This thesis aimed to examine the development of cognitive, linguistic and adaptive skills in children with Down syndrome, with a particular emphasis on the interactions between health and these outcomes. Chapters 3, 4 and 5 describe a longitudinal study in which the cognitive, adaptive and linguistic skills of four- to five-year-old children with Down syndrome were traced over 15 months. Parental interviews about health enabled examination of the links between health and cognitive outcomes. Chapter 6 reports a vocabulary training study, which aimed to look at the relationship between sleep and vocabulary consolidation; a more specific health-cognition link.
Chapters 3 and 4 showed that the cognitive, linguistic and adaptive profile associated with older children and adults with Down syndrome had fully emerged by the age of four, although there was a large degree of variability in the expression of the profile at the individual level. The relationships between different cognitive domains in the children with Down syndrome were largely similar to those in the typically developing group, suggesting that development is delayed rather than disordered. The only exception was the relationship between grammar and vocabulary which was atypical in the children with Down syndrome. Chapter 4 showed that parent-report measures of language can be reliably used as predictors of later objectively measured linguistic skill. In a more detailed investigation of vocabulary skills, Chapter 6 found that children with Down syndrome were able to consolidate new vocabulary over time, achieving similar levels of performance to language matched typically developing controls.
To address questions about the links between health and cognition, Chapter 5 found that childhood hearing difficulties and congenital heart defects were associated with poorer language outcomes between the ages of four- and six-years-old in children with Down syndrome. However, there were no reliable relationships between cognition and either sleep or hospitalisation measures. Furthermore, Chapter 6 failed to find a relationship between sleep and vocabulary consolidation. Implications, both for practitioners and for theoretical models of developmental disability, are discussed.
