Effects of better-ear glimpsing, binaural unmasking, and spectral resolution on spatial release from masking in cochlear-implant users.

Bilateral cochlear-implant (BICI) listeners obtain less spatial release from masking (SRM; speech-recognition improvement for spatially separated vs co-located conditions) than normal-hearing (NH) listeners, especially for symmetrically placed maskers that produce similar long-term target-to-masker ratios at the two ears. Two experiments examined possible causes of this deficit, including limited better-ear glimpsing (using speech information from the more advantageous ear in each time-frequency unit), limited binaural unmasking (using interaural differences to improve signal-in-noise detection), or limited spectral resolution. Listeners had NH (presented with unprocessed or vocoded stimuli) or BICIs. Experiment 1 compared natural symmetric maskers, idealized monaural better-ear masker (IMBM) stimuli that automatically performed better-ear glimpsing, and hybrid stimuli that added worse-ear information, potentially restoring binaural cues. BICI and NH-vocoded SRM was comparable to NH-unprocessed SRM for idealized stimuli but was 14%-22% lower for symmetric stimuli, suggesting limited better-ear glimpsing ability. Hybrid stimuli improved SRM for NH-unprocessed listeners but degraded SRM for BICI and NH-vocoded listeners, suggesting they experienced across-ear interference instead of binaural unmasking. In experiment 2, increasing the number of vocoder channels did not change NH-vocoded SRM. BICI SRM deficits likely reflect a combination of across-ear interference, limited better-ear glimpsing, and poorer binaural unmasking that stems from cochlear-implant-processing limitations other than reduced spectral resolution.

Explaining intelligibility in speech-modulated maskers using acoustic glimpse analysis.

Intelligibility was measured in speech-modulated noise varying in level and temporal modulation rate (TMR). Acoustic analysis measured glimpses available above a local signal-to-noise ratio criterion (LC). The proportion and rate of glimpses were correlated with intelligibility, particularly in relation to masker level or TMR manipulations, respectively. Intelligibility correlations for each metric were maximized at different analysis LCs. Regression analysis showed that both metrics measured at -2 dB LC were required to best explain the total variance (R2 = 0.49) for individual sentence intelligibility. Acoustic conditions associated with recognizing speech in complex maskers are best explained using multidimensional glimpse metrics.

Glimpsing speech interrupted by speech-modulated noise.

Everyday environments frequently present speech in modulated noise backgrounds, such as from a competing talker. Under such conditions, temporal glimpses of speech may be preserved at favorable signal-to-noise ratios during the amplitude dips of the masker. Speech recognition is determined, in part, by these speech glimpses. However, properties of the noise when it dominates the speech may also be important. This study interrupted speech to provide either high-intensity or low-intensity speech glimpses derived from measurements of speech-on-speech masking. These interrupted intervals were deleted and subsequently filled by steady-state noise or one of four different types of noise amplitude modulated by the same or different sentence. Noise was presented at two different levels. Interruption by silence was also examined. Speech recognition was best with high-intensity glimpses and improved when the noise was modulated by missing high-intensity segments. Additional noise conditions detailed significant interactions between the noise level and glimpsed speech level. Overall, high-intensity speech segments, and the amplitude modulation (AM) of the segments, are crucial for speech recognition. Speech recognition is further influenced by the properties of the competing noise (i.e., level and AM) which interact with the glimpsed speech level. Acoustic properties of both speech-dominated and noise-dominated intervals of speech-noise mixtures determine speech recognition.

Modulation masking and glimpsing of natural and vocoded speech during single-talker modulated noise: Effect of the modulation spectrum.

Compared to notionally steady-state noise, modulated maskers provide a perceptual benefit for speech recognition, in part due to preserved speech information during the amplitude dips of the masker. However, overlap in the modulation spectrum between the target speech and the competing modulated masker may potentially result in modulation masking, and thereby offset the release from energetic masking. The current study investigated masking release provided by single-talker modulated noise. The overlap in the modulation spectra of the target speech and the modulated noise masker was varied through time compression or expansion of the competing masker. Younger normal hearing adults listened to sentences that were unprocessed or noise vocoded to primarily limit speech recognition to the preserved temporal envelope cues. For unprocessed speech, results demonstrated improved performance with masker modulation spectrum shifted up or down compared to the target modulation spectrum, except for the most extreme time expansion. For vocoded speech, significant masking release was observed with the slowest masker rate. Perceptual results combined with acoustic analyses of the preserved glimpses of the target speech suggest contributions of modulation masking and cognitive-linguistic processing as factors contributing to performance.