Spectral aliasing in an acoustic spectral ripple discrimination task

Spectral Resolution and Speech Perception in Cochlear Implanted School-Aged Children

OBJECTIVE

Cochlear implantation of prelingually deaf infants provides auditory input sufficient to develop spoken language; however, outcomes remain variable. Inability to participate in speech perception testing limits testing device efficacy in young listeners. In postlingually implanted adults (aCI), speech perception correlates with spectral resolution an ability that relies independently on frequency resolution (FR) and spectral modulation sensitivity (SMS). The correlation of spectral resolution to speech perception is unknown in prelingually implanted children (cCI). In this study, FR and SMS were measured using a spectral ripple discrimination (SRD) task and were correlated with vowel and consonant identification. It was hypothesized that prelingually deaf cCI would show immature SMS relative to postlingually deaf aCI and that FR would correlate with speech identification.

STUDY DESIGN

Cross-sectional study.

SETTING

In-person, booth testing.

METHODS

SRD was used to determine the highest spectral ripple density perceived at various modulation depths. FR and SMS were derived from spectral modulation transfer functions. Vowel and consonant identification was measured; SRD performance and speech identification were analyzed for correlation.

RESULTS

Fifteen prelingually implanted cCI and 13 postlingually implanted aCI were included. FR and SMS were similar between cCI and aCI. Better FR was associated with better speech identification for most measures.

CONCLUSION

Prelingually implanted cCI demonstrated adult-like FR and SMS; additionally, FR correlated with speech identification. FR may be a measure of CI efficacy in young listeners.

Contribution of Verbal Learning & Memory and Spectro-Temporal Discrimination to Speech Recognition in Cochlear Implant Users

OBJECTIVES

Existing cochlear implant (CI) outcomes research demonstrates a high degree of variability in device effectiveness among experienced CI users. Increasing evidence suggests that verbal learning and memory (VL&M) may have an influence on speech recognition with CIs. This study examined the relations in CI users between visual measures of VL&M and speech recognition in a series of models that also incorporated spectro-temporal discrimination. Predictions were that (1) speech recognition would be associated with VL&M abilities and (2) VL&M would contribute to speech recognition outcomes above and beyond spectro-temporal discrimination in multivariable models of speech recognition.

METHODS

This cross-sectional study included 30 adult postlingually deaf experienced CI users who completed a nonauditory visual version of the California Verbal Learning Test-Second Edition (v-CVLT-II) to assess VL&M, and the Spectral-Temporally Modulated Ripple Test (SMRT), an auditory measure of spectro-temporal processing. Participants also completed a battery of word and sentence recognition tasks.

RESULTS

CI users showed significant correlations between some v-CVLT-II measures (short-delay free- and cued-recall, retroactive interference, and "subjective" organizational recall strategies) and speech recognition measures. Performance on the SMRT was correlated with all speech recognition measures. Hierarchical multivariable linear regression analyses showed that SMRT performance accounted for a significant degree of speech recognition outcome variance. Moreover, for all speech recognition measures, VL&M scores contributed independently in addition to SMRT.

CONCLUSION

Measures of spectro-temporal discrimination and VL&M were associated with speech recognition in CI users. After accounting for spectro-temporal discrimination, VL&M contributed independently to performance on measures of speech recognition for words and sentences produced by single and multiple talkers.

LEVEL OF EVIDENCE

3 Laryngoscope, 133:661-669, 2023.

Spectrotemporal Modulation Discrimination in Infants With Normal Hearing

OBJECTIVES

Spectral resolution correlates with speech understanding in post-lingually deafened adults with cochlear implants (CIs) and is proposed as a non-linguistic measure of device efficacy in implanted infants. However, spectral resolution develops gradually through adolescence regardless of hearing status. Spectral resolution relies on two different factors that mature at markedly different rates: Resolution of ripple peaks (frequency resolution) matures during infancy whereas sensitivity to across-spectrum intensity modulation (spectral modulation sensitivity) matures by age 12. Investigation of spectral resolution as a clinical measure for implanted infants requires understanding how each factor develops and constrains speech understanding with a CI. This study addresses the limitations of the present literature. First, the paucity of relevant data requires replication and generalization across measures of spectral resolution. Second, criticism that previously used measures of spectral resolution may reflect non-spectral cues needs to be addressed. Third, rigorous behavioral measurement of spectral resolution in individual infants is limited by attrition. To address these limitations, we measured discrimination of spectrally modulated, or rippled, sounds at two modulation depths in normal hearing (NH) infants and adults. Non-spectral cues were limited by constructing stimuli with spectral envelopes that change in phase across time. Pilot testing suggested that dynamic spectral envelope stimuli appeared to hold infants' attention and lengthen habituation time relative to previously used static ripple stimuli. A post-hoc condition was added to ensure that the stimulus noise carrier was not obscuring age differences in spectral resolution. The degree of improvement in discrimination at higher ripple depth represents spectral frequency resolution independent of the overall threshold. It was hypothesized that adults would have better thresholds than infants but both groups would show similar effects of modulation depth.

DESIGN

Participants were 53 6- to 7-month-old infants and 23 adults with NH with no risk factors for hearing loss who passed bilateral otoacoustic emissions screening. Stimuli were created from complexes with 33- or 100-tones per octave, amplitude-modulated across frequency and time with constant 5 Hz envelope phase-drift and spectral ripple density from 1 to 20 ripples per octave (RPO). An observer-based, single-interval procedure measured the highest RPO (1 to 19) a listener could discriminate from a 20 RPO stimulus. Age-group and stimulus pure-tone complex were between-subjects variables whereas modulation depth (10 or 20 dB) was within-subjects. Linear-mixed model analysis was used to test for the significance of the main effects and interactions.

RESULTS

All adults and 94% of infants provided ripple density thresholds at both modulation depths. The upper range of threshold approached 17 RPO with the 100-tones/octave carrier and 20 dB depth condition. As expected, mean threshold was significantly better with the 100-tones/octave compared with the 33-tones/octave complex, better in adults than in infants, and better at 20 dB than 10 dB modulation depth. None of the interactions reached significance, suggesting that the effect of modulation depth on the threshold was not different for infants or adults.

CONCLUSIONS

Spectral ripple discrimination can be measured in infants with minimal listener attrition using dynamic ripple stimuli. Results are consistent with previous findings that spectral resolution is immature in infancy due to immature spectral modulation sensitivity rather than frequency resolution.

Spectral Resolution Development in Children With Normal Hearing and With Cochlear Implants: A Review of Behavioral Studies

PURPOSE

This review article provides a theoretical overview of the development of spectral resolution in children with normal hearing (cNH) and in those who use cochlear implants (CIs), with an emphasis on methodological considerations. The aim was to identify key directions for future research on spectral resolution development in children with CIs.

METHOD

A comprehensive literature review was conducted to summarize and synthesize previously published behavioral research on spectral resolution development in normal and impaired auditory systems.

CONCLUSIONS

In cNH, performance on spectral resolution tasks continues to improve through the teenage years and is likely driven by gradual maturation of across-channel intensity resolution. A small but growing body of evidence from children with CIs suggests a more complex relationship between spectral resolution development, patient demographics, and the quality of the CI electrode-neuron interface. Future research should aim to distinguish between the effects of patient-specific variables and the underlying physiology on spectral resolution abilities in children of all ages who are hard of hearing and use auditory prostheses.

Bottom-Up Signal Quality Impacts the Role of Top-Down Cognitive-Linguistic Processing During Speech Recognition by Adults with Cochlear Implants

HYPOTHESES

Significant variability persists in speech recognition outcomes in adults with cochlear implants (CIs). Sensory ("bottom-up") and cognitive-linguistic ("top-down") processes help explain this variability. However, the interactions of these bottom-up and top-down factors remain unclear. One hypothesis was tested: top-down processes would contribute differentially to speech recognition, depending on the fidelity of bottom-up input.

BACKGROUND

Bottom-up spectro-temporal processing, assessed using a Spectral-Temporally Modulated Ripple Test (SMRT), is associated with CI speech recognition outcomes. Similarly, top-down cognitive-linguistic skills relate to outcomes, including working memory capacity, inhibition-concentration, speed of lexical access, and nonverbal reasoning.

METHODS

Fifty-one adult CI users were tested for word and sentence recognition, along with performance on the SMRT and a battery of cognitive-linguistic tests. The group was divided into "low-," "intermediate-," and "high-SMRT" groups, based on SMRT scores. Separate correlation analyses were performed for each subgroup between a composite score of cognitive-linguistic processing and speech recognition.

RESULTS

Associations of top-down composite scores with speech recognition were not significant for the low-SMRT group. In contrast, these associations were significant and of medium effect size (Spearman's rho = 0.44-0.46) for two sentence types for the intermediate-SMRT group. For the high-SMRT group, top-down scores were associated with both word and sentence recognition, with medium to large effect sizes (Spearman's rho = 0.45-0.58).

CONCLUSIONS

Top-down processes contribute differentially to speech recognition in CI users based on the quality of bottom-up input. Findings have clinical implications for individualized treatment approaches relying on bottom-up device programming or top-down rehabilitation approaches.

Exponential spectro-temporal modulation generation

Traditionally, real-time generation of spectro-temporally modulated noise has been performed on a linear amplitude scale, partially due to computational constraints. Experiments often require modulation that is sinusoidal on a logarithmic amplitude scale as a result of the many perceptual and physiological measures which scale linearly with exponential changes in the signal magnitude. A method is presented for computing exponential spectro-temporal modulation, showing that it can be expressed analytically as a sum over linearly offset sidebands with component amplitudes equal to the values of the modified Bessel function of the first kind. This approach greatly improves the efficiency and precision of stimulus generation over current methods, facilitating real-time generation for a broad range of carrier and envelope signals.

Role of semantic context and talker variability in speech perception of cochlear-implant users and normal-hearing listeners

This study assessed the impact of semantic context and talker variability on speech perception by cochlear-implant (CI) users and compared their overall performance and between-subjects variance with that of normal-hearing (NH) listeners under vocoded conditions. Thirty post-lingually deafened adult CI users were tested, along with 30 age-matched and 30 younger NH listeners, on sentences with and without semantic context, presented in quiet and noise, spoken by four different talkers. Additional measures included working memory, non-verbal intelligence, and spectral-ripple detection and discrimination. Semantic context and between-talker differences influenced speech perception to similar degrees for both CI users and NH listeners. Between-subjects variance for speech perception was greatest in the CI group but remained substantial in both NH groups, despite the uniformly degraded stimuli in these two groups. Spectral-ripple detection and discrimination thresholds in CI users were significantly correlated with speech perception, but a single set of vocoder parameters for NH listeners was not able to capture average CI performance in both speech and spectral-ripple tasks. The lack of difference in the use of semantic context between CI users and NH listeners suggests no overall differences in listening strategy between the groups, when the stimuli are similarly degraded.