Abstract
The speech of children with developmental apraxia of speech (DAS) is highly unintelligible due to many nonsystematic sound substitutions and distortions. There is ongoing debate about the underlying deficit of the disorder. The ultimate goal of this thesis was to answer this question within the speech production process. For this, a model of speech production was used and the involvement of successive levels of the speech production process in the underlying deficit of DAS was tested in separate experiments, concerning phonetic planning, including the idea of using a syllable repository, motor programming and motor execution. Acoustic analyses, namely second formant values and segment durations, determined possible deviancies in coarticulation (i.e., the influence of the phonetic context on the production of segments) in phonemically correct utterances of children with DAS as compared to normally speaking children. A summary of the results of the experiments revealed that the answer to the fundamental question is clearly positive: both planning and programming are deviant. As part of the planning deficit, also deficits in 'higher' mechanisms, concerning durational control processes and use of prosody, seemed to be at play in DAS. As part of the programming deficit, we also found evidence for problems in the process of automating speech production. From a neuropsychological point of view, these speech motor planning, programming, and automation deficits appeared to be embedded in more general information processing deficits, such as deficits in sequencing abilities and sequential memory. All these results suggested a more generalized problem in sequencing and timing abilities to be underlying the deviant processes of phonetic planning and motor programming. Oral motor development and automation might explain why this generalized problem results in DAS, in that motor timing deficits may manifest themselves at a young age and subsequently frustrate development of other (e.g. phonological) systems
