This project studies auditory stream segregation as an underlying factor for poor speech perception skills in cochlear implant (CI) users by testing normal-hearing adults who listen to CI simulated sounds. Segregation ability was evaluated by behavioral responses to stimulus sequences consisting of two interleaved sets of noise bursts (A and B bursts). The two sets differed in physical attributes of the noise bursts including spectrum, or amplitude modulation (AM) rate, or both. The amount of the difference between the two sets of noise bursts was varied. Speech perception in noise was measured as the AM rate of the noise varied and at different spectral separations between noise and speech. Speech understanding and segregation ability are correlated statistically.
Results show the following: 1. Stream segregation ability increased with greater spectral separation, with no segregation seen when A and B bursts had the same spectrum or when they involved the most overlapping spectra. 2. Larger AM-rate separations were associated with stronger segregation abilities in general. 3. When A and B bursts were different in both spectrum and AM rate, larger AM-rate separations were associated with stronger stream segregation only for the condition that A and B bursts were most overlapping in spectrum. 4. Speech perception in noise decreased as the spectral overlapping of speech and noise increased. 5. Nevertheless, speech perception was not different as the AM rate of the noise varied. 6. Speech perception in both steady-state and modulated noise was found to be correlated with stream segregation ability based on both spectral separation and AM-rate separation.
The findings suggest that spectral separation is a primary/stronger cue for CI listeners to perform stream segregation. In addition, AM-rate separation could be a secondary/weaker cue to facilitate stream segregation. The spectral proximity of noise and speech has a strong effect on CI simulation listeners' speech perception in noise. Although neither the presence of noise modulation nor the modulation rate affected CI simulation listeners' speech understanding, the ability to use the AM-rate cue for segregation is correlated with their speech understanding. The results suggest that CI users could segregate different auditory streams if the spectral and modulation rate differences are large enough; and that their ability to use these cues for stream segregation may be a predictor of their speech perception in noise.