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 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.

Spatial Hearing by Bilateral Cochlear Implant Users With Temporal Fine-Structure Processing

“…Apart from improved speech understanding in noise ( 1 – 5 ), sound localization performance in the bilateral CI condition also improved when compared to unilateral CI [e.g., ( 1 , 5 , 6 )]. However, the benefit of bilateral implantation is not equivalent to binaural hearing, and the performance gap between bilateral CI vs. normal-hearing (NH) listeners is still significant ( 7 , 8 ). Normal-hearing listeners localize sounds in azimuth with high acuity and precision, thanks to the efficient processing of interaural level differences (ILDs) and interaural timing differences (ITDs).…”

Pulsatile Gaussian-Enveloped Tones (GET) Vocoders for Cochlear-Implant Simulation

“…Any improvement is of course limited by the trade-off between temporal and spectral representation. Future studies may explore discrete tones with different envelopes, e.g., Gammatones (de Boer, 1975; Patterson et al, 1987; Ausili et al, 2019). Gammatones are similar in shape to a Gaussian envelope but is asymmetrical with a longer tail.…”

Synchronized Automatic Gain Control in Bilateral Cochlear Implant Recipients Yields Significant Benefit in Static and Dynamic Listening Conditions

“…The fact that even with synchronized AGC, where acoustic ILD is preserved, the scores do not further improve to NH listeners’ performance levels (in a similar experiment, Chandler and Grantham [1992 ] reported average MAA of 1.2° for NH individuals), suggests that restoration of ILD cues or monaural spectral cues together may not be enough to achieve similar accuracy, as NH listeners also have access to ITD cues. In NH listeners, CI simulations impair sound localization by distorting ITD, ILD, and monaural spectral cues ( Ausili et al., 2019 ). A future vocoder study, removing ITD cues, to test the static and dynamic spatial hearing abilities of NH listeners might give us more insights to the contribution of ITD and ILD just noticeable difference (JND) for this particular task.…”