Magnified interaural level differences enhance spatial release from masking in bilateral cochlear implant users

Abstract

ABSTRACTBilateral cochlear implant (BiCI) usage makes binaural benefits a possibility for implant users. Yet, limited access to interaural time difference (ITD) cues and reduced saliency of interaural level difference (ILD) cues restricts perceptual benefits of spatially separating a target from masker sounds for BiCI users. Here, we explore whether magnifying ILD cues improves intelligibility of masked speech for BiCI listeners in a “symmetrical-masker” configuration, which controls for long-term positive target-to-masker ratio (TMR) at the ear nearer the target from naturally occurring ILD cues. We magnified ILDs by estimating moment-to-moment ITDs in 1-octave-wide frequency bands, and applying corresponding ILDs to the target-masker mixtures reaching the two ears at each time in each frequency band. We conducted two experiments, one with NH listeners using vocoded stimuli and one with BiCI users. ILD magnification significantly improved intelligibility in both experiments. BiCI listeners showed no benefit of spatial separation between target and maskers with natural ILDs, even for the largest target-masker separation. Because ILD magnification is applied to the mixed signals at each ear, the strategy does not alter the TMR in either ear at any time; improvements to masked speech intelligibility are thus likely from improved perceptual separation of the competing sources.