The benefit of bilateral versus unilateral cochlear implantation to speech intelligibility in noise

Objectives: To develop a predictive model of spatial release from masking (SRM) for cochlear implantees, and validate this model against data from the literature. To establish the spatial configurations for which the model predicts a large advantage of bilateral over unilateral implantation. To collect data to support these predictions and generate predictions of more typical advantages of bilateral implantation. Design: The model initially assumed that bilateral cochlear implantees had equally effective implants on each side, with which they could perform optimal better-ear listening. Predictions were compared with measurements of SRM, using one and two implants with up to three interfering noises. The effect of relaxing the assumption of equally effective implants was explored. Novel measurements of SRM for eight unilateral implantees were collected, including measurements using speech and noise at azimuths of ± 60 degrees, and compared with prediction. A spatial map of bilateral implant benefit was generated for a situation with one interfering noise in anechoic conditions, and predictions of benefit were generated from binaural room impulse responses in a variety of real rooms. Results: The model accurately predicted data from a previous study for multiple interfering noises in a variety of spatial configurations, even when implants were assumed to be equally effective (r = 0.97). It predicted that the maximum benefit of bilateral implantation was 18 dB. Predictions were little affected if the implants were not assumed to be equally effective. The new measurements supported the 18 dB advantage prediction. The spatial map of predicted benefit showed that, for a listener facing the target voice, bilateral implantees could enjoy an advantage of about 10 dB over unilateral implantees in a wide range of situations. Predictions based on real-room measurements with speech and noise at 1 m showed that large benefits can occur even in reverberant spaces. Conclusions: In optimal conditions, the benefit of bilateral implantation to speech intelligibility in noise can be much larger than has previously been reported. This benefit is thus considerably larger than reported benefits of summation or squelch and is robust in reverberation when the interfering source is close