Phonological Organization in the Production of Novel Words in Typical and Atypical Language Development
A core deficit in preschoolers with developmental language disorder (DLD) is in learning words. Words incorporate both sound patterns and the meanings to which they are mapped. Children with DLD show particular deficits in word form rather semantic aspects of word learning (Dollaghan, 1987; Leonard et al., 2019). Deficits are characterized by increased sound errors relative to peers with typical language development (TD); however, little is known about the stability of these errors over multiple productions of the same word. The objective of this project was to investigate two aspects of phonological word form learning—the variability of syllable sequences and the systematicity of sound feature organization—in 4-5-year-olds with DLD, agematched TD peers, and 2-year-olds with typical language. It was predicted that both children with DLD and younger toddlers would show high levels of phonological variability (e.g., Ferguson & Farwell, 1975). In contrast, toddlers, but not children with DLD, were expected to show systematicity in their errors (e.g., Vihman, 1996). A major aim was to determine whether sound pattern deficits attested in DLD represent a qualitative deficit or are developmental in nature. Toddlers are a critical control group for positioning the findings from children with DLD because it is hypothesized that changes in vocabulary size influence both the variability and systematicity of sound patterns. Therefore, this project also incorporates manipulations to assess the lexical and phonological interface. This dissertation includes comprehensive summaries of three manuscripts and two new studies on phonological organization related to syllable sequence variability and sound feature systematicity in two cohorts of children. Cohort 1 consists of 21 preschoolers with DLD and 21 peers with TD. Cohort 2 consists of 13 two-year-olds with TD and 16 four-year-olds with TD. All children participated in a word learning task in which they imitated multiple repetitions of novel word forms with and without a meaningful object referent. Cohort 2 also produced word forms with a relatively simple phonological structure. Comprehension probes were included to assess learning. Based on broad phonetic transcriptions of each production, segmental accuracy and sound feature accuracy of each form were computed. In addition, novel methods based on network science were developed to assess the variability and systematicity of phonological elements at two different levels: the sequential organization of syllables, and the bundling of phonetic features within syllables. Findings from this project reveal that both TD toddlers and preschoolers with DLD show relatively high levels of syllable sequence variability when compared with TD preschoolers, suggesting that sequential errors in the nonword productions of children with DLD are developmental in origin. The variability in sound organization is also not explained by differences in sound feature systematicity, which was similar across all groups studied. That is, toddlers, preschoolers with DLD, and preschoolers with typical language showed similar patterns of sound feature bundling within syllables. Toddlers showed higher variability than TD preschoolers in words with both complex and relatively simpler structure, suggesting that variability is a core feature of their novel word productions even with varying phonological structure. Further, the variability and systematicity of toddlers’ productions were unaffected by the inclusion of a meaningful referent; however, children with DLD showed increases in the stability of syllable sequences and of sound feature organization when a meaningful referent was added. Thus, children with DLD, but not toddlers, showed interactivity across semantic and phonological levels. These findings suggest a similar developmental trajectory for children with DLD and younger toddlers in both the variability and systematicity of phonological form. In contrast, children with DLD are affected by the inclusion of a referent while toddlers are not, suggesting that this aspect of lexical and phonological interaction is not developmental.