Speech Understanding With Bimodal Stimulation Is Determined by Monaural Signal to Noise Ratios: No Binaural Cue...

To investigate the mechanisms behind binaural and spatial effects in speech understanding for bimodal cochlear implant listeners. In particular, to test our hypothesis that their speech understanding can be characterized by means of monaural signal to noise ratios, rather than complex binaural cue processing such as binaural unmasking.

We applied a semantic framework to characterize binaural and spatial effects in speech understanding on an extensive selection of the literature on bimodal listeners. In addition, we performed two experiments in which we measured speech understanding in different masker types (1) using head-related transfer functions, and (2) while adapting the broadband signal to noise ratios in both ears independently. We simulated bimodal hearing with a vocoder in one ear (the cochlear implant side) and a low-pass filter in the other ear (the hearing aid side). By design, the cochlear implant side was the main contributor to speech understanding in our simulation.

We found that spatial release from masking can be explained as a simple trade-off between a monaural change in signal to noise at the cochlear implant side (quantified as the head shadow effect) and an opposite change in signal to noise at the hearing aid side (quantified as a change in bimodal benefit). In simulated bimodal listeners, we found that for every 1 dB increase in signal to noise ratio at the hearing aid side, the bimodal benefit improved by approximately 0.4 dB in signal to noise ratio.

Although complex binaural cue processing is often implicated when discussing speech intelligibility in adverse listening conditions, performance can simply be explained based on monaural signal to noise ratios for bimodal listeners.