Sound Localization in Real-Time Vocoded Cochlear-Implant Simulations With Normal-Hearing Listeners

Abstract

Bilateral cochlear-implant (CI) users and single-sided deaf listeners with a CI are less effective at localizing sounds thannormal-hearing (NH) listeners. This performance gap is due to the degradation of binaural and monaural sound localizationcues, caused by a combination of device-related and patient-related issues. In this study, we targeted the device-related issuesby measuring sound localization performance of 11 NH listeners, listening to free-field stimuli processed by a real-time CIvocoder. The use of a real-time vocoder is a new approach, which enables testing in a free-field environment. For the NHlistening condition, all listeners accurately and precisely localized sounds according to a linear stimulus–response relationshipwith an optimal gain and a minimal bias both in the azimuth and in the elevation directions. In contrast, when listening withbilateral real-time vocoders, listeners tended to orient either to the left or to the right in azimuth and were unable todetermine sound source elevation. When listening with an NH ear and a unilateral vocoder, localization was impoverished onthe vocoder side but improved toward the NH side. Localization performance was also reflected by systematic variations inreaction times across listening conditions.We conclude that perturbation of interaural temporal cues, reduction of interaurallevel cues, and removal of spectral pinna cues by the vocoder impairs sound localization. Listeners seem to ignore cues thatwere made unreliable by the vocoder, leading to acute reweighting of available localization cues. We discuss how current CIprocessors prevent CI users from localizing sounds in everyday environments.