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. 2014 Nov-Dec;35(6):633-40.
doi: 10.1097/AUD.0000000000000057.

Interaural level differences and sound source localization for bilateral cochlear implant patients

Michael F Dorman  1 , Louise Loiselle, Josh Stohl, William A Yost, Anthony Spahr, Chris Brown, Sarah Cook
Affiliations

Interaural level differences and sound source localization for bilateral cochlear implant patients

Michael F Dorman et al. Ear Hear. 2014 Nov-Dec.

Abstract

Objectives: The aims of this study were (i) to determine the magnitude of the interaural level differences (ILDs) that remain after cochlear implant (CI) signal processing and (ii) to relate the ILDs to the pattern of errors for sound source localization on the horizontal plane.

Design: The listeners were 16 bilateral CI patients fitted with MED-EL CIs and 34 normal-hearing listeners. The stimuli were wideband, high-pass, and low-pass noise signals. ILDs were calculated by passing signals, filtered by head-related transfer functions (HRTFs) to a Matlab simulation of MED-EL signal processing.

Results: For the wideband signal and high-pass signals, maximum ILDs of 15 to 17 dB in the input signal were reduced to 3 to 4 dB after CI signal processing. For the low-pass signal, ILDs were reduced to 1 to 2 dB. For wideband and high-pass signals, the largest ILDs for ±15 degree speaker locations were between 0.4 and 0.7 dB; for the ±30 degree speaker locations between 0.9 and 1.3 dB; for the 45 degree speaker locations between 2.4 and 2.9 dB; for the ±60 degree speaker locations, between 3.2 and 4.1 dB; and for the ±75 degree speaker locations between 2.7 and 3.4 dB. All of the CI patients in all the stimulus conditions showed poorer localization than the normal-hearing listeners. Localization accuracy for the CI patients was best for the wideband and high-pass signals and was poorest for the low-pass signal.

Conclusions: Localization accuracy was related to the magnitude of the ILD cues available to the normal-hearing listeners and CI patients. The pattern of localization errors for the CI patients was related to the magnitude of the ILD differences among loudspeaker locations. The error patterns for the wideband and high-pass signals, suggest that, for the conditions of this experiment, patients, on an average, sorted signals on the horizontal plane into four sectors-on each side of the midline, one sector including 0, 15, and possibly 30 degree speaker locations, and a sector from 45 degree speaker locations to 75 degree speaker locations. The resolution within a sector was relatively poor.

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Figures

Figure 1
Figure 1
Localization accuracy for normal hearing listeners and CI listeners. The left, middle and right panels show results for wideband, high-pass and low-pass signals, respectively. The horizontal bars indicate the group mean scores. The grey zone indicates +/− 1 standard deviation for chance performance.
Figure 2
Figure 2
Group mean localization responses as a function of speaker azimuth for normal hearing (top) and CI patients (bottom) for low-pass, high-pass and wide band signals.. Bubble size is proportional to percent responses. Bubbles ≤ 10% are omitted from the plot. The diagonal line indicates correct responses.
Figure 3
Figure 3
Localization by best performing CI patients. WB = wide band noise. HP = high-pass noise.
See this image and copyright information in PMC

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