Interaural Level Differences and Sound Source Localization for Bilateral Cochlear Implant Patients

Abstract

Objective 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 cochlear implants 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 15–17dB in the input signal were reduced to 3–4dB after CI signal processing. For the low-pass signal, ILDs were reduced to 1–2dB. For wideband and high-pass signals, the largest ILDs for +/− 15 degree speaker locations were between .4 and .7dB; for the +/− 30 degree locations between .9 and 1.3dB; for the 45 degree locations between 2.4 and 2.9dB, for the +/− 60 degree locations, between 3.2 and 4.1dB and for the +/− 75 degree locations between 2.7 and 3.4dB. All of the CI patients in all stimulus conditions showed poorer localization than the normal hearing listeners. Localization accuracy for 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 average, sorted signals on the horizontal plane into four sectors – on each side of the midline, one sector including 0, 15 and possibly 30 degrees, and a sector from 45 degrees to 75 degrees. Resolution within a sector was relatively poor.

Using ILD or ITD Cues for Sound Source Localization and Speech Understanding in a Complex Listening Environment by Listeners With Bilateral and With Hearing-Preservation Cochlear Implants

“…These listeners have access to ILDs but very little access to ITDs and are able to locate sound sources (Grantham, Ashmead, Rickets, Haynes, & Labadie, 2008;Grantham, Ashmead, Ricketts, Labadie, & Haynes, 2007;Schoen, Mueller, Helms, & Nopp, 2005;van Hoesel & Tyler 2003;Wilson, Lawson, Muller, Tyler, & Kiefer, 2003). However, accuracy, on average, is poorer than normal (e.g., Dorman et al, 2014;Grantham et al, 2007). There is no doubt that listeners with bilateral CIs can use ILD cues for sound-source localization (see references above).…”

“…Our experimental design does not provide data that speak to the cues used by the listeners to achieve better-than-chance performance. We can speculate that, in the LP condition, the listeners with bilateral CIs were using the very small ILDs that occur for low-frequency signals (e.g., Dorman et al, 2014). In the HP condition, the listeners with hearing-preservation CIs may have used the differences in level for HP signals at the CI ear and in the ear without the CI to achieve some lateralization of sound sources.…”

“…First, the ILDs available to listeners with CIs are far smaller than the ILDs available to listeners with NH. This is due to the input and output compression inherent in CI signal processing (e.g., Dorman et al, 2014). Second, Grantham et al (2007) found that ILD thresholds ranged from 1 to more than 10 dB for listeners with bilateral CIs.…”

Sound Source Localization and Speech Understanding in Complex Listening Environments by Single-sided Deaf Listeners After Cochlear Implantation

“…One had a 30 dB threshold at 2 kHz. The BCI sample consisted of 16 subjects fit with Med El implants (as described in Dorman et al, 2014) and 11 subjects fit with Cochlear Corporation devices (11). These patients ranged in age from 32 to 79 years.…”

“…The magnitude of the difference is shown in the following example [taken from Dorman et al (in press)]: for normal hearing listeners the ILD at 3 kHz for a sound source at 45 degrees azimuth is approximately 10 dB; at 15 degrees azimuth, the ILD is approximately 3dB. Following CI signal processing, at 45 degrees azimuth, the ILD is 1.6 dB and at 15 degrees it is 0.4 dB (9,11). Thus, SSD-CI patients should experience a distorted representation of signal level as a function of signal azimuth when listening with one normal hearing ear and one deaf ear fitted with a CI.…”