lost its meaning when translated to German and was discarded from the translations, resulting in 99 true–false sentence pairs.…”

“…Participants were asked to repeat all words spoken by the target speaker in the presence of the voice-manipulated masker, and the intelligibility score was determined based on the number of words correctly repeated. In Task 3, a different speech test was administered (sentence verification task [SVT]), which measures overall sentence comprehension ( Adank & Janse, 2009 ; Baddeley et al., 1992 ; El Boghdady et al., 2019 , 2020 ; May et al., 2001 ; Pisoni et al., 1987 ; Saxton et al., 2001 ). In this task, participants were asked to judge whether the target sentence statement, presented simultaneously with a single competing masker, was true or false, without repeating the actual sentence, and both target sentence comprehension accuracy and speed (response times [RTs]) were measured ( Adank & Janse, 2009 ).…”