Sequential stream segregation in normally-hearing and cochlear-implant listeners

Recognition of Sentences With Complex Syntax in Speech Babble by Adolescents With Normal Hearing or Cochlear Implants

PURPOSE

General language abilities of children with cochlear implants have been thoroughly investigated, especially at young ages, but far less is known about how well they process language in real-world settings, especially in higher grades. This study addressed this gap in knowledge by examining recognition of sentences with complex syntactic structures in backgrounds of speech babble by adolescents with cochlear implants, and peers with normal hearing.

DESIGN

Two experiments were conducted. First, new materials were developed using young adults with normal hearing as the normative sample, creating a corpus of sentences with controlled, but complex syntactic structures presented in three kinds of babble that varied in voice gender and number of talkers. Second, recognition by adolescents with normal hearing or cochlear implants was examined for these new materials and for sentence materials used with these adolescents at younger ages. Analyses addressed three objectives: (1) to assess the stability of speech recognition across a multiyear age range, (2) to evaluate speech recognition of sentences with complex syntax in babble, and (3) to explore how bottom-up and top-down mechanisms account for performance under these conditions.

RESULTS

Results showed: (1) Recognition was stable across the ages of 10-14 years for both groups. (2) Adolescents with normal hearing performed similarly to young adults with normal hearing, showing effects of syntactic complexity and background babble; adolescents with cochlear implants showed poorer recognition overall, and diminished effects of both factors. (3) Top-down language and working memory primarily explained recognition for adolescents with normal hearing, but the bottom-up process of perceptual organization primarily explained recognition for adolescents with cochlear implants.

CONCLUSIONS

Comprehension of language in real-world settings relies on different mechanisms for adolescents with cochlear implants than for adolescents with normal hearing. A novel finding was that perceptual organization is a critical factor.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.21965228.

The effects of aging and musicianship on the use of auditory streaming cues

Auditory stream segregation, or separating sounds into their respective sources and tracking them over time, is a fundamental auditory ability. Previous research has separately explored the impacts of aging and musicianship on the ability to separate and follow auditory streams. The current study evaluated the simultaneous effects of age and musicianship on auditory streaming induced by three physical features: intensity, spectral envelope and temporal envelope. In the first study, older and younger musicians and non-musicians with normal hearing identified deviants in a four-note melody interleaved with distractors that were more or less similar to the melody in terms of intensity, spectral envelope and temporal envelope. In the second study, older and younger musicians and non-musicians participated in a dissimilarity rating paradigm with pairs of melodies that differed along the same three features. Results suggested that auditory streaming skills are maintained in older adults but that older adults rely on intensity more than younger adults while musicianship is associated with increased sensitivity to spectral and temporal envelope, acoustic features that are typically less effective for stream segregation, particularly in older adults.

Auditory stream segregation of amplitude-modulated narrowband noise in cochlear implant users and individuals with normal hearing

Voluntary stream segregation was investigated in cochlear implant (CI) users and normal-hearing (NH) listeners using a segregation-promoting objective approach which evaluated the role of spectral and amplitude-modulation (AM) rate separations on stream segregation and its build-up. Sequences of 9 or 3 pairs of A and B narrowband noise (NBN) bursts were presented which differed in either center frequency of the noise band, the AM-rate, or both. In some sequences (delayed sequences), the last B burst was delayed by 35 ms from their otherwise-steady temporal position. In the other sequences (no-delay sequences), the last B bursts were temporally advanced from 0 to 10 ms. A single interval yes/no procedure was utilized to measure participants’ sensitivity (d′) in identifying delayed vs. no-delay sequences. A higher d′ value showed the higher ability to segregate the A and B subsequences. For NH listeners, performance improved with each spectral separation. However, for CI users, performance was only significantly better for the condition with the largest spectral separation. Additionally, performance was significantly poorer for the largest AM-rate separation than for the condition with no AM-rate separation for both groups. The significant effect of sequence duration in both groups indicated that listeners made more improvement with lengthening the duration of stimulus sequences, supporting the build-up effect. The results of this study suggest that CI users are less able than NH listeners to segregate NBN bursts into different auditory streams when they are moderately separated in the spectral domain. Contrary to our hypothesis, our results indicate that AM-rate separation may interfere with the segregation of streams of NBN. Additionally, our results add evidence to the literature that CI users build up stream segregation at a rate comparable to NH listeners, when the inter-stream spectral separations are adequately large.

Speech masking release in Hybrid cochlear implant users: Roles of spectral and temporal cues in electric-acoustic hearing

When compared with cochlear implant (CI) users utilizing electric-only (E-Only) stimulation, CI users utilizing electric-acoustic stimulation (EAS) in the implanted ear show improved speech recognition in modulated noise relative to steady-state noise (i.e., speech masking release). It has been hypothesized, but not shown, that masking release is attributed to spectral resolution and temporal fine structure (TFS) provided by acoustic hearing. To address this question, speech masking release, spectral ripple density discrimination thresholds, and fundamental frequency difference limens (f₀DLs) were evaluated in the acoustic-only (A-Only), E-Only, and EAS listening modes in EAS CI users. The spectral ripple and f₀DL tasks are thought to reflect access to spectral and TFS cues, which could impact speech masking release. Performance in all three measures was poorest when EAS CI users were tested using the E-Only listening mode, with significant improvements in A-Only and EAS listening modes. f₀DLs, but not spectral ripple density discrimination thresholds, significantly correlated with speech masking release when assessed in the EAS listening mode. Additionally, speech masking release correlated with AzBio sentence recognition in noise. The correlation between speech masking release and f₀DLs likely indicates that TFS cues provided by residual hearing were used to obtain speech masking release, which aided sentence recognition in noise.

Sequential stream segregation with bilateral cochlear implants

Sequential stream segregation on the basis of binaural 'ear-of-entry', modulation rate and electrode place-of-stimulation cues was investigated in bilateral cochlear implant (CI) listeners using a rhythm anisochrony detection task. Sequences of alternating 'A' and 'B' bursts were presented via direct electrical stimulation and comprised either an isochronous timing structure or an anisochronous structure that was generated by delaying just the 'B' bursts. 'B' delay thresholds that enabled rhythm anisochrony detection were determined. Higher thresholds were assumed to indicate a greater likelihood of stream segregation, resulting specifically from stream integration breakdown. Results averaged across subjects showed that thresholds were significantly higher when monaural 'A' and 'B' bursts were presented contralaterally rather than ipsilaterally, and that diotic presentation of 'A', with a monaural 'B', yielded intermediate thresholds. When presented monaurally and ipsilaterally, higher thresholds were also found when successive bursts had mismatched rather than matched modulation rates. In agreement with previous studies, average delay thresholds also increased as electrode separation between bursts increased when presented ipsilaterally. No interactions were found between ear-of-entry, modulation rate and place-of-stimulation. However, combining moderate electrode difference cues with either diotic-'A' ear-of-entry cues or modulation-rate mismatch cues did yield greater threshold increases than observed with any of those cues alone. The results from the present study indicate that sequential stream segregation can be elicited in bilateral CI users by differences in the signal across ears (binaural cues), in modulation rate (monaural cues) and in place-of-stimulation (monaural cues), and that those differences can be combined to further increase segregation.

Auditory Stream Segregation and Selective Attention for Cochlear Implant Listeners: Evidence From Behavioral Measures and Event-Related Potentials

The role of the spatial separation between the stimulating electrodes (electrode separation) in sequential stream segregation was explored in cochlear implant (CI) listeners using a deviant detection task. Twelve CI listeners were instructed to attend to a series of target sounds in the presence of interleaved distractor sounds. A deviant was randomly introduced in the target stream either at the beginning, middle or end of each trial. The listeners were asked to detect sequences that contained a deviant and to report its location within the trial. The perceptual segregation of the streams should, therefore, improve deviant detection performance. The electrode range for the distractor sounds was varied, resulting in different amounts of overlap between the target and the distractor streams. For the largest electrode separation condition, event-related potentials (ERPs) were recorded under active and passive listening conditions. The listeners were asked to perform the behavioral task for the active listening condition and encouraged to watch a muted movie for the passive listening condition. Deviant detection performance improved with increasing electrode separation between the streams, suggesting that larger electrode differences facilitate the segregation of the streams. Deviant detection performance was best for deviants happening late in the sequence, indicating that a segregated percept builds up over time. The analysis of the ERP waveforms revealed that auditory selective attention modulates the ERP responses in CI listeners. Specifically, the responses to the target stream were, overall, larger in the active relative to the passive listening condition. Conversely, the ERP responses to the distractor stream were not affected by selective attention. However, no significant correlation was observed between the behavioral performance and the amount of attentional modulation. Overall, the findings from the present study suggest that CI listeners can use electrode separation to perceptually group sequential sounds. Moreover, selective attention can be deployed on the resulting auditory objects, as reflected by the attentional modulation of the ERPs at the group level.

Modulation detection interference in cochlear implant listeners under forward masking conditions

Little is known about cochlear implant (CI) users' ability to process amplitude modulation (AM) under conditions of forward masking (forward-modulation detection/discrimination interference, or F-MDI). In this study, F-MDI was investigated in adult CI listeners using direct electrical stimulation via research interface. The target was sinusoidally amplitude modulated at 50 Hz, and presented to a fixed electrode in the middle of the array. The forward masker was either amplitude modulated at the same rate (AM) or unmodulated and presented at the peak amplitude of its AM counterpart (steady-state peak, SSP). Results showed that the AM masker produced higher modulation thresholds in the target than the SSP masker. The difference (F-MDI) was estimated to be 4.6 dB on average, and did not change with masker-target delays up to 100 ms or with masker-target spatial electrode distances up to eight electrodes. Results with a coherent remote cue presented with the masker showed that confusion effects did not play a role in the observed F-MDI. Traditional recovery from forward masking using the same maskers and a 20-ms probe, measured in four of the subjects, confirmed the expected result: higher thresholds with the SSP masker than the AM masker. Collectively, the results indicate that significant F-MDI occurs in CI users.

The Role of Temporal Cues in Voluntary Stream Segregation for Cochlear Implant Users

The role of temporal cues in sequential stream segregation was investigated in cochlear implant (CI) listeners using a delay detection task composed of a sequence of bursts of pulses (B) on a single electrode interleaved with a second sequence (A) presented on the same electrode with a different pulse rate. In half of the trials, a delay was added to the last burst of the otherwise regular B sequence and the listeners were asked to detect this delay. As a jitter was added to the period between consecutive A bursts, time judgments between the A and B sequences provided an unreliable cue to perform the task. Thus, the segregation of the A and B sequences should improve performance. The pulse rate difference and the duration of the sequences were varied between trials. The performance in the detection task improved by increasing both the pulse rate differences and the sequence duration. This suggests that CI listeners can use pulse rate differences to segregate sequential sounds and that a segregated percept builds up over time. In addition, the contribution of place versus temporal cues for voluntary stream segregation was assessed by combining the results from this study with those from our previous study, where the same paradigm was used to determine the role of place cues on stream segregation. Pitch height differences between the A and the B sounds accounted for the results from both studies, suggesting that stream segregation is related to the salience of the perceptual difference between the sounds.

The Role of Place Cues in Voluntary Stream Segregation for Cochlear Implant Users

Sequential stream segregation by cochlear implant (CI) listeners was investigated using a temporal delay detection task composed of a sequence of regularly presented bursts of pulses on a single electrode (B) interleaved with an irregular sequence (A) presented on a different electrode. In half of the trials, a delay was added to the last burst of the regular B sequence, and the listeners were asked to detect this delay. As a jitter was added to the period between consecutive A bursts, time judgments between the A and B sequences provided an unreliable cue to perform the task. Thus, the segregation of the A and B sequences should improve performance. In Experiment 1, the electrode separation and the sequence duration were varied to clarify whether place cues help CI listeners to voluntarily segregate sounds and whether a two-stream percept needs time to build up. Results suggested that place cues can facilitate the segregation of sequential sounds if enough time is provided to build up a two-stream percept. In Experiment 2, the duration of the sequence was fixed, and only the electrode separation was varied to estimate the fission boundary. Most listeners were able to segregate the sounds for separations of three or more electrodes, and some listeners could segregate sounds coming from adjacent electrodes.