Motor speech disorders and models of speech production

Introduction Fluent speech production by neurologically intact adult speakers is a highly skilled motor ability in the service of human communication. Thus, ‘in one breath, most people are able to realise a sequence of 20 syllablesor 40 phoneme targets … an extraordinary rapid fire sequence of action’ compared to limb movements (Grimme et al. 2011: 14). Yet, despite the speed of speech movements, the spatial precision of articulatory placement is within one millimetre and the temporal precision in coordinating two articulators is within 10 milliseconds (Kent andMoll 1975; Gay et al. 1981). A range of models and theories are available that structure current knowledge about normal speech motor control, provide a basis for practical application of that knowledge, and guide further research in the field (Schmidt 2003). Models of speech motor control can also help us understand, research, assess and deliver interventions for the various types of motor speech disorder that impair the planning and execution of movements of the lungs, larynx and supralaryngeal vocal tract (velum, tongue, lips, jaw) during speech communication. A number of motor speech disorders will be considered in this chapter. Apraxia of speech is a disorder of motor planning in which thespeaker’s ability to translate phonological representations into an articulatory plan is disrupted (Mc Neil et al. 2009). Dysarthria impairs neuromuscular control over the respiratory, laryngeal and supralaryngeal movements of speech (Duffy 2005; Ackermann et al. 2010; Miller 2012). Stuttering is characterized by repeated, prolonged or delayedspeech movements (Max 2004). Chapters elsewhere in this volume describe the presentation and management of these disorders. This chapter instead discusses a selection of models of speech motor control that have emerged from the study of normal speech production and/or motor speech disorders. These models have contributed to our understanding of motor speech disorders or have the potential to do so.