ABSTRACT
This chapter proposes, first, a synthesis of the contemporary premises of speech motor control, and second, an analysis of the neurological disturbances of this function. It is proposed that during language acquisition, the motor system stores contractile and temporal relations that define the articulatory actions of speech. These relations are thought to be the basis of local synergies between muscle groups in various portions of the vocal tract (mediated by the primary motor cortex), and of global synergies that coordinate the actions of the entire vocal tract (mediated by the secondary motor cortex). In the post-acquisitional phase, these synergies are thought to constitute some of the basic building blocks of the articulatory utterance chain.
The relational aspect of these synergies suggests a number of verifiable hypotheses for neurological impairments of language. For instance, it predicts that certain neurological lesions may disrupt the relational aspect of the programming of articulatory events; as a result, speech would become dysfluent and dyscoordinated. These predictions are supported by research on Broca's aphasia, which is characterized by excessive dysfluency and by disruptions of interarticulatory coordination. Further, the hypothesized differentiation of local and global synergies predicts known differences between certain types of aphasia and dysarthria. Specifically, the global phonemic disturbances of conduction aphasia are found to contrast with the typically more local phonetic disturbances of spastic dysarthria.
