Factors affecting binaural fusion in cochlear implant users

For cochlear implant (CI) users, differences in peripheral input patterns across ears may disrupt binaural fusion, thereby limiting binaural perception. The present study investigated factors affecting binaural fusion for speech segregation. Speech recognition thresholds (SRTs) were adaptively measured in 11 simultaneously implanted (SIM) and 10 sequentially implanted (SEQ) bilateral CI (BiCI) users, as well as in 16 CI users with substantial acoustic hearing (AH) in the non-implanted ear (acoustic-electric hearing; "AEH"). Target sentences, spoken by a male talker, were presented alongside two competing sentences spoken by two different male talkers (same-sex maskers) or female talkers (different-sex maskers). The target speech was delivered to one ear, while masker speech was presented either to the same ear as the target (monaural) or to both ears simultaneously (bilateral). The difference in SRTs between bilateral and monaural listening (bilateral-monaural difference; "BMD") was used to indirectly estimate binaural fusion. Results showed higher (poorer) SRTs with bilateral than with monaural listening in SEQ-BiCI, but not in SIM-BiCI users. Across BiCI users, the BMD was negatively correlated with the inter-implant interval. Across AEH CI users, SRTs were statistically significantly better with bilateral than with monaural listening when the target speech was presented to the AH ear, but statistically significantly poorer when the target speech was presented to the CI ear. The impact of binaural fusion on speech segregation was highly asymmetric in SEQ-BiCI and AEH CI users, with a relative advantage when the target speech is presented to the first-implanted ear or the AH ear, respectively.