Both the quantity and quality of the maternal language input are important for early language development. However, depression and anxiety can negatively impact mothers’ engagement with their infants and their infants’ expressive language abilities. Australian mother-infant dyads (N = 30) participated in a longitudinal study examining the effect of maternal language input when infants were 24 and 30 months and maternal depression and anxiety symptoms on vocabulary size. Half the mothers had elevated depression and anxiety symptoms during at least one point in the study (at 6, 12, 18, 24, or 30 months). The results showed that only maternal input measures (word tokens, types, and mean length of utterance) predicted vocabulary size. While no evidence was found that brief periods of maternal depression and anxiety negatively impacted early vocabulary development, the findings highlight the critical importance and possible mitigating effects of maintaining good quality mother–infant interactions during early development.

This longitudinal study assessed three acoustic components of maternal infant-directed speech (IDS) – pitch, affect, and vowel hyperarticulation – in relation to infants’ age and their expressive vocabulary size. These three individual components were measured in IDS addressed to infants at 7, 9, 11, 15, and 19 months (N = 18). All three components were exaggerated at all ages in mothers’ IDS compared to their adult-directed speech. Importantly, the only significant predictor of infants’ expressive vocabulary size at 15 and 19 months was vowel hyperarticulation, but only at 9 months and beyond, not at 7 months, and not pitch or affect at any age. These results set apart vowel hyperarticulation in IDS to infants as the critical IDS component for vocabulary development. Thus IDS, specifically the degree of vowel hyperarticulation therein, is a vehicle by which parents can provide the most optimal speech quality for their infants’ linguistic and communicative development.

Despite the body of research on auditory–visual speech perception in infants and schoolchildren, development in the early childhood period remains relatively uncharted. In this study, English-speaking children between three and four years of age were investigated for: (i) the development of visual speech perception – lip-reading and visual influence in auditory–visual integration; (ii) the development of auditory speech perception and native language perceptual attunement; and (iii) the relationship between these and a language skill relevant at this age, receptive vocabulary. Visual speech perception skills improved even over this relatively short time period. However, regression analyses revealed that vocabulary was predicted by auditory-only speech perception, and native language attunement, but not by visual speech perception ability. The results suggest that, in contrast to infants and schoolchildren, in three- to four-year-olds the relationship between speech perception and language ability is based on auditory and not visual or auditory–visual speech perception ability. Adding these results to existing findings allows elaboration of a more complete account of the developmental course of auditory–visual speech perception.

Two experiments focus on Thai tone perception by native speakers of tone languages (Thai, Cantonese, and Mandarin), a pitch–accent (Swedish), and a nontonal (English) language. In Experiment 1, there was better auditory-only and auditory–visual discrimination by tone and pitch–accent language speakers than by nontone language speakers. Conversely and counterintuitively, there was better visual-only discrimination by nontone language speakers than tone and pitch–accent language speakers. Nevertheless, visual augmentation of auditory tone perception in noise was evident for all five language groups. In Experiment 2, involving discrimination in three fundamental frequency equivalent auditory contexts, tone and pitch–accent language participants showed equivalent discrimination for normal Thai speech, filtered speech, and violin sounds. In contrast, nontone language listeners had significantly better discrimination for violin sounds than filtered speech and in turn speech. Together the results show that tone perception is determined by both auditory and visual information, by acoustic and linguistic contexts, and by universal and experiential factors.

Examples from Chinese, Thai, and Finnish illustrate why researchers cannot always be confident about the precise nature of the word unit. Understanding ambiguities regarding where a word begins and ends, and how to model word recognition when many derivations of a word are possible, is essential for universal theories of reading applied to both developing and expert readers.

Using an infant speech identification (ISI) procedure, English language environment infants, two- and six-year-old children, and adults were tested for their identification of sounds on a native (voiced/voiceless bilabial stop) and a non-native (prevoiced/voiced bilabial stop) speech continuum. Categorical perception of the two contrasts diverged as a function of age, increasing for the native contrast and decreasing for the non-native between two and six years. In Experiment 2, subjects of the same four ages were tested for their identification of a continuum of harmonic tones varying in pitch. Comparison of the results of Experiment i with the essentially continuous perception of this pitch continuum supports the view that the perception of the native contrast becomes more categorical with age, while perception of the non-native contrast becomes less categorical, especially at six years. Experiment 3, in which adults were tested on the three continua with a multi-trial open set procedure, demonstrated that results with the ISI procedure in Experiments 1 and 2 are comparable to results with more traditional methods. The results of the three experiments are discussed in terms of the role of specific linguistic experience in the development of categorical speech perception.

Young infants perceive a variety of speech contrasts, even those that are not relevant in their own language environment. Evidence regarding the loss of perceptual ability with phonologically irrelevant contrasts is reviewed and a theoretical distinction is advanced. It is proposed that the perception of “fragile” contrasts is lost in infancy due to lack of exposure to particular sounds, while the perception of “robust” contrasts is lost around the onset of formal language training due to lack of experience with phonologically irrelevant contrasts. This distinction should provide a heuristic framework for future research on the development of speech perception.