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1-8 Motor Actions

In the study of voluntary movements, the convention is to begin with analysis of a simple movement such as flexion of an elbow. However, voluntary movements that we perform daily are parts of "motor actions" that involve the participation of many body parts and even use of a tool to attain a purposeful goal. Moreover, motor actions also involve perceptual and conceptual activities, for example, in piano playing and dancing. It has been suggested that an "action schema" representing and coding motor actions is expressed in the posterior parietal and premotor (Broadmann's area 6, see Figure 2) cortices (Jeannerod, 1994). For the present purposes, an action schema can be considered to be a cerebral cortical model.

In primates, the premotor cortex expands rostrally from the primary motor cortex. It is generally assumed that the premotor cortex, in particular its dorsal part, plays a major role in computing and controlling complex motor actions (Wise et al., 1997). Moreover, the premotor cortex includes "mirror neurons," which discharge similarly during a motor action performed by the self or by another individual, as discovered by Rizzolatti and his colleagues (Rizzolatti and Craighero, 2004). Based on these and other lines of evidence presented in Chapter 16, we assume that the premotor cortex acts as the controller of motor actions. The premotor controllers act on controlled objects, which nest the primary motor cortex and lower motor systems (see Figure 12C). The postulated action schema is assumed to reside in the temporoparietal cortex and provide a cerebral cortical model to the premotor cortex. The same control system structure may apply to tool use if a tool is represented in an action schema together with body parts (Chapter 15).

Action schema may include two related concepts (actually CNS properties) that are prominent in psychology: the "body schema," which possesses a continually updating map of the self's body shape and postures; and the "motor schema," the self's long-term memory structure capable of being retrieved as a whole, and then controlling the elaboration of motor skills composed of complex actions and movements. Both schemata are acquired or at least further refined by learning (Arbib, 2005; Stamenov, 2005). Along with the model-based control concepts discussed above (Section 5), these body/motor schemata can be considered as components of cerebral cortical models representing the forward and inverse models of the controlled object (Figure 8). These cerebral cortical models are presumably acquired during the initial learning of motor actions. As learning advances, the acquired body schema and motor schemata are transferred to cerebellar internal models (Chapter 15).

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