Speech motor programming in apraxia of speech

Abstract

Unilateral lesions of the left hemisphere can produce a speech motor disorder termed apraxia of speech (AOS). AOS is often characterized as a disturbance of the programming of speech movements, although there is little experimental evidence for this interpretation. The mechanisms of verbal apraxia relate to the preparatory motor processes interfacing between planning and execution stages of speech motor control and are therefore considered relevant in the understanding of the normal processes of spoken language production. We used a simple reaction paradigm to investigate 20 aphasic patients with lesions of the left hemisphere. Ten of them had a concomitant apraxia of speech. Twenty normal subjects served as controls. Subjects were required to produce syllable chains of varying length and varying segmental content. Reaction times (RTs), inter-syllable intervals (ISI) and speech errors (SE) were analyzed. Normal subjects and patients without AOS showed increased RT with increasing utterance length. In contrast, patients with AOS had increased RT and increased ISI in sequences including alternating syllables. No trade-off between speed and articulatory accuracy could be observed. It is concluded that patients with AOS have a specific deficit in assembling motor plans for alternating syllable utterances.