Comparing sound localization deficits in bilateral cochlear-implant users and vocoder simulations with normal-hearing listeners

Abstract

Bilateral cochlear-implant (BiCI) users are less accurate at localizing free-field (FF) sound sources than normal-hearing (NH) listeners. This performance gap is not well understood but is likely due to a combination of compromises in acoustic signal representation by the two independent speech processors and neural degradation of auditory pathways associated with a patient's hearing loss. To exclusively investigate the effect of CI speech encoding on horizontal-plane sound localization, the present study measured sound localization performance in NH subjects listening to vocoder processed and nonvocoded virtual acoustic space (VAS) stimuli. Various aspects of BiCI stimulation such as independently functioning devices, variable across-ear channel selection, and pulsatile stimulation were simulated using uncorrelated noise (Nu), correlated noise (N0), or Gaussian-enveloped tone (GET) carriers during vocoder processing. Additionally, FF sound localization in BiCI users was measured in the same testing environment for comparison. Distinct response patterns across azimuthal locations were evident for both listener groups and were analyzed using a multilevel regression analysis. Simulated implant speech encoding, regardless of carrier, was detrimental to NH localization and the GET vocoder best simulated BiCI FF performance in NH listeners. Overall, the detrimental effect of vocoder processing on NH performance suggests that sound localization deficits may persist even for BiCI patients who have minimal neural degradation associated with their hearing loss and indicates that CI speech encoding plays a significant role in the sound localization deficits experienced by BiCI users.

Keywords: bilateral; cochlear implant; sound localization; vocoder.

Figure 1.

Summary of localization performance. The average RMS error (bars, mean) and standard deviation (error bars, ±SD) for NH and BiCI subjects are plotted. The asterisk indicates statistically significant results for comparison of the average RMS across subjects for each listening condition and post hoc analysis (see text for details).

Figure 2.

Absolute localization error and response distribution across target angles. (a) The across-subject average absolute difference between target and response angles (bars, mean) and standard deviation (error bars, ± SD) for each target location. (b) The average binned responses for each target angle across subjects, with responses placed in the nearest 10 bin.

Figure 3.

Observed data and multilevel regression model fits for effect of listening condition on absolute error across azimuthal locations. For each listening condition (panels), the across-subject average absolute error (point, mean) and standard error (error bars, ± SE) are plotted as a function of the absolute target angle with the solid line representing the model fit. The BiCI_FF model fit was treated as the reference and is plotted (thin line) on each of the panels displaying the NH listening conditions for visual comparison.

Figure 4.

Localization data and response distributions. Responses were binned to the nearest 10° and data point size reflects the number of responses within each bin (bottom left). The number at the bottom right corner of each plot is the RMS error. The small bar graphs next to each localization plot display a histogram of responses binned to the nearest 10°. (a–c) Localization data measured in BiCI users who responses varied by grouping around different spatial location. (d–f) Localization data in NH subjects who exhibited similar patterns of response distributions listening to VC_GET stimuli as the BiCI users in the column above them.