The morphosyntactic development of case in Down syndrome

- Cypriot Greek Down Syndrome adults have a (almost) fully developed Case system.
- A new experimental design combines various experimental methods for data collection and analysis.
- Case is examined on four levels of linguistic analysis: Phonology, Phonetics, Morphology and Syntax.
- We disambiguate morphosyntactic illusions created by articulatory restrictions.
- Use of default case when information on case marking becomes unavailable after spell out.

The experimental research presented in this paper reports on findings from the study of case (syntactic and morphological) in 16 Cypriot Greek adults diagnosed with Down syndrome (DS) and 17 typically developing children. We perform phonetic and phonological analyses alongside morphosyntactic and syntactic analyses on data collected from eight experimental tasks. Results reveal near-ceiling performance for both groups. Syntactically conditioned differences related to structural complexity, problematic use of tense and subject-verb agreement or bundling/DP effects, are not recorded. We establish that the greater majority of differences observed between the two groups are mainly conditioned by articulatory limitations related to the distinct physiology associated with DS, as well as minor differences in their phonological systems. A small residue of differences that are determined to be morphosyntactic in nature is also recorded; these exhibit a clear systematicity in choosing the default case value (nominative), instead of the targeted one. Given the close to ceiling performance with case, morphosyntactic inaccuracies are determined to be neither due to lack of knowledge of the case assignment mechanisms, nor lack of knowledge of inflectional paradigms, but rather a morphological discrepancy, during the Vocabulary Insertion stage.