The nature and time course of motor programming in apraxia of speech

Speech production involves many different muscles at very high speeds, and yet we can speak fluently with very few errors. Speech motor programming refers to the processes that transform an abstract linguistic code into coordinated patterns of muscle activity. Speech motor programming can be impaired secondary to neurological damage, resulting in the disorder called Apraxia of Speech (AOS). This disorder remains poorly understood. In this dissertation, the nature of speech motor programming and AOS is investigated in a model-driven approach to address the following questions: 1) What are the units and processes of speech motor control?; 2) Is speech controlled by a separate motor control system?; 3) Does AOS involve a process-specific deficit?; and 4) Does AOS involve a speech-specific deficit? The research in this dissertation is framed in the context of a recent model of motor programming that delineates two processes occurring at different points in time, namely preprogramming and sequencing. In Experiment 1, a reaction time methodology that has been shown to capture these two processes was used to examine the programming of repeated syllables in unimpaired speakers and in individuals with AOS or aphasia. The results provided support for the hypothesis that AOS involves a deficit in preprogramming but not in sequencing or movement initiation; both processes appeared to be intact in individuals with aphasia. In addition, the results from unimpaired speakers suggested that syllable sequences are programmed as single units. Experiment 2 extended these findings to the programming of finger movement sequences, where it was found that individuals with AOS showed similar preprogramming deficits. Experiments 3 and 4 were designed to examine the influence of rhythm on speech motor programming in unimpaired speakers. The results suggested that rhythmic sequences are preprogrammed faster than non-rhythmic sequences, although methodological concerns preclude strong conclusions. In conclusion, this dissertation contributes to our understanding of speech motor programming in terms of its time course, its units, and its component processes. In addition, the nature of the motor programming deficit in AOS was further delineated to represent a non-speech specific deficit in preprogramming, while sequencing and initiation appear to be intac