Cochlear implant patients' localization using interaural level differences exceeds that of untrained normal hearing listeners.

Bilateral cochlear implant patients are unable to localize as well as normal hearing listeners. Although poor sensitivity to interaural time differences clearly contributes to this deficit, it is unclear whether deficits in terms of interaural level differences are also a contributing factor. In this study, localization was tested while manipulating interaural time and level cues using head-related transfer functions. The results indicate that bilateral cochlear implant users' ability to localize based on interaural level differences is actually greater than that of untrained normal hearing listeners.

The effect of different cochlear implant microphones on acoustic hearing individuals' binaural benefits for speech perception in noise.

OBJECTIVES: Cochlear implant microphones differ in placement, frequency response, and other characteristics such as whether they are directional. Although normal-hearing (NH) individuals are often used as controls in studies examining cochlear implant users' binaural benefits, the considerable differences across cochlear implant microphones make such comparisons potentially misleading. The goal of this study was to examine binaural benefits for speech perception in noise for NH individuals using stimuli processed by head-related transfer functions (HRTFs) based on the different cochlear implant microphones. DESIGN: HRTFs were created for different cochlear implant microphones and used to test participants on the Hearing in Noise Test. Experiment 1 tested cochlear implant users and NH individuals with HRTF-processed stimuli and with sound field (SF) testing to determine whether the HRTFs adequately simulated SF testing. Experiment 2 determined the measurement error and performance-intensity function for the Hearing in Noise Test with NH individuals listening to stimuli processed with the various HRTFs. Experiment 3 compared NH listeners' performance across HRTFs to determine how the HRTFs affected performance. Experiment 4 evaluated binaural benefits for NH listeners using the various HRTFs, including ones that were modified to investigate the contributions of interaural time and level cues. RESULTS: The results indicated that the HRTFs adequately simulated SF testing for the Hearing in Noise Test. They also demonstrated that the test-retest reliability and performance-intensity function were consistent across HRTFs, and that the measurement error for the test was 1.3 dB, with a change in signal-to-noise ratio of 1 dB reflecting a 10% change in intelligibility. There were significant differences in performance when using the various HRTFs, with particularly good thresholds for the HRTF based on the directional microphone when the speech and masker were spatially separated, emphasizing the importance of measuring binaural benefits separately for each HRTF. Evaluation of binaural benefits indicated that binaural squelch and spatial release from masking were found for all HRTFs, and binaural summation was found for all but one HRTF, although binaural summation was less robust than the other types of binaural benefits. In addition, the results indicated that neither interaural time nor level cues dominated binaural benefits for the NH participants. CONCLUSIONS: This study provides a means to measure the degree to which cochlear implant microphones affect acoustic hearing with respect to speech perception in noise. It also provides measures that can be used to evaluate the independent contributions of interaural time and level cues. These measures provide tools that can aid researchers in understanding and improving binaural benefits in acoustic hearing individuals listening via cochlear implant microphones.

The use of interaural time and level difference cues by bilateral cochlear implant users.

While considerable evidence suggests that bilateral cochlear implant (CI) users' sound localization abilities rely primarily on interaural level difference (ILD) cues, and only secondarily, if at all, on interaural time difference (ITD) cues, this evidence has largely been indirect. This study used head-related transfer functions (HRTFs) to independently manipulate ITD and ILD cues and directly measure their contribution to bilateral CI users' localization abilities. The results revealed a strong reliance on ILD cues, but some CI users also made use of ITD cues. The results also suggest a complex interaction between ITD and ILD cues.