Lateralization of virtual sound sources with a binaural cochlear-implant sound coding strategy inspired by the medial olivocochlear reflex.

ABSTRACT

Many users of bilateral cochlear implants (BiCIs) localize sound sources less accurately than do people with normal hearing. This may be partly due to using two independently functioning CIs with fixed compression, which distorts and/or reduces interaural level differences (ILDs). Here, we investigate the potential benefits of using binaurally coupled, dynamic compression inspired by the medial olivocochlear reflex; an approach termed "the MOC strategy" (Lopez-Poveda et al., 2016, Ear Hear 37e138-e148). Twelve BiCI users were asked to localize wideband (125-6000 Hz) noise tokens in a virtual horizontal plane. Stimuli were processed through a standard (STD) sound processing strategy (i.e., involving two independently functioning sound processors with fixed compression) and three different implementations of the MOC strategy one with fast (MOC1) and two with slower contralateral control of compression (MOC2 and MOC3). The MOC1 and MOC2 strategies had effectively greater inhibition in the higher than in the lower frequency channels, while the MOC3 strategy had slightly greater inhibition in the lower than in the higher frequency channels. Localization was most accurate with the MOC1 strategy, presumably because it provided the largest and less ambiguous ILDs. The angle error improved slightly from 25.3° with the STD strategy to 22.7° with the MOC1 strategy. The improvement in localization ability over the STD strategy disappeared when the contralateral control of compression was made slower, presumably because stimuli were too short (200 ms) for the slower contralateral inhibition to enhance ILDs. Results suggest that some MOC implementations hold promise for improving not only speech-in-noise intelligibility, as shown elsewhere, but also sound source lateralization.