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

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