The role of periodicity in the perception of masked speech with simulated and real cochlear implants

In normal hearing, periodic sounds are much less effective maskers of speech than aperiodic ones [Steinmetzger and Rosen (2015). J. Acoust. Soc. Am. 138, 3586–3599]. Here, it is shown that this masker-periodicity benefit is strongly reduced in noise-vocoder simulations of cochlear implants (CIs) and almost absent with real CIs, which helps explain why CI users face such difficulties when attempting to understand speech in noisy environments. Furthermore, when there were no F0-related periodicity cues in the target speech, CI users performed markedly worse with maskers whose envelopes were adjusted to be the inverse of the target sentence, while no such effect was observed with simulated CIs. Moreover, neither group of listeners showed a masking release with these maskers, although they were designed to maximise glimpsing opportunities. In both cases, the difference between simulated and real CIs is thought to be caused by current spread across the CI’s electrode array: Firstly, CI users seem unable to perceive the random envelope modulations characteristic for aperiodic sounds, which diminishes the contrast between the aperiodic and periodic maskers. Secondly, the segregation of speech and masker is complicated further by spectral smearing, explaining the more prominent role of periodicity cues in the target speech.