Abstract/Summary

Speech tends to be the most challenging developmental difficulty for children with Down syndrome (DS). Studies on the development of phonology in this special population are currently limited and based on the sound mastery model of typically developing (TD) children. The available studies on phonological development in English-speaking children with DS have been confined either to examining one type of speech data, have small speech samples with limited representativeness, have included fewer participants, or rarely compare children with DS to another group. Additionally, most of the available speech intervention programmes have focused either on developing daily living communication skills or articulation at the expense of phonology for highly unintelligible children with DS. Therefore, developing a research study which combines single word (SW) and connected speech (CS) production data helps identify the difficult areas in phonology that may affect speech development in DS within the age range tested in the present study. Enhancing the phonological abilities of children with DS by adopting phonology-based intervention models to treat phonology-based speech problems facilitates learning of new sounds and using these sounds in correct contexts. The present study fills this gap in the current knowledge. The thesis is divided into two parts: a cross-sectional group study and a multiple baseline single subject case study. The group study aims at explaining the development of phonological systems of children with DS by analysing both SW and CS samples to determine the differences between children with DS and TD children in terms of: consonants and vowels acquisition, effect of consonant position on production accuracy and type and frequency of occurrence of phonological processes. The case study aims at evaluating the effectiveness of a phonological pattern-based cycles approach in improving the speech production accuracy of a child with DS and, in return, increasing the child’s speech intelligibility. In the group study, the comparison of the developmental trajectories of children with DS and TD children showed significant differences between the two groups in terms of onset and rate of development when producing single word/connected speech. There were also differences within DS group between SW and CS samples in that there were more sound errors/phonological processes in CS than in SW samples. By and large, it was found that children with DS develop their phonological systems in a similar way to TD children, but at a slower pace and also have a delayed onset of development of the tested phonological aspects. The findings of the case study revealed that a phonology-based intervention model, such as the cycles procedure, was effective in reducing the occurrence of the target phonological processes and increasing sound production accuracy and speech intelligibility.