The role of infant-directed speech in language development of infants with hearing loss

It is estimated that approximately two out of every 1000 infants worldwide are born with unilateral or bilateral hearing loss (HL). Congenital HL, which refers to HL present at birth, has major negative effects on infants’ speech and language acquisition. Although such negative effects can be mediated by early access to hearing devices and intervention, the majority of children with HL have delayed language development in comparison with their normal-hearing (NH) peers. The aim of this thesis was to provide a deeper empirical understanding of the acoustic features in infant-directed speech (IDS) to infants with HL compared to infants with NH of the same chronological and the same hearing age. The three specific objectives were set for this thesis. The first objective is to investigate the effects of HL and the degree of hearing experience on the acoustic features of IDS. The second objective is to assess adjustments in IDS features across development in IDS to infants with HL, as they acquire more hearing experience. The third objective is to evaluate the role of specific IDS components such as vowel hyperarticulation and exaggerated prosody in lexical processing in infants with NH from six to 18 months of age, at both neural and behavioural levels. This was achieved by conducting four experiments. The first experiment used a cross-sectional design that assessed the acoustic features in IDS to infants with HL with a specific focus on whether and how infants’ chronological age and hearing age may affect these features. Experiment 2 included a longitudinal investigation that focused on the acoustic features of IDS to infants with HL and infants with NH of the same hearing age. We sought to identify how infants’ changing linguistic needs may shape maternal IDS across development. Experiments 3 and 4 focused on lexical processing in six-, 10-, and 18-month-old infants, whereby we aimed to identify the role of specific IDS features in facilitating lexical processing in infants with NH at different stages of language acquisition. The results of this thesis demonstrated that mothers adjust their IDS to infants with HL in a similar manner as in IDS to infants with NH. However, some differences are evident in the production of the corner vowels /i/ and /u/. These differences exist even when controlling for the amount of hearing experience had by infants with HL. Additionally, findings demonstrated a relation between vowel production in IDS and infants’ receptive vocabulary indicating that the exaggeration in vowel production in maternal IDS may play a fostering role in infants’ language acquisition. This linguistic role was confirmed as vowel hyperarticulation was also found to facilitate lexical processing at the neural level in 10-month-old infants. However, with regard to older infants (18 months), our findings demonstrated that natural IDS with heightened pitch and vowel hyperarticulation represents the richest input that facilitates infants’ speech processing. In summary, the findings of this thesis suggest that congenital HL in infants affects maternal production of vowels in IDS resulting in less clear vowel categories. This may result from mothers adjusting their vowel production according to infants’ reduced vowel discrimination abilities, thus, adjusting their IDS to infants’ linguistic competence. Additionally, receptive vocabulary seems not to be affected by this, indicating the role of other cues for building a lexicon in infants with HL that warrant further investigation. Furthermore, the findings suggest that pitch and vowel hyperarticulation in IDS play significant roles in facilitating lexical processing in the first two years of life

Acoustic features of infant-directed speech to infants with hearing loss

Published Online: 03 December 2020This study investigated the effects of hearing loss and hearing experience on the acoustic features of infant-directed speech (IDS) to infants with hearing loss (HL) compared to controls with normal hearing (NH) matched by either chronological or hearing age (experiment 1) and across development in infants with hearing loss as well as the relation between IDS features and infants' developing lexical abilities (experiment 2). Both experiments included detailed acoustic analyses of mothers' productions of the three corner vowels /a, i, u/ and utterance-level pitch in IDS and in adult-directed speech. Experiment 1 demonstrated that IDS to infants with HL was acoustically more variable than IDS to hearing-age matched infants with NH. Experiment 2 yielded no changes in IDS features over development; however, the results did show a positive relationship between formant distances in mothers' speech and infants' concurrent receptive vocabulary size, as well as between vowel hyperarticulation and infants' expressive vocabulary. These findings suggest that despite infants' HL and thus diminished access to speech input, infants with HL are exposed to IDS with generally similar acoustic qualities as are infants with NH. However, some differences persist, indicating that infants with HL might receive less intelligible speech.This research was supported by HEARing Cooperative Research Centre Grant No. 82631, “The Seeds of Language Development,” to D.B. The second author's work is supported by the Basque Government through the BERC 2018-2021 program and by the Spanish Ministry of Science and Innovation through the Ramon y Cajal Research Fellowship PID2019-105528GA-I00. We would like to thank all the parents and infants for participating in the study; “The Shepherd Centre” in Sydney and Wollongong; “Hear and Say” in Brisbane for their help in recruitment of participants with HL; and Benjawan Kasisopa, Maria Christou-Ergos, Hana Zjakic, and Scott O'Loughlin for their assistance with data collection

Language development in infants with hearing loss: Benefits of infant-directed speech

Available online 2 February 2022The majority of infants with permanent congenital hearing loss fall significantly behind their normal hearing peers in the development of receptive and expressive oral communication skills. Independent of any prosthetic intervention (“hardware”) for infants with hearing loss, the social and linguistic environment (“software”) can still be optimal or sub-optimal and so can exert significant positive or negative effects on speech and language acquisition, with far-reaching beneficial or adverse effects, respectively. This review focusses on the nature of the social and linguistic environment of infants with hearing loss, in particular others’ speech to infants. The nature of this “infant-directed speech” and its effects on language development has been studied extensively in hearing infants but far less comprehensively in infants with hearing loss. Here, literature on the nature of infant-directed speech and its impact on the speech perception and language acquisition in infants with hearing loss is reviewed. The review brings together evidence on the little-studied effects of infant-directed speech on speech and language development in infants with hearing loss, and provides suggestions, over and above early screening and external treatment, for a natural intervention at the level of the carer-infant microcosm that may well optimize the early linguistic experiences and mitigate later adverse effects for infants born with hearing loss.We would like to thank the Hearing Cooperative Research Centre, Australia for the grant 82631, “The Seeds of Language Development”, to the 2nd author. The 3rd author’s work was supported by the Basque Government, Basque Country, Spain through the BERC 2018–2021 program and by the Spanish Ministry of Science and Innovation through the Ramon y Cajal Research Fellowship, PID2019–105528GA-I00

Neural processing of hyper-and hypo-articulated vowels in Infant-Directed Speech

When addressing infants, adults use a speech register known as infant-directed speech (IDS). Compared to adult-directed speech (ADS), IDS has a number of distinctive acoustic and linguistic features. Vowel hyperarticulation, the expansion of the acoustic space between the corner vowels /i,u,a/, is one feature specifically proposed to facilitate language acquisition processes. Interestingly, the presence of vowel hyperarticulation in IDS appears to be dependent on the infant’s communicative and linguistic needs. Mothers do not hyperarticulate vowels in IDS to infants with hearing loss (Lam & Kitamura, 2010) or infants at-risk for dyslexia (Kalashnikova et al., 2018), indicating that infants’ ability to hear and process speech can influence speakers’ IDS to them. Given the important role of vowel hyperarticulation in early language acquisition, it is of interest to investigate the effects of IDS with hypo-articulated vowels on infants’ early linguistic processing.This study investigated whether there is a neurophysiological difference in the processing of hyper- and hypo-articulated vowels in IDS by comparing the electroencephalographic (EEG) signatures of typical IDS with hyperarticulated vowels (hyper-IDS), IDS that lacks vowel hyperarticulation (hypo-IDS), and ADS in 9 month-old-infants (N = 12). Event-related potentials (ERPs) were recorded while infants listened to familiar words in hyper-IDS, hypo-IDS, and ADS registers. If hyper-IDS facilitates infants’ early lexical processing, we expected it to elicit a different pattern in brain potentials compared to hypo-IDS and ADS.Regarding electrophysiological measures, mean amplitudes were calculated in the 250-500ms time window measured from word onset, since ERP amplitudes in this window have been proposed to reflect increased semantic processing (Kidd et al., 2018; Zangl & Mills, 2007). A Speech (hyper-IDS, hypo-IDS, ADS) x Antpost (frontal, central, parietal, occipital) x Laterality (left, right) ANOVA yielded a main effect of Speech (F(2, 22) = 7.054, p = .004, p2 = .391) and Speech x Antpost x Laterality interaction (F(6, 66) = 2.771, p = .018, p2 = .201), meaning that the factor Speech interacted with topographical factors. Hypo-IDS was found to elicit a broadly distributed negativity compared to hyper-IDS and ADS respectively, which gave rise to more positive amplitudes in the same time window. Taken together, these results suggest a decrease in lexical processing for hypo-IDS.These findings indicate different brain responses to IDS with and without vowel hyperarticulation in early language processing, supporting the assumption that vowel hyperarticulation in IDS influences infants’ early linguistic processing. Given that early brain potentials occurring at 200-500ms have previously been suggested to reflect increased lexical or semantic processing, these findings indicate that infants are sensitive to the specific acoustic qualities of IDS, which facilitate semantic processing (Junge et al., 2014). Importantly, the current findings demonstrate that IDS with hyperarticulated vowels provides infants with a rich linguistic signal. When hyperarticulation is absent, lexical processing appears to be impeded. This is an especially important insight with regard to infants with hearing loss who do not have access to this feature in IDS, but who may instead rely more on other perceptual advantages offered by this register

The Effect of Older Sibling, Postnatal Maternal Stress, and Household Factors on Language Development in Two to Four-Year-Old Children

Previous literature has shown that family structure affects language development. Here, factors relating to older siblings (their presence in the house, sex, and age gap), mothers (maternal stress), and household size and residential crowding were assessed to systematically examine the different roles of these factors. Data from mother–child dyads in a Singaporean birth cohort, (677–855 dyads; 52% males; 58% to 61% Chinese, 20% to 24% Malay, 17% to 19% Indian) collected when children were 24, 48, and 54 months old, were analyzed. There was a negative effect of having an older sibling, moderated by the siblings’ age gap, but not by the older sibling’s sex, nor household size or residential crowding. Maternal stress affected language outcomes in some analyses but not others. Implications for understanding the possible effects of family structure on language development are discusse