Abstract
Speech intelligibility in multitalker settings is challenging for most cochlear implant (CI) users. One possibility for this limitation is the suboptimal representation of vocal cues in implant processing, such as the fundamental frequency (F0), and the vocal tract length (VTL). Previous studies suggested that while F0 perception depends on spectrotemporal cues, VTL perception relies largely on spectral cues. To investigate how spectral smearing in CIs affects vocal cue perception in speech-on-speech (SoS) settings, adjacent electrodes were simultaneously stimulated using current steering in 12 Advanced Bionics users to simulate channel interaction. In current steering, two adjacent electrodes are simultaneously stimulated forming a channel of parallel stimulation. Three such stimulation patterns were used: Sequential (one current steering channel), Paired (two channels), and Triplet stimulation (three channels). F0 and VTL just-noticeable differences (JNDs; Task 1), in addition to SoS intelligibility (Task 2) and comprehension (Task 3), were measured for each stimulation strategy. In Tasks 2 and 3, four maskers were used: the same female talker, a male voice obtained by manipulating both F0 and VTL (F0+VTL) of the original female speaker, a voice where only F0 was manipulated, and a voice where only VTL was manipulated. JNDs were measured relative to the original voice for the F0, VTL, and F0+VTL manipulations. When spectral smearing was increased from Sequential to Triplet, a significant deterioration in performance was observed for Tasks 1 and 2, with no differences between Sequential and Paired stimulation. Data from Task 3 were inconclusive. These results imply that CI users may tolerate certain amounts of channel interaction without significant reduction in performance on tasks relying on voice perception. This points to possibilities for using parallel stimulation in CIs for reducing power consumption.
Highlights
Speech intelligibility in multitalker settings is challenging for most cochlear implant (CI) users
Paired and Triplet stimulation were expected to yield lower performance than Sequential stimulation. With this to-masker ratio (TMR), and this may affect the generalizability of our results, we estimated that only better performers would be able to yield performance sufficiently away from floor to have a chance to observe the effects of masker voice and stimulation pattern
The data demonstrate that even though there is a large variability in performance across the CI participants for each stimulation strategy, there appears to be a trend for decreasing SoS intelligibility scores as the amount of channel interaction increases
Summary
Speech intelligibility in multitalker settings is challenging for most cochlear implant (CI) users. Unlike normal-hearing (NH) listeners, who have been shown in the literature to benefit from spectral dips or temporal modulations in a fluctuating masker to obtain release-from-masking (Cullington & Zeng, 2008; Duquesnoy, 1983; Festen & Plomp, 1990; Gustafsson & Arlinger, 1994; Nelson et al, 2003), CI users do not seem to make use of such dips Evidence for this comes from a number of studies which have demonstrated that CI users have more difficulty understanding speech in the presence of a fluctuating competing speech masker compared to a steady-state noise masker (Cullington & Zeng, 2008; Stickney et al, 2004). NH listeners start experiencing more difficulties “listening in the dips” of the masker; CI users usually find this situation more favorable in comparison to scenarios involving a single-talker masker (Chen et al, 2020; e.g., Cullington & Zeng, 2008)
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