Understanding the contributions of prosodic phonology to morphological development: Implications for children with Specific Language Impairment

Abstract

A growing body of research with typically developing children has begun to show that the acquisition of grammatical morphemes interacts not only with a developing knowledge of syntax, but also with developing abilities at the interface with prosodic phonology. In particular, a Prosodic Licensing approach to these issues provides a framework for understanding why children’s early use of grammatical morphemes is so variable. This in turn provides a means for making predictions, given the prosodic structure of a particular language and the location of a particular grammatical morpheme, if it is likely to be produced or not. Given that many of the patterns of development found in typically developing children are found in older children with Specific Language Impairment (SLI) as well, the Prosodic Licensing Hypothesis should provide a better understanding of some of the variable use of grammatical morphemes found in children with SLI.

Keywords

Grammatical morphemes language acquisition phonology/morphology interface prosodic phonology Specific Language Impairment

Introduction

Children’s early use of grammatical morphemes is notoriously inconsistent. That is, the same child may use the same morpheme sometimes, but not always. This variable use of grammatical morphemes can persist for some time in typically developing (TD) children. This is documented by the relatively late acquisition of 3rd person singular morphemes, and the even more protracted acquisition of the past tense (e.g. Brown, 1973). Certain allomorphs of the same morpheme exhibit even more delayed acquisition, as found for the syllabic allomorphs (e.g., brushes [brʌʃ+əz]), added [æd+əd]) (e.g., Berko, 1958). Thus, even when the syntax is held constant, children show variable use of the same grammatical morpheme due to the phonologically/prosodically different contexts in which the morpheme appears. This points to the fact that children’s phonological/prosodic development is still incomplete. Thus, just as researchers have posited impoverished syntactic representations as the root cause for the incomplete use of morpho-syntax (e.g., Wexler, 1994), incomplete representations for prosodic morphology also explain much of children’s variable use of grammatical morphemes (Demuth & McCullough, 2009; Gerken, 1996). Of course, a complete understanding of how grammatical morphology develops will need a combined model that incorporates both syntactic and prosodic/phonological factors and probably many others (e.g., frequency effects, discourse factors, working memory/utterance length and complexity). The goal of this review is not to provide a complete model of this process, but to illuminate the prosodic/phonological factors that play a role in morphological acquisition – factors that are typically overlooked in syntactic accounts of morphological development.

Learning the phonology of one’s language is a part of learning its grammar. Thus, grammatical development includes phonological development, not just syntax. Phonological development is not just segments, but also higher-level aspects of prosodic phonology. The Prosodic Licensing Hypothesis (Demuth, 2014; Demuth & McCullough, 2009) outlined below thus differs from the Surface Hypothesis (Leonard, 2014; Leonard, Eyer, Bedore, & Grela, 1997) in that it appeals to phonological structure and the development of phonological representations. In particular, it proposes that language learners will produce ‘unmarked’ phonological structures first, and only later acquire more ‘marked’ phonological structures (cf. Demuth, 1995). This allows us to make predictions about the types of grammatical morphemes that might present a challenge for early learners. Problems that persist that cannot be accounted for due to the prosodic phonology of the language and/or the grammatical morphemes themselves must then be accounted for by appealing to other factors (e.g., frequency). Critically, we explore these issues in contexts where the syntax is held constant. Thus, although syntactic factors undoubtedly play a role in understanding how and when different grammatical morphemes are ‘acquired’, phonological/prosodic factors help to account for the variable acquisition curve typically found along the way. These are not mere ‘performance’ factors, but are part of language learners’ grammatical competence.

The goal of this article is therefore to provide an overview of these phonological/prosodic effects on TD children’s production of grammatical morphemes, and to show how some of these effects appear to account for some of the variable use of grammatical morphemes in the speech of children with Specific Language Impairment (SLI) as well. Since some of the research with TD children was inspired by research coming out of Heather van der Lely’s lab, some of the SLI results will be discussed in tandem. Van der Lely and colleagues were always open to exploring the linguistic limitations of children with SLI that went beyond syntax to include both phonology and morphology as well (Marshall, Marinis, & van der Lely, 2007; Marshall & van der Lely, 2006, 2012; van der Lely & Ullman, 2001). This has opened the field to years of rich discussion and collaboration. The findings of Marshall and van der Lely (2007), in particular, have stimulated much of our own research into the acquisition of inflectional morphology in both TD children and those with SLI. However, there has been much more research on prosodic morphology in TD children. It remains to be seen if and how some of the findings from younger TD children generalize to older children with SLI. We leave this for future researchers to explore cross-linguistically, with both TD and SLI populations.

Prosodic phonology

Much research on the acquisition of phonology has traditionally focused on the acquisition of segments. However, research on children’s acquisition has increasingly begun to examine phonological development at higher levels of prosodic structure (e.g., syllable, prosodic word, phonological phrase) and the implications for understanding why children produce the particular truncated early word shapes and utterances they do. This has been brought about in part by approaches to phonology such as Prosodic Phonology (e.g., Selkirk, 1984, 1996) and Optimality Theory (Prince & Smolensky, 2004), providing a framework for exploring the course of phonological development above the level of the segment. Further developments, such as increasing attention to the relationship between perception and production abilities (e.g., Sundara, Demuth, & Kuhl, 2011) and the acoustic analysis of child speech (e.g., Demuth & McCullough, 2009), have begun to lay the groundwork for thinking of children’s early speech productions in terms of a developmental model of speech planning and production (Demuth, 2014; Theodore, Demuth, & Shattuck-Hufnagel, 2015). This is critical for understanding the course of phonological and morphological development not only in children with TD language, but also for those with SLI.

To investigate the structure of children’s early syllables, words and morphemes it is useful to consider the Prosodic Hierarchy illustrated in (1) (Nespor & Vogel, 1986; Selkirk, 1984, 1996), where moras and syllables compose to form Feet and Prosodic Words (PWs), which in turn are embedded in higher-level Phonological Phrases (PPs) and Intonational Phrases (IPs).

(1) The Prosodic HierarchyUtt (Phonological Utterance)

Syllables are composed of an onset and a rhyme, as in (2). The rhyme consists of an obligatory nucleus, and an optional coda, i.e., one or two moras. Thus, monomoraic syllables contain only a nucleus, whereas bimoraic syllables may contain either a vowel plus coda consonant (dog), a diphthong (play), or a long/tense vowel (see). More consonants at the beginning of the word are typically thought to be part of a branching onset, whereas more consonants at the end of the word are thought to be syllabified at the higher level of the syllable (see Kirk & Demuth, 2005, for further discussion).

(2) Basic Syllable Structure

Only some languages permit complex onsets and codas, forming consonant clusters. The types of consonant clusters permitted vary depending on the language. However, most consonant clusters obey the Sonority Sequencing Principle (SSP), where sonority is greatest in the nucleus, and decreases toward the edges of the syllable (Clements, 1990; Selkirk, 1984). This is captured by the Sonority Hierarchy in (3), where each sound can be categorized based on one of seven manners of articulation (Ladefoged, 1993). More sonorous segments tend to fill the nucleus of the syllable, and less sonorous segments tend to fill onset and coda positions. For example, in the word blend /blɛnd/, /ɛ/ is a vowel, /b/ and /d/ are stops; /l/ and /n/ are a liquid and nasal, which are both more sonorous than a stop, but less sonorous than a vowel.

(3) The Sonority Hierarchy

Thus, learning ‘phonology’ involves developing phonological representations at all levels of the Prosodic Hierarchy. As theories of prosodic phonology have become more detailed, and interactions between prosodic phonology and grammatical morphology have become more transparent (e.g., Selkirk, 1996), a reassessment of the role of prosodic phonology and morphology in the speech of children becomes a very interesting enterprise.

The Prosodic Licensing Hypothesis (Demuth, 2014; Demuth & McCullough, 2009; Lleó, 2003; Lleó & Demuth, 1999) therefore contrasts with the Surface Hypothesis (Leonard et al., 1997) primarily in that the former appeals to prosodic phonology and the development of phonological representations. Although both accounts observe that unstressed grammatical morphemes present a challenge for learners, the two theories differ in understanding why this is the case. The Surface Hypothesis proposes that children with SLI have a perceptual deficit combined with limitations on processing capacity that renders short, unstressed grammatical morphemes particularly challenging. In contrast, the Prosodic Licensing Hypothesis suggests that grammatical morphemes will be more likely to be produced in ‘unmarked’ phonological contexts where they are ‘easier’ to produce. These include rhythmically ‘unmarked’ foot structures (e.g., for English, disyllabic trochaic feet), simple syllable structures and the end phonological phrases, where phrase-final lengthening provides more time for producing more complex syllable structures (and inflectional morphemes). These issues are developed below.

Prosodic effects on the acquisition of grammatical morphemes – feet

Drawing on insights from the Prosodic Hierarchy, researchers in the 1990s began to examine children’s acquisition of grammatical morphemes (cf. Demuth, 1994, 2001; Gerken, 1994, 1996; Lleó, 2001; Lleó & Demuth, 1999). They found that some of the variability in children’s production of grammatical morphemes is predictably constrained by aspects of children’s developing phonological and prosodic representations (cf. Demuth, 2014; Demuth & McCullough, 2009; Lleó, 2003). Although initially found to explain the variable omission of articles in English (Demuth & McCullough, 2009; Gerken, 1996), this approach has also been shown to account for the variable omission of determiners and articles in French (Bassano et al., 2013; Bassano, Maillochon, & Mottet, 2008; Demuth & Tremblay, 2008), Spanish (Demuth, Patrolia, Song, & Masapollo, 2012; Lleó, 2001; Lleó & Demuth, 1999) and Italian (Giusti & Gozzi, 2006). It also explains the variable omission of noun class prefixes in the Bantu language Sesotho (Demuth & Ellis, 2009). Thus, the Prosodic Licensing Hypothesis helps to account for some of the early variable production of function words cross-linguistically, even in languages with very different prosodic structures. In all cases there is earlier production of morphemes that can be prosodified as part of a prosodic unit, often a disyllabic foot (see Demuth, 2015 for review).

Note that the term ‘prosodic’ as used above refers to the prosodic (phonological) units of the Prosodic Hierarchy (Selkirk, 1996). This is different from the notion of ‘perceptual salience’, where increased pitch (or other acoustic factors, such as longer duration) may enhance the perception of a morpheme. Several researchers have noted that intonation or tone/pitch accent such as that found in Swedish may serve to ‘prosodically highlight’ certain grammatical morphemes, promoting earlier acquisition due to greater ‘perceptual salience’ (e.g., Hansson, Nettelbladt, & Leonard, 2003; Peters & Strömqvist, 1996; Sundström, Samuelsson, & Lyxell, 2014). Although perceptual salience is a critical prerequisite for the acquisition of grammatical morphemes, both for TD children (cf. Davies, Xu Rattanasone, & Demuth, in submission; Sundara et al., 2011) and for children with SLI (cf. Criddle & Durkin, 2001), we suggest that it is not sufficient; even well-perceived morphemes are omitted in early speech. Thus, the Prosodic Licensing Hypothesis strives to better understand variability in production of the same grammatical morpheme, where issues of phonological/prosodic representations and prosodic (boundary-related) context play a role. In all the cases mentioned above, it appears that unmarked prosodic structures such as disyllabic feet provide one of the best prosodic contexts for early inclusion of grammatical morphemes.

Prosodic effects on the acquisition of grammatical morphemes – syllable structure

The Prosodic Licensing of grammatical morphemes can also occur at the level of the syllable, as in the case of inflectional morphemes that may only be realized as a segment. Some children exhibit syllable structure (phonotactic) restrictions on the acquisition of English 3rd person –s (e.g., Stemberger & Bernhardt, 1997). Thus, in both spontaneous speech and controlled elicited imitation tasks, 2-year-olds are more likely to produce this grammatical morpheme when it occurs as a simple coda consonant than when it forms part of a consonant cluster (e.g., sees vs. hits) (Song, Sundara, & Demuth, 2009), and similar findings are reported for plurals and possessive morphemes (cf. Mealings & Demuth, 2014a; Theodore, Demuth, & Shattuck-Hufnagel, 2011, 2012). These effects are particularly prominent utterance medially compared to utterance finally, where phrase-final lengthening provides more time to completely produce all segments in the final syllable (e.g., The dogs barked vs. I saw the dogs) (cf. Hsieh, Leonard, & Swanson, 1999). Interestingly, Polite (2011) reports that pre-schoolers with SLI show similar effects in spontaneous speech, producing more plurals with words ending in a vowel (e.g., pea+s [z]) than in a consonant, where the plural morpheme is part of a consonant cluster (e.g. dog+s [gz]).

Some research has found that the prosodic characteristics of words in different positions within the utterance may influence children’s abilities to both perceive and produce grammatical inflections. For instance, using a sentence imitation task, Dalal and Loeb (2005) found that 5-year-olds with SLI were more likely to produce the past tense morpheme in utterance final as opposed to utterance medial position. The authors suggested that the higher perceptual salience of words or morphemes in utterance final position due to phrase-final lengthening facilitates morpheme processing and production (though see Marshall and van der Lely, 2007). Although we have shown that perception of 3rd person singular –s is better for typically developing 2-year-olds utterance finally compared to utterance medially (cf. Sundara et al., 2011), we suggest that the increased duration utterance finally, which makes the phonotactics simpler (i.e., less ‘marked’), may facilitate morpheme production as well, in both TD children and in older children with SLI.

Consider, for example, the sequence ‘dogs bark’. Although the acoustic cues to plural /z/ are shorter and may be masked by the surrounding consonant utterance medially, making it less perceptually salient compared to utterance finally, the articulatory context is also much more complex, where three consonants [gzb] must be produced in close succession compared to only [gz] utterance finally. Thus, articulatory planning challenges also contribute to lower production of utterance-medial grammatical morphemes (cf. Theodore et al., 2015 for discussion). Given that English is an SVO language, where verbs occur utterance medially about 75% of the time, it is not surprising that English-speaking children take time to fully acquire 3rd person singular –s (cf. Hsieh et al., 1999; Song et al., 2009).

Marshall and van der Lely (2007) also showed that, for 9- to 16-year-old children with Grammatical SLI, production accuracy for past tense morphemes diminished as coda complexity increased from one coda consonant (e.g., sewed [so+d]) to a two coda cluster (e.g., yelled [jɛl+d]) to a three coda cluster (e.g., danced [dæns+t]). These findings suggested that similar phonotactic complexity effects might be found with other grammatical morphemes. We therefore examined 3rd person singular –s productions by TD 2-year-olds, and found that they were likewise much more likely to produce the morpheme when it occurred as a singleton coda compared to when it was part of a more phonologically marked consonant cluster (i.e., cries vs. eats) (Song et al., 2009). Interestingly, this was true in both elicited imitations and in spontaneous speech, suggesting that this is a robust effect.

Researchers working with both TD and SLI populations have also found that the sonority of the coda cluster segments matters, with children being more likely to use the past tense with sonorant stem-final consonants (e.g., called) compared to an obstruent (e.g., kicked) (Oetting & Horohov, 1997), where the Sonority Hierarchy shown in (3) is violated. Recent research has expanded these findings, showing that both phonotactic probability and neighborhood density influence productions of children with SLI (e.g., Coady, 2013; Munson, Kurtz, & Windsor, 2005). Riches (2015) suggests that this behavior may reflect a low-level speech or articulation difficulty that surfaces in the context of challenging clusters. This is consistent with the Prosodic Licensing Hypothesis notion of phonologically simple, ‘unmarked’, forms being earlier acquired.

Other factors influencing the production of grammatical morphemes

The suggestion that phonotactic probability and neighborhood density might play a role in morpheme production is an interesting one. On one hand, these might be considered to be effects of markedness. That is, the higher frequency of a form in the grammar a child hears might actually be considered to be less marked than forms that occur less often, even if they go against sonority considerations (cf. Stites, Demuth, and Kirk, 2004 for discussion). Interestingly, however, Berko (1958) found that TD children had an especially protracted acquisition of syllabic allomorphs (e.g., wash+es). Thus, adding a morphemic syllable to the word appears to be more difficult than producing it with a consonant cluster (cf. Marchman, Wulfeck, & Ellis Weismer, 1999; Mealings, Cox, & Demuth, 2013). This might be surprising, given that such forms are part of an unmarked trochaic foot, and therefore might be predicted to be acquired early. Note also that the morpheme is an entire syllable, rather than merely a segment: it might therefore also be considered to be more perceptually salient. This raises questions about how such forms are acquired by children with SLI.

To explore this issue, Tomas, Demuth, Smith-Lock, and Petocz (2015) examined the use of morphological inflections, including 3rd person singular –s, past tense –ed and possessive –s, in 5-year-olds diagnosed with SLI. Although the use of inflections was high for both simple codas and consonant clusters (e.g., cries, eats), these children showed extremely low use of syllabic allomorphs across all morpheme types (e.g., dresses, padded, horse’s). Note that this is not a simple articulatory problem: All these children had passed an articulatory screening test, repeating nonce words with the target sound combinations in non-morphemic contexts.

Tomas et al.’s (2015) results are also interesting for another reason. Recall that researchers have proposed that there are significant problems with the syntactic processes involved in marking tense for children with SLI (e.g., Rice & Wexler, 1996; Rice, Wexler, & Cleave, 1995). However, Tomas et al. showed that these 5-year-olds with SLI had problems with nominal morphology as well. Although segmental morphemes were produced around 80% of the time for the verbal present and past tense, and 70% for the possessive, the use of the syllabic allomorphs was low for both verbal and nominal morphemes (20% for verbs [present and past], and 10% for nouns [possessives]). This suggests that children with SLI may have problems not only with verbal tense morphology, but with nominal grammatical morphology as well. The question is why?

We have previously shown that TD children’s early productions generally reflect the frequency with which different prosodic syllable and word structures appear in the input they hear (e.g., Levelt, Schiller, & Levelt, 2000; Roark & Demuth, 2000). Tomas et al. (2015) therefore calculated the frequency of syllabic vs. segmental allomorphs in child-directed speech to 2-year-olds, using the Providence Corpus (Demuth, Culbertson, & Alter, 2006; cf. CHILDES database, MacWhinney, 2000). Out of 1407 total items, syllabic allomorphs constituted 5% of all present tense inflections (e.g., kisses), 17% of all past tense inflections (e.g., knitted) and 10% of all possessive inflections (e.g., Josh’s). This suggests that the low overall frequency of these allomorphs may help account for their later acquisition, both for TD children and those with SLI.

Researchers have long known that lexical frequency plays an important role in psycholinguistic processing (e.g., MacDonald, Pearlmutter, & Seidenberg, 1994), and infant speech perception studies show that infants are sensitive to the frequency of the segments and prosodic structures they hear (e.g., Anderson, Morgan, & White, 2003). It has also long been known that 3- to 5-year-old children’s representation of familiar, high-frequency words is more robust in both perception and production than that of novel and low-frequency words (Edwards, Beckman, & Munson, 2004). It may be the case that morphological frequency plays a role in morphological acquisition as well, despite Brown’s (1973) claims to the contrary. However, such a proposal would need to be combined with that of the prosodic and phonological factors discussed above. Demuth (2001) suggests that language learners may be keeping track of the statistics of structures at all levels of the Prosodic Hierarchy, as well as the segmental interactions therein. This could also include lexical issues of neighborhood density (Edwards et al., 2004; Storkel, 2004). Thus, some of the variability found in the acquisition of morphemes may be explained by the frequency with which these occur in the lexicon, at least within a given prosodic context (see de Bree and Kerkhoff, 2010 for further discussion along this line). Controlling for prosodic context may provide a clearer understanding of children’s phonological competence and the factors that contribute to variability in production. Thus, children with SLI may have delayed acquisition at the higher levels of prosodic phonology, which has a significant impact on their acquisition of grammatical morphology.

It is also possible that children find it more difficult to produce similar segments separated by only the short reduced vowel schwa (e.g., added [ædəd], brushes [brʌʃəz]), or simply have not yet learned about the Obligatory Contour Principle (OCP), prohibiting similar elements (e.g., tones, segments) from occurring adjacent to one another. Although McCarthy (1986) suggests that this constraint is part of Universal Grammar, Odden (1988) suggests it must be learned; this suggests that it may also be more phonologically ‘marked’, and thus later acquired.

Processing factors may also contribute to the variable use of grammatical morphemes. Valian (1991) found that TD children between the ages of 1;10 and 2;8 were more likely to omit grammatical morphemes such as pronominal subjects in longer utterances, and Song et al. (2009) found that 3rd person singular morphemes were omitted significantly more often in longer utterances by 2- to 3-year-olds in everyday speech. Mealings and Demuth (2014b) found similar effects in a more controlled experimental context, especially when the morphemes occurred utterance medially.

Conclusion

The field of phonological acquisition has been significantly influenced by recent developments in prosodic phonology and constraint-based approaches such as Optimality Theory. But there is still the problem of understanding the mechanisms underlying phonological development, and its impact on the acquisition of grammatical morphemes. Van der Lely and colleagues proposed the Computational Grammatical Complexity (CGC) model (e.g., Marshall & van der Lely, 2007), and we have showed here how more marked phonological and prosodic syllables, words and contexts all influence the likelihood that an inflectional morpheme will be produced. These findings suggest that children’s acquisition of grammatical morphemes is closely tied to the development of prosodic phonology. Given that many grammatical morphemes are unstressed prosodic clitics (copulas, auxiliary verbs, as well as syllabic inflections), their acquisition is dependent on the development of higher-level prosodic structures. This suggests that phonological factors may also help explain the prolonged omission of prosodically weak (unstressed) tense-bearing auxiliary verbs such as ‘do’ (e.g., Where (does) the girl swim?) (though see Rice & Blossom, 2012). The Prosodic Licensing Hypothesis therefore provides a framework for exploring the role of higher-level prosodic representations in understanding grammatical morpheme omission, and how this changes over time. It also provides a principled means for making predictions about the course of grammatical morpheme acquisition within and across languages. Ultimately, what is needed is a full model of how such prosodic phenomena interact with both frequency and processing effects during language development.

Footnotes

Acknowledgements

We thank the editors and two reviewers for helpful comments.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was funded in part by the following grants: NIH R01HD057606, ARC FL130100014, ARC CE110001021.

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