Intonation is one of the most important prosodic processes in spoken language, involving a wide range of variables and semantic-expressive potential. It is the direct manifestation of pitch changes (level, shape and direction) throughout the sentence; its main acoustic parameter is the fundamental frequency (f0), including all its components such as pitch range, scaling, or alignment between segments and tonal events. In speech, the intonation fulfils a number of functions, some of which are clearly linguistic in nature, such as the realisation of the modality or information structure of utterances, others are pragmatic or even emotional in nature. This includes the transmission of affective and attitudinal meanings, but also the possibility for intonation to convey all those indirect contents that allow the utterance to be better adapted to the communicative context, also expressing (im)politeness, irony, falsehood, etc.
Despite this apparent complexity, brought about by a combination of functions and meanings, intonation has an early development. Prosodic manifestations emerge in infants before the onset of the speech, triggering a kind of order of precedence over segmental production (Snow & Balog, 2002). This is evidenced by the ability of newborns to modulate their crying from the first few months of life, and subsequently by the first syllabic formations they produce. For this reason, according to the implicational principles of language acquisition, prosodic phenomenology, as a precocious component, is also one of the linguistic domains most resistant to control in adult second language acquisition.
Studies have demonstrated that the development of intonation is typically precocious, manifesting itself at a very early age during childhood. Nevertheless, apart from linguistic or individual differences, there is a gradual acquisition of the phonological and phonetic aspects of intonation by children. The acquisition of certain prosodic features, such as pitch contour contrasts or the emotional significance of intonation, occurs earlier in the learning process. Conversely, the development of other features, including narrow or contrastive focus, and epistemic meanings shared between the speaker and listener, takes place at a later stage. The extant literature on this subject is substantial. For further insight into this subject, readers are directed to consult the works of Kehoe (2013) and Prieto and Esteve-Gibert (2018).
Research on the acquisition of prosodic processes in the first language is now numerous and varied, partly due to the inherently interdisciplinary nature of the subject. Indeed, the topic has been approached from different angles and for diverse purposes, thus attracting interest not only from the field of linguistics, but also from other sciences, including evolutionary psychology, neuroscience and language rehabilitation techniques. In this multidisciplinary field, and with particular reference to linguistics, there has been an imbalance in the level of attention paid to prosodic production and perception. This issue will be discussed in the following sections.
Over the years, the amount of research on prosodic production has been far greater than that on comprehension. There is now a well-established tradition of research on the production of intonation acquisition in children, including both contour-based approaches that are designed to discern intonation contours and, more recently, analyses addressed within the phonological model of AM intonation.
A substantial corpus of research has been conducted in this domain, yielding the identification of behaviours that were heretofore merely hypothesised. Production studies have demonstrated that prosodic information assumes a pivotal role even in early childhood, aiding in the identification of word boundaries and the facilitation of the retrieval of semantic and pragmatic meanings. A substantial body of research has analysed the prosodic forms of infants earliest productions, crying, babbling, and then composing of words and phrases, also from a longitudinal perspective. Research findings show that prosody adheres to a systematic developmental trajectory that corresponds to the age of the child. However, the outcomes of the research reveal certain differences, which are probably due to methodological protocols that prioritise controlled tasks, or to specific language-related factors.
According to Wells et al. (2004), children as young as 5 years of age can use intonation effectively for a variety of functions, including phrasing and the focus realisation. Nevertheless, even if these skills are achieved in the preschool years, the prosodic system continues to develop and becomes more complex in the subsequent years, especially in terms of comprehension and expression. For instance, from a phonological perspective, the use of nuclear pitch accents precedes that of pre-nuclear ones (Chen & Fikkert, 2007), whereas the adult-like distribution of pitch accents is acquired by the end of the second year.
Other differences have been identified when a comparison is drawn between the intonation production of children and that of adults. In this direction, the alignment and the scaling of nuclear falling pitch accents are not uniform in children of different languages when compared to adults. In detail, Chen and Fikkert (2007) analysed spontaneous production from Dutch children aged 16/25 months, showing that the intonation inventory (pitch accents and boundary tones) is acquired within the initial 24 months of life, when the stage of lexical and phrasal development is also reached (two-word stage). This finding was confirmed by Frota et al. (2016) in their study on European Portuguese children. This threshold seems to be even lower in Frota and Vigario’s (2008) study of Spanish and Catalan children, which showed that the intonation inventory is well managed in production before the onset of spoken language.
Staying with this theme, De Ruiter (2014) found that German children aged between 5 and 7 years old produce a distinct intonation structure when engaging in storytelling tasks, than adults, despite their capacity to signal the informational status of lexical items (new or given) through intonation, mirroring that of adults. For Italian, Zanchi et al. (2016) used an AM phonological approach to analyse the production of 20 children (mean age 44 months) and eight adults during a storytelling task. The results show that, compared to adults, there are no differences in the production of the nuclear pitch accent, typically H+L*, nor in the intonation phrasing; anyway, the percentage of rising continuation contours, which is pragmatically relevant for the narrative purposes, is significantly lower in children, due to the predominance of falling patterns.
The acquisition of prosody in L1 is a complex phenomenon involving the simultaneous activation of multiple components. Prosody is not only associated with other components of language, but is also intricately linked to the paralinguistic functions of language. Intonation plays a fundamental role in the manifestation of states of mind, attitudes and emotions. For example, children can identify primary emotions such as happiness, anger and sadness simply by listening to prosody and interpreting facial expressions. From the first few months of life, babies are able to use vocal cues, intonation, gestures, facial cues and eye contact intentionally to communicate emotions. According to Armstrong & Hübscher (2018), infants can already produce and understand intonation to convey affect and emotion at a prelinguistic stage.
Studies on the early perception of intonation forms are scarce when compared to that on production. Within this research domain, scholars have chiefly concentrated on the discrimination of pitch accents and the contrast between declarative and yes/no question intonation patterns. They have also considered the opposition between native and non-native languages. An interesting element that has been at the centre of debate among researchers is the temporal imbalance observed in children under one year of age between the ability to produce prosodic features and the ability to discriminate them perceptually. The process of acquiring prosody in infants is based on auditory stimuli, which is why they are sensitive to the sounds of their mother tongue from a very early age. Several studies have shown that prelinguistic infants are able to recognise their mother tongue based on prosody alone (among all, Mehler et al., 1988). In the first few months of life, infants demonstrate the capacity to discern the sounds of their mother tongue, including the stress patterns and rhythmic metrics that differentiate it from other languages.
The results show that perceptual sensitivity to intonation contrasts precedes the production of contours, resulting in a form of temporal incongruity. Studies on typologically distant languages posit that infants are capable of discerning the rhythm, stress patterns and intonation contours of their native language from the initial months of life. Furthermore, they demonstrate the capacity to differentiate lexical-prosodic contrasts in tonal languages. Diverse studies have provided evidence in this direction. For instance, 12-month-old children have been shown to be sensitive to discriminating between declaratives and interrogatives in a listening task (Geffen & Mintz, 2015). The distinction between falling and rising intonation contours occurs in European Portuguese children between the ages of 4 and 9 months, whereas the realisation of pitch-accent contrasts would not be complete until the age of one year (Frota et al., 2014). In this respect, infants appear to be more sensitive to rising, high and dynamic intonation patterns. In addition, the discrimination of new, narrow or contrastive focus does not really occur until the child is 12 months old (Butler et al., 2016). This suggests the existence of a kind of intonational development in perception that starts very early in children and involves the more salient features first.
There is broad agreement that intonation is a component that emerges during the early developmental stages of a child. However, further research is required to enhance our understanding of the manner in which intonation, and more generally prosody, interacts with the other linguistic components. The order of acquisition of prosodic aspects, and in particular intonational aspects, in both production and perception, has not yet been defined in all its manifestations. This is due to the constraining effect that intonational features have on other linguistic levels, from syntax to semantics or pragmatics. The results obtained are not always comparable, probably due to the methodologies employed or the impact of some language-specific aspects such as rhythm and stress patterns. Furthermore, in contrast to the observations made at the level of segmental development, the potential implications of the delay in mastering intonation features during the early years of life as a precursor of subsequent language delays or difficulties remain to be elucidated.
For this reason, defining how prosody develops in children is indeed a fundamental step in understanding what prosodic behaviours are in special populations. In this regard, the initial focus should be on the study of the acquisition of intonation in clinical populations afflicted with language delay or specific speech pathologies (both primary and secondary aetiologies). In many cases, the alteration of some intonational domains or the delay of certain prosodic patterns can be considered as precursors of speech disorders, even in the prelinguistic stage. In this vein, a number of empirical studies have investigated the prosodic changes in different atypical groups of children and adults, such as people with autism, hearing loss, genetic syndromes or motor speech impairments. Focusing on the single domain of intonation, the presence of unusual contours, changes in phrasing, pitch range, speech rate and duration are the most common features associated with a condition of atypical prosody (Stokajovick & Setter, 2011; Peppé, 2018).
The field of research concerned with the acquisition of prosody by children learning a non-native language and in bilingual contexts is of equal interest. Experimental studies demonstrate that L1 intonation exerts a significant influence, shaping the development of prosodic manifestations in a second language even in adulthood. However, if the process of L2 acquisition begins during childhood, when perceptual sensitivity is still high, the mastery of linguistic features, including prosody, which is based on auditory aspects, will be enhanced (Mennen, 2015).
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