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Childhood apraxia of speech (CAS) is a highly controversial clinical entity with respect to both clinical signs and underlying neuro-motor deficit. This chapter analyzes the origin of these controversies and offers an alternative clinical approach and direction for research based on a review of the literature and studies conducted in our own laboratory. The review shows evidence for a deficit at each of the psycholinguistic and neuro-motor stages from lexical retrieval to motor execution as the primary, proximal origin of CAS. This lack of convergence together with the reported associations rather than dissociations between psycholinguistic and neuro-motor functions during development forms a serious challenge for the modular view on the cognitive architecture of the infant speech production system. An alternative view on clinical classification, the issue of co-morbidity, and research in speech pathology are discussed based on longitudinal analyses of developmental trajectories. By way of example, a computational modelling study is presented in which characteristics of CAS are simulated by manipulating parameters of the DIVA (Directions Into Velocities of Articulators) model.

A central issue in studies on developmental speech disorders, especially with regard to childhood apraxia of speech (CAS) and phonological disorder (PD) is the distinction between phonological and motoric processes. Acoustic studies suggest that children with CAS produce incorrect realizations of correctly selected phonemes, whereas the opposite is postulated for children with PD. Thus conceived, the underlying impairment is located at different levels of speech production in these two groups of children.

This chapter gives an overview of research aiming to determine at which levels of speech production the deficits of children with developmental apraxia of speech might be located. We conclude that two levels might be involved in developmental apraxia of speech, namely syllable planning (including the syllabary) and motor programming. Next, the results of two experiments are discussed, each focussing on one of these two levels. The first experiment, in which the syllable structure was manipulated, demonstrated no systematic durational patterns in children with developmental apraxia of speech, a result that is interpreted as signifying a deficit in syllable planning, specifically in using prosodic parameters in articulation. The second experiment showed a deviant effect of a bite-block in children with developmental apraxia of speech as compared to normally speaking children, which supported the interpretation of a motor programming deficit in developmental apraxia of speech. Our conclusion is that in developmental apraxia of speech both syllable planning and motor programming are involved. Insufficient durational control plays a central role at both of these levels.

Contains fulltext : 19265_deveapofs.pdf (Publisher’s version ) (Open Access) 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 176 p.