Issues contrasting adult acquired versus developmental apraxia of speech.

Acquired and developmental apraxia of speech (AOS and DAS) are defined as disorders of the transition from an abstract phonological code into motor commands. However, the natural course of these disorders differs substantially because of the fundamental difference in the developmental stage at which the apraxia expresses itself. In normal and pathological development alike, development of language and speech is an interactive process, involving speech motor control, perception, and psycholinguistic processes. Infant speech develops from random babbling and sensomotoric learning, followed by more abstract phonological acquisition. Supposing that the core deficit of DAS comprises a reduced sensomotoric learning capacity explains a large part of the symptomatology of DAS in the psycholinguistic domain because of the impact on phonological, perceptual, and higher-level processes. This contrasts with adults with AOS, who already have acquired stable top-down processes. Implications for clinical management are discussed.