Bimodal benefit depends on the performance difference between a cochlear implant and a hearing aid.

Objectives The present study characterizes the relationship between bimodal benefit and hearing aid (HA) performance, cochlear implant (CI) performance, and the difference in the performances of the two devices. Methods Fourteen adult bimodal listeners participated in the study. Consonant, vowel, and sentence recognition were measured in quiet and noise (at a +5 and +10 dB signal-to-noise ratio (SNR)) with an HA alone, a CI alone, and with the combined use of an HA and CI in each listener. Speech and noise were presented directly in front of the listener. Results The correlation analyses showed that bimodal benefit was significantly associated with the difference in performances of a CI and an HA in all testing materials, with HA-alone performance in vowel recognition, and with CI-alone performance in sentence recognition. However, regression analyses showed that the independent contribution of the difference in performance across ears to bimodal benefit was significant, irrespective of the testing material or the SNR: the smaller the difference, the greater the benefit. Further, the independent contributions of HA-only performance and CI-alone performance were not significant factors in predicting the existence of bimodal benefit across testing materials and SNRs when the effect of the difference between CI and HA performance was removed from the model. Conclusion The results suggest that bimodal benefit is limited by how effectively the modalities integrate, rather than HA-only or CI-alone performance, and that this integration is facilitated when the performances of the modalities are similar.