Spectral Ripple Discrimination in Normal-Hearing Infants

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.

Behavioral characterization of the cochlear amplifier lesion due to loss of function of stereocilin (STRC) in human subjects

Loss of function of stereocilin (STRC) is the second most common cause of inherited hearing loss. The loss of the stereocilin protein, encoded by the STRC gene, induces the loss of connection between outer hair cells and tectorial membrane. This only affects the outer hair cells (OHCs) function, involving deficits of active cochlear frequency selectivity and amplifier functions despite preservation of normal inner hair cells. Better understanding of cochlear features associated with mutation of STRC will improve our knowledge of normal cochlear function, the pathophysiology of hearing impairment, and potentially enhance hearing aid and cochlear implant signal processing. Nine subjects with homozygous or compound heterozygous loss of function mutations in STRC were included, age 7-24 years. Temporal and spectral modulation perception were measured, characterized by spectral and temporal modulation transfer functions. Speech-in-noise perception was studied with spondee identification in adaptive steady-state noise and AzBio sentences with 0 and -5 dB SNR multitalker babble. Results were compared with normal hearing (NH) and cochlear implant (CI) listeners to place STRC-/- listeners' hearing capacity in context. Spectral ripple discrimination thresholds in the STRC-/- subjects were poorer than in NH listeners (p < 0.0001) but remained better than for CI listeners (p < 0.0001). Frequency resolution appeared impaired in the STRC-/- group compared to NH listeners but did not reach statistical significance (p = 0.06). Compared to NH listeners, amplitude modulation detection thresholds in the STRC-/- group did not reach significance (p=  0.06) but were better than in CI subjects (p < 0.0001). Temporal resolution in STRC-/- subjects was similar to NH (p = 0.98) but better than in CI listeners (p = 0.04). The spondee reception threshold in the STRC-/- group was worse than NH listeners (p = 0.0008) but better than CI listeners (p = 0.0001). For AzBio sentences, performance at 0 dB SNR was similar between the STRC-/- group and the NH group, 88 % and 97 % respectively. For -5 dB SNR, the STRC-/- performance was significantly poorer than NH, 40 % and 85 % respectively, yet much better than with CI who performed at 54 % at +5 dB SNR in children and 53 % at + 10 dB SNR in adults. To our knowledge, this is the first study of the psychoacoustic performance of human subjects lacking cochlear amplification but with normal inner hair cell function. Our data demonstrate preservation of temporal resolution and a trend to impaired frequency resolution in this group without reaching statistical significance. Speech-in-noise perception compared to NH listeners was impaired as well. All measures were better than those in CI listeners. It remains to be seen if hearing aid modifications, customized for the spectral deficits in STRC-/- listeners can improve speech understanding in noise. Since cochlear implants are also limited by deficient spectral selectivity, STRC-/- hearing may provide an upper bound on what could be obtained with better temporal coding in electrical stimulation.

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.

Infant Pitch and Timbre Discrimination in the Presence of Variation in the Other Dimension

Adult listeners perceive pitch with fine precision, with many adults capable of discriminating less than a 1 % change in fundamental frequency (F0). Although there is variability across individuals, this precise pitch perception is an ability ascribed to cortical functions that are also important for speech and music perception. Infants display neural immaturity in the auditory cortex, suggesting that pitch discrimination may improve throughout infancy. In two experiments, we tested the limits of F0 (pitch) and spectral centroid (timbre) perception in 66 infants and 31 adults. Contrary to expectations, we found that infants at both 3 and 7 months were able to reliably detect small changes in F0 in the presence of random variations in spectral content, and vice versa, to the extent that their performance matched that of adults with musical training and exceeded that of adults without musical training. The results indicate high fidelity of F0 and spectral-envelope coding in infants, implying that fully mature cortical processing is not necessary for accurate discrimination of these features. The surprising difference in performance between infants and musically untrained adults may reflect a developmental trajectory for learning natural statistical covariations between pitch and timbre that improves coding efficiency but results in degraded performance in adults without musical training when expectations for such covariations are violated.

The contribution of spectral processing to the acquisition of phonological sensitivity by adolescent cochlear implant users and normal-hearing controls

This study tested the hypotheses that (1) adolescents with cochlear implants (CIs) experience impaired spectral processing abilities, and (2) those impaired spectral processing abilities constrain acquisition of skills based on sensitivity to phonological structure but not those based on lexical or syntactic (lexicosyntactic) knowledge. To test these hypotheses, spectral modulation detection (SMD) thresholds were measured for 14-year-olds with normal hearing (NH) or CIs. Three measures each of phonological and lexicosyntactic skills were obtained and used to generate latent scores of each kind of skill. Relationships between SMD thresholds and both latent scores were assessed. Mean SMD threshold was poorer for adolescents with CIs than for adolescents with NH. Both latent lexicosyntactic and phonological scores were poorer for the adolescents with CIs, but the latent phonological score was disproportionately so. SMD thresholds were significantly associated with phonological but not lexicosyntactic skill for both groups. The only audiologic factor that also correlated with phonological latent scores for adolescents with CIs was the aided threshold, but it did not explain the observed relationship between SMD thresholds and phonological latent scores. Continued research is required to find ways of enhancing spectral processing for children with CIs to support their acquisition of phonological sensitivity.

Music and psychoacoustic perception abilities in cochlear implant users with auditory neuropathy spectrum disorder

OBJECTIVE

Auditory neuropathy spectrum disorder (ANSD) is a condition wherein the pre-neural or cochlear outer hair cell activity is intact, but the neural activity in the auditory nerve is disrupted. Cochlear implant (CI) can be beneficial for subjects with ANSD; however, little is known about the music perception and psychoacoustic abilities of CI users with ANSD. Music perception in CI users is a multidimensional and complex ability requiring the contribution of both auditory and nonauditory abilities. Even though auditory abilities lay the foundation, the contribution of patient-related variables such as ANSD may affect the music perception. This study aimed to evaluate the psychoacoustic and music perception abilities of CI recipients with ANSD.

STUDY DESIGN

Twelve CI users with ANSD and twelve age- and gendermatched CI users with sensorineural hearing loss (SNHL) were evaluated. Music perception abilities were measured using the Turkish version of the Clinical Assessment of Music Perception (T-CAMP) test. Psychoacoustic abilities were measured using the spectral ripple discrimination (SRD) and temporal modulation transfer function (TMTF) tests. In addition, the age of diagnosis and implantation was recorded.

RESULTS

Pitch direction discrimination (PDD), timbre recognition, SRD, and TMTF performance of CI users with ANSD were concordant with those reported in previous studies, and differences between ANSD and SNHL groups were not statistically significant. However, the ANSD group performed poorly compared with SNHL group in melody recognition subtest of T-CAMP, and the difference was statistically significant.

CONCLUSION

CI can prove beneficial for patients with ANSD with respect to their music and psychoacoustic abilities, similar to patients with SNHL, except for melody recognition. Recognition of melodies requires both auditory and non-auditory abilities, and ANSD may have an extensive but subtle effect in the life of CI users.

Spectral aliasing in an acoustic spectral ripple discrimination task

Spectral ripple discrimination tasks are commonly used to probe spectral resolution in cochlear implant (CI), normal-hearing (NH), and hearing-impaired individuals. In addition, these tasks have also been used to examine spectral resolution development in NH and CI children. In this work, stimulus sine-wave carrier density was identified as a critical variable in an example spectral ripple-based task, the Spectro-Temporally Modulated Ripple (SMR) Test, and it was demonstrated that previous uses of it in NH listeners sometimes used values insufficient to represent relevant ripple densities. Insufficient carry densities produced spectral under-sampling that both eliminated ripple cues at high ripple densities and introduced unintended structured interference between the carriers and intended ripples at particular ripple densities. It was found that this effect produced non-monotonic psychometric functions for NH listeners that would cause systematic underestimation of thresholds with adaptive techniques. Studies of spectral ripple detection in CI users probe a density regime below where this source of aliasing occurs, as CI signal processing limits dense ripple representation. While these analyses and experiments focused on the SMR Test, any task in which discrete pure-tone carriers spanning frequency space are modulated to approximate a desired pattern must be designed with the consideration of the described spectral aliasing effect.

Reducing Simulated Channel Interaction Reveals Differences in Phoneme Identification Between Children and Adults With Normal Hearing

OBJECTIVES

Channel interaction, the stimulation of overlapping populations of auditory neurons by distinct cochlear implant (CI) channels, likely limits the speech perception performance of CI users. This study examined the role of vocoder-simulated channel interaction in the ability of children with normal hearing (cNH) and adults with normal hearing (aNH) to recognize spectrally degraded speech. The primary aim was to determine the interaction between number of processing channels and degree of simulated channel interaction on phoneme identification performance as a function of age for cNH and to relate those findings to aNH and to CI users.

DESIGN

Medial vowel and consonant identification of cNH (age 8-17 years) and young aNH were assessed under six (for children) or nine (for adults) different conditions of spectral degradation. Stimuli were processed using a noise-band vocoder with 8, 12, and 15 channels and synthesis filter slopes of 15 (aNH only), 30, and 60 dB/octave (all NH subjects). Steeper filter slopes (larger numbers) simulated less electrical current spread and, therefore, less channel interaction. Spectrally degraded performance of the NH listeners was also compared with the unprocessed phoneme identification of school-aged children and adults with CIs.

RESULTS

Spectrally degraded phoneme identification improved as a function of age for cNH. For vowel recognition, cNH exhibited an interaction between the number of processing channels and vocoder filter slope, whereas aNH did not. Specifically, for cNH, increasing the number of processing channels only improved vowel identification in the steepest filter slope condition. Additionally, cNH were more sensitive to changes in filter slope. As the filter slopes increased, cNH continued to receive vowel identification benefit beyond where aNH performance plateaued or reached ceiling. For all NH participants, consonant identification improved with increasing filter slopes but was unaffected by the number of processing channels. Although cNH made more phoneme identification errors overall, their phoneme error patterns were similar to aNH. Furthermore, consonant identification of adults with CI was comparable to aNH listening to simulations with shallow filter slopes (15 dB/octave). Vowel identification of earlier-implanted pediatric ears was better than that of later-implanted ears and more comparable to cNH listening in conditions with steep filter slopes (60 dB/octave).

CONCLUSIONS

Recognition of spectrally degraded phonemes improved when simulated channel interaction was reduced, particularly for children. cNH showed an interaction between number of processing channels and filter slope for vowel identification. The differences observed between cNH and aNH suggest that identification of spectrally degraded phonemes continues to improve through adolescence and that children may benefit from reduced channel interaction beyond where adult performance has plateaued. Comparison to CI users suggests that early implantation may facilitate development of better phoneme discrimination.

Spectral ripple discrimination in children with auditory processing disorder

Objective: The purpose of this study was to examine developmental trends in spectral ripple discrimination (SRD) and to compare the performance of typically developing children to children with auditory processing disorder (APD). Study design: Cross-sectional study. Study sample: Fifteen children with APD, as well as 17 typically developing children and 14 adults reporting no listening or academic difficulties participated. Results: Typically developing children showed poor SRD thresholds compared to adults, indicating prolonged maturation of spectral shape recognition. Both typically developing children and APD children showed a maturational trend in SRD, but a General Linear Model fit to their thresholds showed that children with APD displayed SRD thresholds that were significantly poorer than those of typically developing children when controlling for age. This suggests that in APD children, SRD maturation lags behind typically developing children. Conclusion: Poor spectral ripple discrimination may explain some of the listening difficulties experienced by children with APD.

Amplitude modulation detection and temporal modulation cutoff frequency in normal hearing infants

The goal of this study was to determine if temporal modulation cutoff frequency was mature in three-month-old infants. Normal-hearing infants and young adults were tested in a single-interval forced-choice observer-based psychoacoustic procedure. Two parameters of the temporal modulation transfer function (TMTF) were estimated to separate temporal resolution from amplitude modulation sensitivity. The modulation detection threshold (MDT) of a broadband noise amplitude modulated at 10 Hz estimated the y-intercept of the TMTF. The cutoff frequency of the TMTF, measured at a modulation depth 4 dB greater than the MDT, provided an estimate of temporal resolution. MDT was obtained in 27 of 33 infants while both MDT and cutoff frequency was obtained in 15 infants and in 16 of 16 adults. Mean MDT was approximately 10 dB poorer in infants compared to adults. In contrast, mean temporal modulation cutoff frequency did not differ significantly between age groups. These results suggest that temporal resolution is mature, on average, by three months of age in normal hearing children despite immature sensitivity to amplitude modulation. The temporal modulation cutoff frequency approach used here may be a feasible way to examine development of temporal resolution in young listeners with markedly immature sensitivity to amplitude modulation.

The Relationship Between Environmental Sound Awareness and Speech Recognition Skills in Experienced Cochlear Implant Users

HYPOTHESIS

1) Environmental sound awareness (ESA) and speech recognition skills in experienced, adult cochlear implant (CI) users will be highly correlated, and, 2) ESA skills of CI users will be significantly lower than those of age-matched adults with normal hearing.

BACKGROUND

Enhancement of ESA is often discussed with patients with sensorineural hearing loss as a potential benefit of implantation and, in some cases, ESA may be a major motivating factor. Despite its ecological validity and patients' expectations, ESA remains largely a presumed skill. The relationship between ESA and speech recognition is not well-understood.

METHODS

ESA was assessed in 35 postlingually deaf, experienced CI users and a control group of 41 age-matched, normal hearing listeners using the validated, computerized familiar environmental sounds test-identification (FEST-I) and a diverse speech recognition battery. Demographic and audiological factors as well as nonverbal intelligence quotient (IQ)/nonverbal reasoning and spectral resolution were assessed.

RESULTS

Six of the 35 experienced CI users (17%) demonstrated FEST-I accuracy within the range of the NH controls. Among CI users all correlations between FEST-I accuracy and speech recognition scores were strong. Chronological age at the time of testing, duration of deafness, spectral resolution, and nonverbal IQ/nonverbal reasoning were strongly correlated with FEST-I accuracy. Partial correlation analysis showed that correlations between FEST-I and speech recognition measures remained significant when controlling for the demographic and audiological factors.

CONCLUSION

Our findings reinforce the hypothesis that ESA and speech perception share common underlying processes rather than reflecting truly separate auditory domains.