The Relationship Between Monaural and Binaural Spectral Resolution in Bilateral Cochlear Implant Users

The goal of the present study was to investigate how monaural performance, interaural asymmetry, and the timing of bilateral implantation (simultaneous, sequential) contribute to binaural spectral ripple discrimination and binaural summation in bilateral cochlear implant (BiCI) users.

Twenty Mandarin-speaking adult BiCI users participated in the study. Of the 20 participants, 10 underwent sequential bilateral cochlear implantation, and 10 underwent simultaneous bilateral cochlear implantation. Spectral resolution was assessed using the Spectral-temporally Modulated Ripple Test (SMRT) under both monaural and binaural listening conditions. Binaural summation benefit was quantified as the difference between binaural SMRT scores and monaural SMRT scores in the better performing ear. Interaural performance asymmetry was quantified as the difference in monaural SMRT scores between each cochlear implant (CI) ear tested individually. Binaural and monaural spectral resolutions were compared in terms of SMRT scores, with binaural summation further examined as a function of interaural performance asymmetry.

There was no significant difference in binaural or monaural SMRT scores between the simultaneously implanted BiCI (SIM-BiCI) and sequentially implanted BiCI (SEQ-BiCI) groups. Greater interaural performance asymmetry was strongly associated with reduced binaural summation in the SIM-BiCI group (r = -.91, p < .001) and the SEQ-BiCI group (r = -.89, p < .001). SMRT scores with both CIs were significantly correlated with SMRT scores with the poorer performing CI (r = .70, p < .001). Binaural SMRT scores were significantly predicted by a linear combination of SMRT scores with the poorer ear (p < .001) and the laterality of the poorer ear (right or left ear; p = .022).

While there was no significant difference in binaural summation between SIM-BiCI and SEQ-BiCI users, binaural summation was strongly and negatively associated with interaural performance asymmetry. This suggests that the binaural benefit for spectral resolution is primarily limited by interaural differences in spectral resolution. Binaural SMRT performance was largely dependent on the spectral resolution of the poorer performing CI, with additional improvement associated with better spectral resolution in the right CI.