Measuring the effects of spectral smearing and enhancement on speech recognition in noise for adults and children.

Broadened auditory filters associated with sensorineural hearing loss have clearly been shown to diminish speech recognition in noise for adults, but far less is known about potential effects for children. This study examined speech recognition in noise for adults and children using simulated auditory filters of different widths. Specifically, 5 groups (20 listeners each) of adults or children (5 and 7 yrs), were asked to recognize sentences in speech-shaped noise. Seven-year-olds listened at 0 dB signal-to-noise ratio (SNR) only; 5-yr-olds listened at +3 or 0 dB SNR; and adults listened at 0 or -3 dB SNR. Sentence materials were processed both to smear the speech spectrum (i.e., simulate broadened filters), and to enhance the spectrum (i.e., simulate narrowed filters). Results showed: (1) Spectral smearing diminished recognition for listeners of all ages; (2) spectral enhancement did not improve recognition, and in fact diminished it somewhat; and (3) interactions were observed between smearing and SNR, but only for adults. That interaction made age effects difficult to gauge. Nonetheless, it was concluded that efforts to diagnose the extent of broadening of auditory filters and to develop techniques to correct this condition could benefit patients with hearing loss, especially children.

Benefits of preserving stationary and time-varying formant structure in alternative representations of speech: implications for cochlear implants.

Cochlear implants have improved speech recognition for deaf individuals, but further modifications are required before performance will match that of normal-hearing listeners. In this study, the hypotheses were tested that (1) implant processing would benefit from efforts to preserve the structure of the low-frequency formants and (2) time-varying aspects of that structure would be especially beneficial. Using noise-vocoded and sine-wave stimuli with normal-hearing listeners, two experiments examined placing boundaries between static spectral channels to optimize representation of the first two formants and preserving time-varying formant structure. Another hypothesis tested in this study was that children might benefit more than adults from strategies that preserve formant structure, especially time-varying structure. Sixty listeners provided data to each experiment: 20 adults and 20 children at each of 5 and 7 years old. Materials were consonant-vowel-consonant words, four-word syntactically correct, meaningless sentences, and five-word syntactically correct, meaningful sentences. Results showed that listeners of all ages benefited from having channel boundaries placed to optimize information about the first two formants, and benefited even more from having time-varying structure. Children showed greater gains than adults only for time-varying formant structure. Results suggest that efforts would be well spent trying to design processing strategies that preserve formant structure.

Do adults with cochlear implants rely on different acoustic cues for phoneme perception than adults with normal hearing?

PURPOSE Several acoustic cues specify any single phonemic contrast. Nonetheless, adult, native speakers of a language share weighting strategies, showing preferential attention to some properties over others. Cochlear implant (CI) signal processing disrupts the salience of some cues: In general, amplitude structure remains readily available, but spectral structure less so. This study asked how well speech recognition is supported if CI users shift attention to salient cues not weighted strongly by native speakers. METHOD Twenty adults with CIs participated. The /bɑ/-/wɑ/ contrast was used because spectral and amplitude structure varies in correlated fashion for this contrast. Adults with normal hearing weight the spectral cue strongly but the amplitude cue negligibly. Three measurements were made: labeling decisions, spectral and amplitude discrimination, and word recognition. RESULTS Outcomes varied across listeners: Some weighted the spectral cue strongly, some weighted the amplitude cue, and some weighted neither. Spectral discrimination predicted spectral weighting. Spectral weighting explained the most variance in word recognition. Age of onset of hearing loss predicted spectral weighting but not unique variance in word recognition. CONCLUSION The weighting strategies of listeners with normal hearing likely support speech recognition best, so efforts in implant design, fitting, and training should focus on developing those strategies.

Low-frequency signals support perceptual organization of implant-simulated speech for adults and children.

OBJECTIVE: Using signals processed to simulate speech received through cochlear implants and low-frequency extended hearing aids, this study examined the proposal that low-frequency signals facilitate the perceptual organization of broader, spectrally degraded signals. DESIGN: In two experiments, words and sentences were presented in diotic and dichotic configurations as four-channel noise-vocoded signals (VOC-only), and as those signals combined with the acoustic signal below 0.25 kHz (LOW-plus). Dependent measures were percent correct recognition, and the difference between scores for the two processing conditions given as proportions of recognition scores for VOC-only. The influence of linguistic context was also examined. STUDY SAMPLE: Participants had normal hearing. In all, 40 adults, 40 seven-year-olds, and 20 five-year-olds participated. RESULTS: Participants of all ages showed benefits of adding the low-frequency signal. The effect was greater for sentences than words, but no effect of diotic versus dichotic presentation was found. The influence of linguistic context was similar across age groups, and did not contribute to the low-frequency effect. Listeners who had poorer VOC-only scores showed greater low-frequency effects. CONCLUSION: The benefit of adding a low-frequency signal to a broader, spectrally degraded signal derives in some part from its facilitative influence on perceptual organization of the sensory input.

Improving speech-in-noise recognition for children with hearing loss: potential effects of language abilities, binaural summation, and head shadow.

OBJECTIVE: This study examined speech recognition in noise for children with hearing loss, compared it to recognition for children with normal hearing, and examined mechanisms that might explain variance in children's abilities to recognize speech in noise. DESIGN: Word recognition was measured in two levels of noise, both when the speech and noise were co-located in front and when the noise came separately from one side. Four mechanisms were examined as factors possibly explaining variance: vocabulary knowledge, sensitivity to phonological structure, binaural summation, and head shadow. STUDY SAMPLE: Participants were 113 eight-year-old children. Forty-eight had normal hearing (NH) and 65 had hearing loss: 18 with hearing aids (HAs), 19 with one cochlear implant (CI), and 28 with two CIs. RESULTS: Phonological sensitivity explained a significant amount of between-groups variance in speech-in-noise recognition. Little evidence of binaural summation was found. Head shadow was similar in magnitude for children with NH and with CIs, regardless of whether they wore one or two CIs. Children with HAs showed reduced head shadow effects. CONCLUSION: These outcomes suggest that in order to improve speech-in-noise recognition for children with hearing loss, intervention needs to be comprehensive, focusing on both language abilities and auditory mechanisms.

Explaining coherence in coherence masking protection for adults and children.

Coherence masking protection (CMP) is the phenomenon in which a low-frequency target (typically a first formant) is labeled accurately in poorer signal-to-noise levels when combined with a high-frequency cosignal, rather than presented alone. An earlier study by the authors revealed greater CMP for children than adults, with more resistance to disruptions in harmonicity across spectral components [Nittrouer and Tarr (2011). Atten. Percept. Psychophys. 73, 2606-2623]. That finding was interpreted as demonstrating that children are obliged to process speech signals as broad spectral patterns, regardless of the harmonic structure of the spectral components. The current study tested three alternative, auditory explanations for the observed coherence of target + cosignal: (1) unique spectral shapes of target + cosignal support labeling, (2) periodicity of target + cosignal promotes coherence, and (3) temporal synchrony across target + cosignal reinforces temporal expectancies. Adults, eight-year-olds, and five-year-olds labeled stimuli in five conditions: F1 only and F1 + a constant cosignal (both used previously) were benchmarks for comparing thresholds for F1 + 3 new cosignals. Children again showed greater CMP than adults, but none of the three hypotheses could explain their CMP. It was again concluded that children are obliged to recognize speech signals as broad spectral patterns.

Amplitude rise time does not cue the /ba/-/wa/ contrast for adults or children.

PURPOSE: Previous research has demonstrated that children weight the acoustic cues to many phonemic decisions differently than do adults and gradually shift those strategies as they gain language experience. However, that research has focused on spectral and duration cues rather than on amplitude cues. In the current study, the authors examined amplitude rise time (ART; an amplitude cue) and formant rise time (FRT; a spectral cue) in the /b/-/w/ manner contrast for adults and children, and related those speech decisions to outcomes of nonspeech discrimination tasks. METHOD: Twenty adults and 30 children (ages 4-5 years) labeled natural and synthetic speech stimuli manipulated to vary ARTs and FRTs, and discriminated nonspeech analogs that varied only by ART in an AX paradigm. RESULTS: Three primary results were obtained. First, listeners in both age groups based speech labeling judgments on FRT, not on ART. Second, the fundamental frequency of the natural speech samples did not influence labeling judgments. Third, discrimination performance for the nonspeech stimuli did not predict how listeners would perform with the speech stimuli. CONCLUSION: Even though both adults and children are sensitive to ART, it was not weighted in phonemic judgments by these typical listeners.

Children weight dynamic spectral structure more than adults: evidence from equivalent signals.

Earlier work using sine-wave and noise-vocoded signals suggests that dynamic spectral structure plays a greater role in speech recognition for children than adults [Nittrouer and Lowenstein. (2010). J. Acoust. Soc. Am. 127, 1624-1635], but questions arise concerning whether outcomes can be compared because sine waves and wide noise bands are different in nature. The current study addressed that question using narrow noise bands for both signals, and applying a difference ratio to index the contribution made by dynamic spectral structure. Results replicated earlier findings, supporting the idea that dynamic spectral structure plays a critical role in speech recognition, especially for children.

Emergent literacy in kindergartners with cochlear implants.

OBJECTIVE: A key ingredient to academic success is being able to read. Deaf individuals have historically failed to develop literacy skills comparable with those of their normal-hearing (NH) peers, but early identification and cochlear implants (CIs) have improved prospects such that these children can learn to read at the levels of their peers. The goal of this study was to examine early, or emergent, literacy in these children. METHOD: Twenty-seven deaf children with CIs, who had just completed kindergarten were tested on emergent literacy, and on cognitive and linguistic skills that support emergent literacy, specifically ones involving phonological awareness, executive functioning, and oral language. Seventeen kindergartners with NH and eight with hearing loss, but who used hearing aids served as controls. Outcomes were compared for these three groups of children, regression analyses were performed to see whether predictor variables for emergent literacy differed for children with NH and those with CIs, and factors related to the early treatment of hearing loss and prosthesis configuration were examined for children with CIs. RESULTS: The performance of children with CIs was roughly 1 SD or more below the mean performance of children with NH on all tasks, except for syllable counting, reading fluency, and rapid serial naming. Oral language skills explained more variance in emergent literacy for children with CIs than for children with NH. Age of first implant explained moderate amounts of variance for several measures. Having one or two CIs had no effect, but children who had some amount of bimodal experience outperformed children who had none on several measures. CONCLUSIONS: Even deaf children who have benefitted from early identification, intervention, and implantation are still at risk for problems with emergent literacy that could affect their academic success. This finding means that intensive language support needs to continue through at least the early elementary grades. Also, a period of bimodal stimulation during the preschool years can help boost emergent literacy skills to some extent.

Coherence masking protection for mid-frequency formants by adults and children.

Coherence masking protection (CMP) refers to the phenomenon in which a target formant is labeled at lower signal-to-noise levels when presented with a stable cosignal consisting of two other formants than when presented alone. This effect has been reported primarily for adults with first-formant (F1) targets and F2/F3 cosignals, but has also been found for children, in fact in greater magnitude. In this experiment, F2 was the target and F1/F3 was the cosignal. Results showed similar effects for each age group as had been found for F1 targets. Implications for auditory prostheses for listeners with hearing loss are discussed.

Coherence masking protection for speech in children and adults.

In three experiments, we tested the hypothesis that children are more obliged than adults to fuse components of speech signals and asked whether the principle of harmonicity could explain the effect or whether it is, instead, due to children's implementing speech-based mechanisms. Coherence masking protection (CMP) was used, which involves labeling a phonetically relevant formant (the target) presented in noise, either alone or in combination with a stable spectral band (the cosignal) that provides no additional information about phonetic identity and is well outside the critical band of the target. Adults and children (8 and 5 years old) heard stimuli that were either synthetic speech or hybrids consisting of sine wave targets and synthetic cosignals. The target and cosignal either shared a common harmonic structure or did not. An adaptive procedure located listeners' thresholds for accurate labeling. Lower thresholds when the cosignal is present indicate CMP. Younger children demonstrated CMP effects that were both larger in magnitude and less susceptible to disruptions in harmonicity than those observed for adults. The conclusion was that children are obliged to integrate spectral components of speech signals, a perceptual strategy based on their recognition of when all components come from the same generator.