Bilateral Versus Unilateral Cochlear Implantation in Adult Listeners: Speech-On-Speech Masking and Multitalker Localization

Abstract

Binaural hearing helps normal-hearing listeners localize sound sources and understand speech in noise. However, it is not fully understood how far this is the case for bilateral cochlear implant (CI) users. To determine the potential benefits of bilateral over unilateral CIs, speech comprehension thresholds (SCTs) were measured in seven Japanese bilateral CI recipients using Helen test sentences (translated into Japanese) in a two-talker speech interferer presented from the front (co-located with the target speech), ipsilateral to the first-implanted ear (at +90° or -90°), and spatially symmetric at ±90°. Spatial release from masking was calculated as the difference between co-located and spatially separated SCTs. Localization was assessed in the horizontal plane by presenting either male or female speech or both simultaneously. All measurements were performed bilaterally and unilaterally (with the first implanted ear) inside a loudspeaker array. Both SCTs and spatial release from masking were improved with bilateral CIs, demonstrating mean bilateral benefits of 7.5 dB in spatially asymmetric and 3 dB in spatially symmetric speech mixture. Localization performance varied strongly between subjects but was clearly improved with bilateral over unilateral CIs with the mean localization error reduced by 27°. Surprisingly, adding a second talker had only a negligible effect on localization.

Keywords: better-ear glimpsing; bilateral benefit; cochlear implants; localization; spatial release from masking.

Figure 1.

(a to c) The three different noise conditions applied in the speech comprehension test. The target source is indicated by the gray-filled loudspeakers and noise sources are indicated by the open loudspeakers. Note that the spatially asymmetric condition shown in panel (b) represents the case when the left ear is tested in the unilateral condition (as indicated by the dot) and needs to be mirrored for the right ear. T = Target speech; D = speech distractor.

Figure 2.

Graphical user interface for the localization and voice gender identification experiment provided to the subjects on a handheld touch screen (iPad). The highlighted loudspeaker and listener buttons indicate the 13 source directions that were tested.

Figure 3.

Mean and individual SCTs obtained in the three different background noise configurations in the unilateral (left panel) and bilateral (right panel) condition. The data of Subject 4 (left-pointing triangles in round brackets) is neither considered in the mean value (solid circles) nor in the subsequent statistical analysis. Col = co-located; SA = spatially asymmetric; SS = spatially symmetric.

Figure 4.

Mean and individual SRM (left panel) as well as the bilateral benefit obtained in the three different background noise configurations (right panel). Data of Subject 4 are not considered here. Col = co-located; SA = spatially asymmetric; SS = spatially symmetric; Uni = unilateral CIs; Bi = bilateral CIs.

Figure 5.

Horizontal localization performance for two example subjects (s1 and s7) for the unilateral (left panels) as well as the bilateral condition (right panels). The circles indicate individual trials and are shifted horizontally within the gray-and-white shaded area for clarity.

Figure 6.

Individual RMS localization errors are shown for the unilateral (open symbols) and the bilateral (filled symbols) condition, with the data for the single-talker condition plotted on the left and for the two-talker condition on the right of the dashed lines. In each condition, the RMS errors were averaged across the male and female voices.