An integrative model of speech motor control: A response to Ziegler

Background: Ziegler (2003) reviews neural and behavioural evidence to support a task-dependent model of motor control, whereby motor speech disorders represent impairment of a sensory-motor system specialised for speech. Based on this approach, Ziegler argues against the examination of volitional or novel nonspeech motor activities to gain insight into motor control for speech. Aims: The primary objective of this paper is to discuss Ziegler's (2003) conceptualisation of the task-dependent model and present an alternative integrative model. In the latter, speech and volitional nonspeech motor control are integrated into the functioning of a more general motor system where neural and behavioural systems demonstrate areas of overlap. Studies of the nervous system, the evolutionary foundations of speech in great apes, behaviour, and motor learning are presented to support an integrative model of motor control. Main contribution: Neurological and evolutionary evidence strongly suggest that neural networks are flexible, multifaceted, multifunctional, and overlapping in function. It is highly likely that a higher-level behaviour like speech involves networks that are similarly multifunctional and overlapping with other motor functions. Ziegler's concept of task-dependent motor control may relate as much to parameters such as complexity, familiarity, and automaticity of task performance as to the nonspeech-speech distinction. Studies are reviewed that support the inclusion of nonspeech motor tasks in assessment of the disordered speech motor control system. Specific nonspeech tasks clearly facilitate differential diagnosis and provide insight into the functioning and breakdown of the motor system for related but more complex speech behaviours. Finally, motor learning studies are discussed with particular reference to how these might inform models of motor control. Conclusions: This response cautions against the seemingly premature acceptance of a model assuming separate sensory motor systems for volitional nonspeech motor activities and speech. Continued research, without the limitations imposed by such an assumption, is required to enhance understanding of component parts of complex behaviours and how those components interact in normal and damaged systems at different levels of performance. These data have the potential to form the basis for new, more effective approaches to treatment of motor speech disorders.