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 degrees 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.

Figures

Fig. 1

Sound localization results are shown for 17 individual subjects. Data are compared for three listening modes: left ear alone, right ear alone, and bilateral. Within each plot, the average (±SD) reported location is shown as a function of the actual source position.

Fig. 2

Effect of listening to target speech presented from 0° in the presence of babble whose location was varied (front, left, and right) is shown for three listening conditions (bilateral, left ear, and right ear) and compared for 3 and 6 mo. Because the criterion of >3.1 was applied to all differences as a measure of significance, here the vertical bars represent the total number of subjects showing each type of benefit, as described in the text.

Fig. 3

The magnitude of bilateral advantage (mean ± SD) is compared for the 3 and 6 mo intervals. The data are categorized into the types of benefits that might occur under conditions in which listeners used both devices compared with either ear alone, including binaural summation, binaural squelch, and head shadow effects. Top and bottom panels show results for cases in which subjects used the left or right ear alone, respectively. Data for the babble locations on the right or left are collapsed for the squelch and summation effects.

Fig. 4

Change in the SRTs for individual subjects as they transition from 3 to 6 mo of bilateral experience is plotted. The nine plots show data for the three listening modes (left ear, right ear, and bilateral) and the three spatial configurations of the babble (front, right, left). Within each panel, the absolute difference in SRT between the two time intervals is shown, with zero being unity, positive values indicating decreased SRTs over time, and negative values indicating increased SRTs over time. Bars falling to the right of the vertical dashed line in each plot indicate that the difference is greater than the 3.1 dB critical difference for statistically meaningful improved performance. In one case (bottom-left panel), the axes were altered to reflect a different range of performance.

Fig. 5

Data from Figure 4 are summarized to show the overall change in SRTs from the 3 mo interval to the 6 mo interval for the nine combinations of listening mode and spatial configuration of the babble. These box plots show the medians (horizontal lines), 25th and 75th interquartile ranges (gray boxes), and SDs.

Fig. 6

Correlations between sound localization RMS data obtained when listeners used both devices (abscissa) and SRT values (ordinate). The SRT data in each panel were obtained in one of the nine conditions; left to right are the three listening modes (left ear, right ear, and bilateral) and from top to bottom are the three spatial configurations for the babble (front, right, left). Within each panel, the r (top) and p (bottom) values are shown.