The influence of neurodevelopmental disorders on language acquisition: the generalisation of morphological rules in atypical populations

ABSTRACT Language is so natural to us that we rarely reflect on its complexity. However, not all children acquire language easily, and, when language is impaired, children may experience severe limitations in their communicative abilities. This study aims at analysing the lexical and grammatical behaviour of 10 children with neurodevelopmental disorders (NDs; mean: 8.1 years; range: 3.7-11.7 ) and compares it with that of 14 typically developing (TD) children (mean: 32 months; range: 25-36) matched for lexical age. The issue is addressed both in a theoretical and a practical perspective: on one hand, we try to understand whether ND children use language in a generative way; on the other, the intention is to contribute to the clinical identification of markers of language impairment. Lexical competence was assessed using the Italian version of the MacArthur-Bates Communicative Development Inventories (Primo Vocabolario del Bambino – PVB, Caselli et al., 2015). Morphosyntactic competence was assessed through the analysis of spontaneous speech productions and through various elicitation tasks. ND children’s productions display a general shortage of functors compared to their typically developing peers. The quantitative analysis of spontaneous speech productions shows slightly higher MLU values for the ND group (3.1 words - SD = 1) than the TD group (2.3 words - SD = 0.83), due to ND children’s older age, whereas type-token ratio values are almost identical between the samples (ND = 0.36 - SD = 0.123; TD = 0.38 - SD = 0.122). ND children tend to produce slightly more open-class words, although proportions are rather similar between the two groups. The qualitative analysis applied to ND children’s productions highlights a low degree of sentence complexity, below the one expected from normative data on language-matched peers. Finally, elicitation tasks present an overall negative performance, especially in free morphology tasks. In summary, children with neurodevelopmental disorders may experience difficulties in using functional morphemes and morphosyntactic rules. From a clinical perspective, our results converge with the markers of language impairment observed for Italian in previous studies, namely difficulties in clitic production and in 3rd person plural inflection.