Speech motor control is task-specific: Evidence from dysarthria and apraxia of speech

Background : This work was motivated by recent attempts at explaining apraxia of speech as a general motor disorder which involves both speech and nonspeech movements (Ballard, Granier, & Robin, 2000). It also relates to some more recent theoretical accounts of the diagnostic value of nonspeech tasks, which postulate that such tasks may help to isolate single motor components and examine their individual contributions to a complex motor speech impairment (e.g., Folkins et al., 1995; Robin, Solomon, Moon, & Folkins, 1997). These approaches are implicitly based on a model which assumes that motor control of the organs implicated in speaking is independent of the particular motor task that is imposed on them. Aims : Two alternative models are discussed: a task-independent model of motor control, which postulates a universal sensory-motor system controlling all motor functions of the speech apparatus, irrespective of their purpose, and a task-dependent model, which postulates different sensory-motor control systems for vegetative functions, emotional expression, and speech. The motor subsystems distinguished in the task-dependent model are considered separate in the sense that (1) they involve distinct sensory-motor patterns, (2) they are based on distinct and specialised neural circuitries, and (3) their impairments through brain lesions are dissociated from each other and from impairments of the nonspeech voluntary motor functions of the same musculature. Main Contribution : Evidence for the task-dependent model is provided in three major sections: a section on vegetative motor functions, a section on emotional expressive motor functions, and a section on voluntary nonspeech motor activities. In each section, clinical and experimental dissociations between speech and nonspeech tasks are reviewed and the physiological and neuro-anatomical underpinnings of these observations are discussed. In a general discussion, the nature of speech as a learned motor skill and the relation of speech motor control to an auditory reference frame are highlighted. Conclusions : It is claimed that both the dysarthrias and apraxia of speech should be considered disorders of a sensory-motor system specialised for speaking. Impairments of speech and nonspeech movements should be kept separate.