Discrimination Of Speech Sounds Research Articles

A novel paradigm for measuring prediction abilities in a rat model using a speech-sound discrimination task

The ability to predict and respond to upcoming stimuli is a critical skill for all animals, including humans. Prediction operates largely below conscious awareness to allow an individual to recall previously encountered stimuli and prepare an appropriate response, especially in language. The ability to predict upcoming words within typical speech patterns aids fluent comprehension, as conversational speech occurs quickly. Individuals with certain neurodevelopmental disorders such as autism and dyslexia have deficits in their ability to generate and use predictions. Rodent models are often used to investigate specific aspects of these disorders, but there is no existing behavioral paradigm that can assess prediction capabilities with complex stimuli like speech sounds. Thus, the present study modified an existing rapid speech sound discrimination paradigm to assess whether rats can form predictions of upcoming speech sound stimuli and utilize them to improve task performance. We replicated prior work showing that rats can discriminate between speech sounds presented at rapid rates. We also saw that rats responded exclusively to the target at slow speeds but began responding to the predictive cue in anticipation of the target as the speed increased, suggesting that they learned the predictive value of the cue and adjusted their behavior accordingly. This prediction task will be useful in assessing prediction deficits in rat models of various neurodevelopmental disorders through the manipulation of both genetic and environmental factors.

A novel paradigm for measuring prediction abilities in a rat model using a speech-sound discrimination task

The ability to predict and respond to upcoming stimuli is a critical skill for all animals, including humans. Prediction operates largely below conscious awareness to allow an individual to recall previously encountered stimuli and prepare an appropriate response, especially in language. The ability to predict upcoming words within typical speech patterns aids fluent comprehension, as conversational speech occurs quickly. Individuals with certain neurodevelopmental disorders such as autism and dyslexia have deficits in their ability to generate and use predictions. Rodent models are often used to investigate specific aspects of these disorders, but there is no existing behavioral paradigm that can assess prediction capabilities with complex stimuli like speech sounds. Thus, the present study modified an existing rapid speech sound discrimination paradigm to assess whether rats can form predictions of upcoming speech sound stimuli and utilize them to improve task performance. We replicated prior work showing that rats can discriminate between speech sounds presented at rapid rates. We also saw that rats responded exclusively to the target at slow speeds but began responding to the predictive cue in anticipation of the target as the speed increased, suggesting that they learned the predictive value of the cue and adjusted their behavior accordingly. This prediction task will be useful in assessing prediction deficits in rat models of various neurodevelopmental disorders through the manipulation of both genetic and environmental factors.

Improving quality of life in the elderly: hearing loss treatment with cochlear implants

BackgroundHearing loss impacts health-related quality of life and general well-being and was identified in a Lancet report as one of the largest potentially modifiable factors for the prevention of age-related dementia. There is a lack of robust data on how cochlear implant treatment in the elderly impacts quality of life. The primary objective was to measure the change in health utility following cochlear implantation in individuals aged ≥ 60 years.MethodsThis study uniquely prospectively recruited a large multinational sample of 100 older adults (mean age 71.7 (SD7.6) range 60–91 years) with severe to profound hearing loss. In a repeated-measures design, pre and post implant outcome measures were analysed using mixed-effect models. Health utility was assessed with the Health Utilities Index Mark III (HUI3). Subjects were divided into groups of 60–64, 65–74 and 75 + years.ResultsAt 18 months post implant, the mean HUI3 score improved by 0.13 (95%CI: 0.07–0.18 p < 0.001). There was no statistically significant difference in the HUI3 between age groups (F[2,9228] = 0.53, p = 0.59). The De Jong Loneliness scale reduced by an average of 0.61 (95%CI: 0.25–0.97 p < 0.014) and the Lawton Instrumental Activities of Daily Living Scale improved on average (1.25, 95%CI: 0.85–1.65 p < 0.001). Hearing Handicap Inventory for the Elderly Screening reduced by an average of 8.7 (95%CI: 6.7–10.8, p < 0.001) from a significant to mild-moderate hearing handicap. Age was not a statistically significant factor for any of the other measures (p > 0.20). At baseline 90% of participants had no or mild depression and there was no change in mean depression scores after implant. Categories of Auditory perception scale showed that all subjects achieved a level of speech sound discrimination without lip reading post implantation (level 4) and at least 50% could use the telephone with a known speaker.ConclusionsBetter hearing improved individuals’ quality of life, ability to communicate verbally and their ability to function independently. They felt less lonely and less handicapped by their hearing loss. Benefits were independent of age group. Cochlear implants should be considered as a routine treatment option for those over 60 years with bilateral severe to profound hearing loss.Trial RegistrationClinicalTrials.gov (http://www.clinicaltrials.gov/), 7 March 2017, NCT03072862.

Can speech perception deficits cause phonological impairments? Evidence from short-term memory for ambiguous speech.

Poor performance on phonological tasks is characteristic of neurodevelopmental language disorders (dyslexia and/or developmental language disorder). Perceptual deficit accounts attribute phonological dysfunction to lower-level deficits in speech-sound processing. However, a causal pathway from speech perception to phonological performance has not been established. We assessed this relationship in typical adults by experimentally disrupting speech-sound discrimination in a phonological short-term memory (pSTM) task. We used an automated audio-morphing method (Rogers & Davis, 2017) to create ambiguous intermediate syllables between 16 letter name-letter name ("B"-"P") and letter name-word ("B"-"we") pairs. High- and low-ambiguity syllables were used in a pSTM task in which participants (N = 36) recalled six- and eight-letter name sequences. Low-ambiguity sequences were better recalled than high-ambiguity sequences, for letter name-letter name but not letter name-word morphed syllables. A further experiment replicated this ambiguity cost (N = 26), but failed to show retroactive or prospective effects for mixed high- and low-ambiguity sequences, in contrast to pSTM findings for speech-in-noise (SiN; Guang et al., 2020; Rabbitt, 1968). These experiments show that ambiguous speech sounds impair pSTM, via a different mechanism to SiN recall. We further show that the effect of ambiguous speech on recall is context-specific, limited, and does not transfer to recall of nonconfusable items. This indicates that speech perception deficits are not a plausible cause of pSTM difficulties in language disorders. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Identifying Developmental Language Disorder in Deaf Children with Cochlear Implants: A Case Study of Three Children.

(1) Background: While spoken language learning delays are assumed for deaf and hard of hearing (DHH) children after cochlear implant (CI), many catch up with their hearing peers. Some DHH children with CIs, however, show persistent delays in language, despite protective factors being in place. This suggests a developmental language disorder (DLD). However, at present there is little consensus on how to diagnose DLD in DHH children. (2) Methods: Given the lack of consensus in this area, a set of case studies provides an appropriate first step. The goal of this paper is to show the plausibility of a DLD diagnosis, following careful analysis of protective and risk factors. A retrospective case study review was conducted for three children. Their long-term language outcomes up to four years after CI were considered in the context of access to sound, speech sound discrimination, social skills and non-verbal cognition. (3) Results: It was possible to posit DLD in one child who had experienced good access to sound, alongside good speech discrimination abilities and social development, and normal non-verbal cognition, but who presented with severe language learning difficulties. (4) Conclusions: Finding markers for DLD in DHH children is important for diagnosis and intervention. The implications for clinical practice are discussed.

EEG Alpha Band Responses Reveal Amplification Benefits in Infants with Hearing Loss.

Our objective was to examine the effects of hearing aid amplification on auditory detection and discrimination in infants who were hard of hearing (IHH) using a physiological measure of auditory perception. We recorded EEG from 41 sleeping IHH aged 1.04 to 5.62 months while presenting auditory stimuli in a mismatch response paradigm. Responses were recorded during two listening conditions for each participant: aided and unaided. Temporal envelopes of the mismatch response in the EEG alpha band (6-12 Hz) were extracted from the latent, time-frequency transformed data. Aided alpha band responses were greater than unaided responses for the deviant trials but were not different for the standard trials. Responses to the deviant trials were greater than responses to the standard trials for the aided conditions but were not different for the unaided conditions. These results suggest that the alpha band mismatch can be used to examine both detection and discrimination of speech and non-speech sounds in IHH. With further study, the alpha band mismatch could expand and refine our abilities to validate hearing aid fittings at younger ages than current clinical protocols allow.

Foreign speech sound discrimination and associative word learning lead to a fast reconfiguration of resting-state networks

Learning new words in an unfamiliar language is a complex endeavor that requires the orchestration of multiple perceptual and cognitive functions. Although the neural mechanisms governing word learning are becoming better understood, little is known about the predictive value of resting-state (RS) metrics for foreign word discrimination and word learning attainment. In addition, it is still unknown which of the multistep processes involved in word learning have the potential to rapidly reconfigure RS networks. To address these research questions, we used electroencephalography (EEG), measured forty participants, and examined scalp-based power spectra, source-based spectral density maps and functional connectivity metrics before (RS1), in between (RS2) and after (RS3) a series of tasks which are known to facilitate the acquisition of new words in a foreign language, namely word discrimination, word-referent mapping and semantic generalization. Power spectra at the scalp level consistently revealed a reconfiguration of RS networks as a function of foreign word discrimination (RS1 vs. RS2) and word learning (RS1 vs. RS3) tasks in the delta, lower and upper alpha, and upper beta frequency ranges. Otherwise, functional reconfigurations at the source level were restricted to the theta (spectral density maps) and to the lower and upper alpha frequency bands (spectral density maps and functional connectivity). Notably, scalp RS changes related to the word discrimination tasks (difference between RS2 and RS1) correlated with word discrimination abilities (upper alpha band) and semantic generalization performance (theta and upper alpha bands), whereas functional changes related to the word learning tasks (difference between RS3 and RS1) correlated with word discrimination scores (lower alpha band). Taken together, these results highlight that foreign speech sound discrimination and word learning have the potential to rapidly reconfigure RS networks at multiple functional scales.

The Use of Heart Rate Responses Extracted From Functional Near-Infrared Spectroscopy Data as a Measure of Speech Discrimination Ability in Sleeping Infants.

Cardiac responses (e.g., heart rate changes) due to an autonomous response to sensory stimuli have been reported in several studies. This study investigated whether heart rate information extracted from functional near-infrared spectroscopy (fNIRS) data can be used to assess the discrimination of speech sounds in sleeping infants. This study also investigated the adaptation of the heart rate response over multiple, sequential stimulus presentations. fNIRS data were recorded from 23 infants with no known hearing loss, aged 2 to 10 months. Speech syllables were presented using a habituation/dishabituation test paradigm: the infant's heart rate response was first habituated by repeating blocks of one speech sound; then, the heart rate response was dishabituated with the contrasting (novel) speech sound. This stimulus presentation sequence was repeated for as long as the infants were asleep. The group-level average heart rate response to the novel stimulus was greater than that to the habituated first sound, indicating that sleeping infants were able to discriminate the speech sound contrast. A significant adaptation of the heart rate responses was seen over the session duration. The dishabituation response could be a valuable marker for speech discrimination, especially when used in conjunction with the fNIRS hemodynamic response.

Incidental and Multimodal High Variability Phonetic Training: Potential, Limits, and Future Directions

AbstractScholars have extensively investigated the effectiveness of high variability phonetic training (HVPT), that is, identification and discrimination of second language speech sounds produced by multiple speakers followed by trial‐by‐trial feedback. Building on the notion of incidental and multimodal learning in cognitive psychology (e.g., Lim &amp; Holt, 2011), we developed a new, HVPT‐based videogame paradigm in which participants aimed to shoot clay targets as fast as possible while being guided to learn sound cues as a by‐product of planned learning. Focusing on the speech acquisition of 58 Japanese English‐as‐a‐foreign‐language learners, the current study set out to test the pedagogical potential and limits of the incidental HVPT approach. According to the results of statistical analyses, the effectiveness of incidental HVPT can be more clearly observed if it focuses on more learnable targets (e.g., acquisition of English [æ]–[ʌ] rather than [r]–[l] contrasts) with gains being more generalizable from trained to new speakers’ voices and from perception to production dimensions.

The effects of positive emotional gesture guidance on speech sound discrimination in Thai children with ASD: A mismatch negativity study

The effects of positive emotional gesture guidance on speech sound discrimination in Thai children with ASD: A mismatch negativity study

Speech sound discrimination by Mongolian gerbils

The present study establishes the Mongolian gerbil (Meriones unguiculatus) as a model for investigating the perception of human speech sounds. We report data on the discrimination of logatomes (CVCs - consonant-vowel-consonant combinations with outer consonants /b/, /d/, /s/ and /t/ and central vowels /a/, /aː/, /ɛ/, /eː/, /ɪ/, /iː/, /ɔ/, /oː/, /ʊ/ and /uː/, VCVs - vowel-consonant-vowel combinations with outer vowels /a/, /ɪ/ and /ʊ/ and central consonants /b/, /d/, /f/, /g/, /k/, /l/, /m/, /n/, /p/, /s/, /t/ and /v/) by gerbils. Four gerbils were trained to perform an oddball target detection paradigm in which they were required to discriminate a deviant CVC or VCV in a sequence of CVC or VCV standards, respectively. The experiments were performed with an ICRA-1 noise masker with speech-like spectral properties, and logatomes of multiple speakers were presented at various signal-to-noise ratios. Response latencies were measured to generate perceptual maps employing multidimensional scaling, which visualize the gerbils’ internal maps of the sounds. The dimensions of the perceptual maps were correlated to multiple phonetic features of the speech sounds for evaluating which features of vowels and consonants are most important for the discrimination. The perceptual representation of vowels and consonants in gerbils was similar to that of humans, although gerbils needed higher signal-to-noise ratios for the discrimination of speech sounds than humans. The gerbils’ discrimination of vowels depended on differences in the frequencies of the first and second formant determined by tongue height and position. Consonants were discriminated based on differences in combinations of their articulatory features. The similarities in the perception of logatomes by gerbils and humans renders the gerbil a suitable model for human speech sound discrimination.

Testes para a avaliação comportamental das habilidades auditivas centrais em indivíduos com perdas auditivas neurossensorial: uma revisão integrativa

ABSTRACT Purpose: to identify the most used tests and the changed hearing skills in the central auditory processing of individuals with sensorineural hearing loss. Methods: an integrative literature review conducted between March and August 2021 in PubMed/MEDLINE, LILACS, SciELO, IBECS, and Index Psi Scientific Journals. The review included studies addressing central hearing skills with central auditory processing behavioral tests in subjects with mild to moderate sensorineural hearing loss. Two independent reviewers made the study selection, data extraction, and qualitative synthesis. The selection aimed at verifying whether the studies answered the research question. Literature Review: 39 studies were found for full-text reading; after applying the eligibility criteria, 13 studies were included in the review. Participants diagnosed with sensorineural hearing loss predominantly had hearing changes in verbal tests, in comparison with normal standards for individuals without hearing loss. Thus, the most changed skills are the ones that depend on speech sound discrimination. Conclusion: the Staggered Spondaic Word Test predominated among the most used behavioral tests, while the most researched central auditory skills were binaural integration and binaural separation.

İnkomplet partisyon tip II bulunan unilateral ve bimodal koklear implant kullanıcılarının konuşma, uzaysal algı ve işitme kalitesinin değerlendirilmesi

Evaluation of the speech, spatial and qualities of hearing in unilateral and bimodal cochlear implant users with incomplete partition type II Objective: The aim of this study was to perform a scale-based assessment of the hearing abilities of cochlear implant users with IP type II malformation and normal cochlea, including discrimination, orientation, and positioning of speech and environmental sounds in their environment. Material and Methods: A total of 30 cochlear implant users, 15 participants with IP-II inner ear anomalies and 15 participants with normal cochlea, aged 18-55 years were included in this study. All participants included in the study are unilateral CI and bimodal users. Participants were assessed using the Speech, Spatial, and Hearing Qualities Scale (SSQ). Results: When cochlear implant users with IP-II malformations and cochlear implant users with normal cochlea individually scored auditory abilities, the results were similar. It was observed that bimodal listeners tended to have all subdomains and overall SSQ scores compared to use of unilateral CI. Conclusion: It is very important to benefit from the advantages of binaural hearing in hearing sounds. It is thought that the use of scales as well as routine audiological evaluation batteries in the follow-up processes of cochlear implant users will be beneficial for audiologists. Keywords: Cochlear implant, speech perception, spatial perception, hearing quality

Speech Processing in Autism Spectrum Disorder: An Integrative Review of Auditory Neurophysiology Findings.

Purpose Investigations into the nature of communication disorders in autistic individuals increasingly evaluate neural responses to speech stimuli. This integrative review aimed to consolidate the available data related to speech and language processing across levels of stimulus complexity (from single speech sounds to sentences) and to relate it to the current theories of autism. Method An electronic database search identified peer-reviewed articles using event-related potentials or magnetoencephalography to investigate auditory processing from single speech sounds to sentences in autistic children and adults varying in language and cognitive abilities. Results Atypical neural responses in autistic persons became more prominent with increasing stimulus and task complexity. Compared with their typically developing peers, autistic individuals demonstrated mostly intact sensory responses to single speech sounds, diminished spontaneous attentional orienting to spoken stimuli, specific difficulties with categorical speech sound discrimination, and reduced processing of semantic content. Atypical neural responses were more often observed in younger autistic participants and in those with concomitant language disorders. Conclusions The observed differences in neural responses to speech stimuli suggest that communication difficulties in autistic individuals are more consistent with the reduced social interest than the auditory dysfunction explanation. Current limitations and future directions for research are also discussed.

Auditory Brainstem Responses Predict Behavioral Deficits in Rats with Varying Levels of Noise-Induced Hearing Loss

Intense noise exposure is a leading cause of hearing loss, which results in degraded speech sound discrimination ability, particularly in noisy environments. The development of an animal model of speech discrimination deficits due to noise induced hearing loss (NIHL) would enable testing of potential therapies to improve speech sound processing. Rats can accurately detect and discriminate human speech sounds in the presence of quiet and background noise. Further, it is known that profound hearing loss results in functional deafness in rats. In this study, we generated rats with a range of impairments which model the large range of hearing impairments observed in patients with NIHL. One month after noise exposure, we stratified rats into three distinct deficit groups based on their auditory brainstem response (ABR) thresholds. These groups exhibited markedly different behavioral outcomes across a range of tasks. Rats with moderate hearing loss (30 dB shifts in ABR threshold) were not impaired in speech sound detection or discrimination. Rats with severe hearing loss (55 dB shifts) were impaired at discriminating speech sounds in the presence of background noise. Rats with profound hearing loss (70 dB shifts) were unable to detect and discriminate speech sounds above chance level performance. Across groups, ABR threshold accurately predicted behavioral performance on all tasks. This model of long-term impaired speech discrimination in noise, demonstrated by the severe group, mimics the most common clinical presentation of NIHL and represents a useful tool for developing and improving interventions to target restoration of hearing.

Examining the success of three kinds of cross-language similarity in predicting English listeners’ discrimination of Danish vowels

Several methods have been used in second language speech research to predict nonnative listeners’ discrimination of foreign language speech sounds. The present study examined how well three of these methods predict the discrimination of the four tightly spaced Danish unrounded front vowels by native listeners of Standard Southern British English (SSBE), whose four front vowels are evenly spaced along the close-open dimension. Experiment 1 examined the ecphoric similarity of the Danish and English vowels in a perceptual assimilation task in which L1 SSBE listeners used native labels and goodness-of-fit ratings in identifying the Danish vowels. Experiment 2 examined the perceptual similarity of the English and Danish vowels in a graded discrimination task in which the same listeners rated the degree of dissimilarity of Danish-English vowel pairs. The third prediction of discriminability was derived from an acoustic comparison of the Danish and English vowels in the F1/F2 space. The same participants as in Experiments 1 and 2 then discriminated three Danish front vowel contrasts. None of the three methods was found to fully successfully predict discrimination of the Danish vowels by the SSBE listeners. The presentation will discuss reasons for the lack of predictive power of the three widely used methods.

Repeated Parental Singing During Kangaroo Care Improved Neural Processing of Speech Sound Changes in Preterm Infants at Term Age.

Preterm birth carries a risk for adverse neurodevelopment. Cognitive dysfunctions, such as language disorders may manifest as atypical sound discrimination already in early infancy. As infant-directed singing has been shown to enhance language acquisition in infants, we examined whether parental singing during skin-to-skin care (kangaroo care) improves speech sound discrimination in preterm infants. Forty-five preterm infants born between 26 and 33 gestational weeks (GW) and their parents participated in this cluster-randomized controlled trial (ClinicalTrials ID IRB00003181SK). In both groups, parents conducted kangaroo care during 33–40 GW. In the singing intervention group (n = 24), a certified music therapist guided parents to sing or hum during daily kangaroo care. In the control group (n = 21), parents conducted standard kangaroo care and were not instructed to use their voices. Parents in both groups reported the duration of daily intervention. Auditory event-related potentials were recorded with electroencephalogram at term age using a multi-feature paradigm consisting of phonetic and emotional speech sound changes and a one-deviant oddball paradigm with pure tones. In the multi-feature paradigm, prominent mismatch responses (MMR) were elicited to the emotional sounds and many of the phonetic deviants in the singing intervention group and in the control group to some of the emotional and phonetic deviants. A group difference was found as the MMRs were larger in the singing intervention group, mainly due to larger MMRs being elicited to the emotional sounds, especially in females. The overall duration of the singing intervention (range 15–63 days) was positively associated with the MMR amplitudes for both phonetic and emotional stimuli in both sexes, unlike the daily singing time (range 8–120 min/day). In the oddball paradigm, MMRs for the non-speech sounds were elicited in both groups and no group differences nor connections between the singing time and the response amplitudes were found. These results imply that repeated parental singing during kangaroo care improved auditory discrimination of phonetic and emotional speech sounds in preterm infants at term age. Regular singing routines can be recommended for parents to promote the development of the auditory system and auditory processing of speech sounds in preterm infants.

Behavioral effect of mismatch negativity neurofeedback on foreign language learning.

Listening is critical for foreign language learning. Listening difficulties can occur because of an inability to perceive or recognize sounds while listening to speech, whereas successful listening can boost understanding and improve speaking when learning a foreign language. Previous studies in our laboratory revealed that EEG-neurofeedback (NF) using mismatch negativity event-related brain potential successfully induced unconscious learning in terms of auditory discrimination of speech sounds. Here, we conducted a feasibility study with a small participant group (NF group and control group; six participants each) to examine the practical effects of mismatch negativity NF for improving the perception of speech sounds in a foreign language. Native Japanese speakers completed a task in which they learned to perceive and recognize spoken English words containing the consonants “l” or “r”. Participants received neurofeedback training while not explicitly attending to auditory stimuli. The results revealed that NF training significantly improved the proportion of correct in discrimination and recognition trials, even though the training time for each word pair was reduced to 20% of the training time reported in our previous study. The learning effect was not affected by training with three pairs of words with different vowels. The current results indicate that NF resulted in long-term learning that persisted for at least 2 months.

Structural variation in the temporal lobe predicts learning and retention of non-native speech sounds

ABSTRACT Studies of non-native speech sound learning report considerable individual variability in learning new sounds and retaining them in memory. The current study tested whether individual variation in brain structure (measured using MRI) accounts for differences in learning or retention of non-native speech sounds. Fifty-seven participants were tested on identification and discrimination of difficult non-native speech sounds in the evening before training, after training, and tested again the next morning. Surface area and volume of the left superior temporal gyrus positively predicted discrimination learning, whereas surface area of the left transverse temporal gyrus negatively predicted overnight improvement of identification. Hippocampal volume as well as gyrification of bilateral transverse temporal gyri positively predicted overnight improvement of discrimination. Findings suggest that individual differences in non-native speech sound learning can be traced to differences in brain structure supporting perception, while differences in retention are linked to the structure of hippocampal regions important for memory consolidation.

Infants’ neural speech discrimination predicts individual differences in grammar ability at 6 years of age and their risk of developing speech-language disorders

The ‘sensitive period’ for phonetic learning posits that between 6 and 12 months of age, infants’ discrimination of native and nonnative speech sounds diverge. Individual differences in this dynamic processing of speech have been shown to predict later language acquisition up to 30 months of age, using parental surveys. Yet, it is unclear whether infant speech discrimination could predict longer-term language outcome and risk for developmental speech-language disorders, which affect up to 16 % of the population. The current study reports a prospective prediction of speech-language skills at a much later age—6 years-old—from the same children’s nonnative speech discrimination at 11 months-old, indexed by MEG mismatch responses. Children’s speech-language skills at 6 were comprehensively evaluated by a speech-language pathologist in two ways: individual differences in spoken grammar, and the presence versus absence of speech-language disorders. Results showed that the prefrontal MEG mismatch response at 11 months not only significantly predicted individual differences in spoken grammar skills at 6 years, but also accurately identified the presence versus absence of speech-language disorders, using a machine-learning classification. These results represent new evidence that advance our theoretical understanding of the neurodevelopmental trajectory of language acquisition and early risk factors for developmental speech-language disorders.