Masking release and modulation interference in cochlear implant and simulation listeners

Purpose: To examine the effects of temporal and spectral interference of masking noise on sentence recognition for listeners with cochlear implants (CI) and normal-hearing persons listening to vocoded signals that simulate signals processed through a CI (NH-Sim). Method: NH-Sim and CI listeners participated in the experiments using speech and noise that were processed by bandpass filters. Depending on the experimental condition, the spectra of the maskers relative to that of speech were set to be completely embedded with, partially overlapping, or completely separate from, the speech. The maskers were either steady or amplitude modulated and were presented at +10 dB signal-to-noise ratio. Results: NH-Sim listeners experienced progressively more masking as the masker became more spectrally overlapping with speech, whereas CI listeners experienced masking even when the masker was spectrally remote from the speech signal. Both the NH-Sim and CI listeners experienced significant modulation interference when noise was modulated at a syllabic rate (4 Hz), suggesting that listeners may experience both modulation interference and masking release. Thus, modulated noise has mixed and counteracting effects on speech perception. Conclusion: When the NH-Sim and CI listeners with poor spectral resolution were tested using syllabic-like rates of modulated noise, they tended to integrate or confuse the noise with the speech, causing an increase in speech errors. Optional training programs might be useful for CI listeners who show more difficulty understanding speech in noise.