Language and number in Down syndrome: the complex developmental trajectory from infancy to adulthood.

This paper examines language and number understanding in infants with Down syndrome and Williams syndrome and compares infant performance to that of adults. The cross-syndrome/cross-domain studies demonstrate that the pattern of performance of infants with Down syndrome and Williams syndrome on two tasks assessing language and number cannot be derived from the pattern of proficiencies and impairments in the adult phenotypic outcome. Single word comprehension was assessed using a visual preference paradigm. All groups (Williams syndrome, Down syndrome, chronological age and mental age-matched controls) looked longer at the stimuli which matched the verbal label but the infants with Down syndrome and Williams syndrome were equally delayed (equivalent to their mental age controls). The similarity between the infants with Down syndrome and those with Williams syndrome did not parallel the difference present in the adult phenotypes, where vocabulary skill in Down syndrome is significantly lower than that in Williams syndrome. Number was assessed using a novelty preference paradigm, in which infants were familiarised with displays of 2 objects and then presented with 2 versus 3 objects. Infants with Williams syndrome discriminated between the familiar and novel numerosities. Infants with Down syndrome did not. Again, the difference between the Down syndrome and Williams syndrome infant groups did not parallel the pattern seen in the adult phenotypes, where individuals with Down syndrome performed better than those with Williams syndrome. It is therefore crucial to characterise the infant state, in order to understand fully the developmental trajectories of atypical groups.