Factors affecting masking release in cochlear-implant vocoded speech

Abstract

Cochlear-implant (CI) listeners generally perform better when listening to speech in steady-state noise than in fluctuating maskers, and the reasons for that are unclear. The present study presents a new hypothesis for the observed absence of release from masking. When listening to speech in fluctuating maskers (e.g., competing talkers), CI users cannot fuse the pieces of the message over temporal gaps because they are not able to perceive reliably the acoustic landmarks introduced by obstruent consonants (e.g., stops). These landmarks are evident in spectral discontinuities associated with consonant closures and releases and are posited to aid listeners determine word/syllable boundaries. To test this hypothesis, normal-hearing (NH) listeners were presented with vocoded (6-22 channels) sentences containing clean obstruent segments, but corrupted (by steady noise or fluctuating maskers) sonorant segments (e.g., vowels). Results indicated that NH listeners performed better with fluctuating maskers than with steady noise even when speech was vocoded into six channels. This outcome suggests that having access to the acoustic landmarks provided by the obstruent consonants enables listeners to integrate effectively pieces of the message glimpsed over temporal gaps into one coherent speech stream.