Sentence recognition in noise promoting or suppressing masking release by normal-hearing and cochlear-implant listeners

Abstract

Normal-hearing (NH) listeners maintain robust speech understanding in modulated noise by "glimpsing" portions of speech from a partially masked waveform-a phenomenon known as masking release (MR). Cochlear implant (CI) users, however, generally lack such resiliency. In previous studies, temporal masking of speech by noise occurred randomly, obscuring to what degree MR is attributable to the temporal overlap of speech and masker. In the present study, masker conditions were constructed to either promote (þMR) or suppress (ÀMR) masking release by controlling the degree of temporal overlap. Sentence recognition was measured in 14 CI subjects and 22 young-adult NH subjects. Normal-hearing subjects showed large amounts of masking release in the þMR condition and a marked difference between þMR and ÀMR conditions. In contrast, CI subjects demonstrated less effect of MR overall, and some displayed modulation interference as reflected by poorer performance in modulated maskers. These results suggest that the poor performance of typical CI users in noise might be accounted for by factors that extend beyond peripheral masking, such as reduced segmental boundaries between syllables or words. Encouragingly, the best CI users tested here could take advantage of masker fluctuations to better segregate the speech from the background.

Masking Release and Modulation Interference in Cochlear Implant and Simulation Listeners

“…One of the common findings from the studies of CI users is individual variability in their speech recognition scores regardless of speech materials or noise conditions (e.g., Fu & Galvin, 2008;Gifford, Shallop, & Peterson, 2008;Kwon et al, 2012), which was also found in the current study. Table 6 shows the performance of individual CI listeners in several conditions: quiet with full-spectrum speech and low-pass filtered speech, remote noise for the three AM conditions, and embedded noise for the same AM conditions.…”

“…Previous studies have reported that NH listeners' speech recognition scores could improve by as much as 80 percentage points when noise was modulated versus steady (Jin & Nelson, 2006). However, significant masking release reduction or no masking release has been found in cochlear implant (CI) users or in NH listeners identifying vocoded speech that simulates speech processed by a CI device (NH-Sim; Fu & Nogaki, 2004;Kwon, Perry, Wilhelm, & Healy 2012;Nelson & Jin, 2004;Nelson, Jin, Carney, & Nelson, 2003;Qin & Oxenham, 2003;Stickney, Zeng, Litovsky, & Assmann, 2004). For example, Nelson and colleagues (Nelson & Jin, 2004;Nelson et al, 2003) compared the performance of three listener groups (NH, CI, and NH-Sim) for sentence recognition in the presence of different masking noises, including steady-state noise and gated noise modulated at different frequencies.…”

“…In studies of spatial masking, when speech and noise were presented from different locations, NH listeners as well as CI and NH-Sim listeners showed spatial release from masking (Ihlefeld et al 2010;Loizou et al, 2009). Furthermore, Kwon et al (2012) showed that when the envelopes of modulated noise were less temporally overlapped with those of speech signals, ,50% of CI participants were able to benefit from the dips of modulated noise, resulting in significant masking release. These results suggest that if speech and noise are perceptually segregated, CI listeners would be able to experience masking release.…”

Evidence for independent time-unit processing of speech using noise promoting or suppressing masking release

“…It was found that temporally interpolated noise, even at very high levels, had little effect on sentence recognition using masking-release conditions similar to those of Kwon et al [(2012). J. Acoust.…”

“…These studies indicate that intelligibility is increased when fluctuations are introduced to a masker (e.g., Miller, 1947b;Carhart et al, 1966), thus providing glimpses of speech in the masker dips. Kwon et al (2012) examined MR under conditions specifically designed to promote or suppress it. In one condition (-MR), masker fluctuations occurred simultaneously with speech-energy fluctuations, thus maximizing the masking of speech by noise and minimizing MR.…”