Speech Motor Planning in Apraxia of Speech and Aphasia

Apraxia of speech (AOS) is a motor speech disorder that poses significant obstacles to a person's ability to communicate and take part in everyday life. Agreement exists between current theories of AOS that the impairment affects the speech motor planning stage, where linguistic representations are transformed into speech movements, but they disagree on the specific nature of the breakdown at this processing level. A more detailed understanding of this impairment is essential for developing targeted, effective treatment approaches and for identifying the appropriate candidates for these treatments. The study of AOS is complicated by the fact that this disorder rarely occurs in isolation but is commonly accompanied by various degrees of aphasia (a language impairment) and/or dysarthria (a neuromuscular impairment of speech motor control). In addition, the behavioral similarities of AOS and its closest clinical neighbor, aphasia with phonemic paraphasias, undermine the usefulness of traditional methods, such as perceptual error analysis, in the study of both disorders. The purpose of this dissertation was to test three competing hypotheses about the specific nature of the speech motor planning impairment in AOS in a systematic sequence of three reaction time experiments. This research was formulated in the context of a well-established theoretical framework of speech production and it combines psycholinguistic reaction time paradigms with a cognitive neuropsychological approach. The results of the three experiments provide evidence that one component of the speech motor planning impairment in AOS involves difficulty with selecting the intended motor program for articulation. Furthermore, this difficulty appears to be intensified by simultaneously activated alternative speech motor programs that compete with the target program for selection. These findings may prove useful as a theoretically-motivated basis for improving diagnostic tools and treatment protocols for people with AOS and aphasia, thus enhancing clinical decision-making. Such translational and clinical research aimed at developing sensitive and specific diagnostic tools and improving treatment approaches is the ultimate long-term objective of this research program