Having Two Ears Facilitates the Perceptual Separation of Concurrent Talkers for Bilateral and Single-Sided Deaf Cochlear Implantees

Abstract

Objectives: Listening to speech with multiple competing talkers requires the perceptual separation of the target voice from the interfering background. Normal-hearing listeners are able to take advantage of perceived differences in the spatial locations of competing sound sources to facilitate this process. Previous research suggests that bilateral (BI) cochlear-implant (CI) listeners cannot do so, and it is unknown whether single-sided deaf (SSD) CI users (one acoustic and one CI ear) have this ability. This study investigated whether providing a second ear via cochlear implantation can facilitate the perceptual separation of targets and interferers in a listening situation involving multiple competing talkers.

Design: BI-CI and SSD-CI listeners were required to identify speech from a target talker mixed with one or two interfering talkers. In the baseline monaural condition, the target speech and the interferers were presented to one of the CIs (for the BI-CI listeners) or to the acoustic ear (for the SSD-CI listeners). In the bilateral condition, the target was still presented to the first ear but the interferers were presented to both the target ear and the listener's second ear (always a CI), thereby testing whether CI listeners could use information about the interferer obtained from a second ear to facilitate perceptual separation of the target and interferer.

Results: Presenting a copy of the interfering signals to the second ear improved performance, up to 4 to 5 dB (12 to 18 percentage points), but the amount of improvement depended on the type of interferer. For BI-CI listeners, the improvement occurred mainly in conditions involving one interfering talker, regardless of gender. For SSD-CI listeners, the improvement occurred in conditions involving one or two interfering talkers of the same gender as the target. This interaction is consistent with the idea that the SSD-CI listeners had access to pitch cues in their normal-hearing ear to separate the opposite-gender target and interferers, while the BI-CI listeners did not.

Conclusions: These results suggest that a second auditory input via a CI can facilitate the perceptual separation of competing talkers in situations where monaural cues are insufficient to do so, thus partially restoring a key advantage of having two ears that was previously thought to be inaccessible to CI users.

Fig. 1

Results of experiment 1 showing speech-identification performance as a function of TMR. The target signal was always presented monaurally. The interferers were presented monaurally to the same ear as the target (monaural condition) or diotically (bilateral condition). Conditions where bilateral performance was significantly better than monaural performance are identified by asterisks (for individual TMRs) and large open stars (for data pooled across TMRs). Error bars indicate ±1 standard error of the mean across listeners.

Fig. 2

Results of experiment 1 showing the magnitude of the perceptual-separation advantage for each interferer type, averaged across TMR, for (a) the NH listeners presented with unprocessed stimuli, (b) the BI-CI listeners, (c) the SSD-CI listeners, (d) NH listeners in the bilateral-vocoded condition, and (e) NH listeners in the SSD-vocoded condition. Error bars indicate ±1 standard error of the mean across listeners.

Fig. 3

Estimates of the magnitude of the perceptual-separation advantage in experiment 1 for individual CI listeners (small points) and mean data for the vocoder conditions presented to NH listeners (white squares). Asterisks indicate the BI-CI listener who demonstrated significant interference when the interferer signals were presented to the second CI; this listener was excluded from the computation of the group-mean average (horizontal bars). Error bars indicate ±1 standard error of the mean across listeners.

Fig. 4

Mean keyword-identification performance as a function of TMR for the subset of BI-CI and SSD-CI listeners tested in experiment 3. The color and number spoken by the interfering talkers (brown or black; nine or ten) were not items included in the response set, precluding the use of lexical information in the interferer-only ear to produce an improvement in performance relative to the monaural condition. Conditions where bilateral performance was significantly better than monaural performance are identified by asterisks (for individual TMRs) and large open stars (for data pooled across TMRs). Error bars indicate ±1 standard error of the mean across listeners.