Spatial Hearing and Speech Intelligibility in Bilateral Cochlear Implant Users

Abstract

Objective The abilities to localize sounds and segregate speech from interfering sounds in a complex auditory environment were studied in a group of adults who use bilateral cochlear implants. The first aim of the study was to investigate the change in speech intelligibility under bilateral and unilateral listening modes as a function of bilateral experience during the first 6 mo of activation. The second aim was to look at whether localization and speech intelligibility in the presence of interfering speech are correlated and if the relationship is specific to the bilateral listening mode. The third aim was to examine whether sound lateralization (right versus left) emerges before sound localization within a hemifield. Design Participants were 17 native English speaking adults with postlingual deafness. All subjects received the Nucleus 24 Contour implant in both ears, either during the same surgery or during two separate surgeries that were no more than 1 mo apart. Both devices for each subject were activated at the same time, regardless of surgical approach. Speech intelligibility was measured at 3 and 6 mo after activation. Target speech was presented at 0° in front. Testing was conducted in quiet and in the presence of four-talker babble. The babble was located on the right, on the left, or in front (colocated with the target). Sound localization abilities were measured at the 3 mo interval. All testing was conducted under three listening modes: left ear alone, right ear alone, or bilateral. Results On the speech-in-babble task, benefit of listening with two ears compared with one was greater when going from 3 to 6 mo of experience. This was evident when the target speech and interfering speech were spatially separated, but not when they were presented from the same location. At 3 mo postactivation of bilateral hearing, 82% of subjects demonstrated bilateral benefit when right/left discrimination was evaluated. In contrast, 47% of subjects showed a bilateral benefit when sound localization was evaluated, suggesting that directional hearing might emerge in a two-step process beginning with discrimination and converging on more fine-grained localization. The bilateral speech intelligibility scores were positively correlated with sound localization abilities, so that listeners who were better able to hear speech in babble were generally better able to identify source locations. Conclusions During the early stages of bilateral hearing through cochlear implants in postlingually deafened adults, there is an early emergence of spatial hearing skills. Although nearly all subjects can discriminate source locations to the right versus left, less than half are able to perform the more difficult task of identifying source locations in a multispeaker array. Benefits for speech intelligibility with one versus two implants improve with time, in particular when spatial cues are used to segregate speech and competing noise. Localization and speech-in-noise abilities in this group of patients are somewhat correlated.

The role of envelope statistics in detecting changes in interaural correlation

“…Although CIs were not originally designed for bilateral use (e.g., the processors are not synchronized across the two ears, which results in random ITDs), more and more people are becoming bilaterally implanted. Studies suggest that bilateral CI users can understand speech slightly better than unilateral CI users, especially in noise (e.g., Schleich et al, 2004;Litovsky et al, 2009).…”

Sensitivity to interaural envelope correlation changes in bilateral cochlear-implant users

“…Because binaural hearing provides the advantages of better sound localization and speech understanding in noise, it is not surprising that the clinical incidence of bilateral cochlear implants (CIs) has increased Litovsky et al, 2009). However, benefits of binaural hearing are maximized when ITDs, ILDs, and interaural decorrelation cues are preserved and salient.…”

Spatial release from masking in children with bilateral cochlear implants and with normal hearing: Effect of target-interferer similarity

“…It is reasonable to surmise that lack of coordination between the two CIs may render the symmetrical condition an impossible one for achieving SRM, in contrast with the asymmetrical condition, which does promote SRM Litovsky et al, 2009;Misurelli and Litovsky, 2012;van Hoesel and Tyler, 2003). Previous studies on the cocktail party problem in hearing impaired listeners and CI users have quantified benefits with reference to whether one or both ears are stimulated.…”