Toward a theory of apractic syndromes

Theoretical development on human motor behavior has occurred largely independently of data on pathological movement disorders. This paper represents an initial attempt to interface findings from studies of apraxia and those of normal motor behavior with a view to formulating a common theoretical framework. Such an integration may ultimately aid in understanding the nature of skill acquisition and provide insights into the organization of motor systems. Three putative theoretical models of movement control are discussed with reference to apractic syndromes. The most commonly accepted view—the hierarchy—possesses properties such as linear transitivity and unidirectionality of information flow that render it inadequate in explaining functional plasticity in the central nervous system. The heterarchy, which incorporates reciprocity of function and circular transitivity, is a more likely candidate but suffers from an inability to regulate the degrees of freedom of the system. Our favored candidate is the coalition model which embodies heterarchical principles, but in addition, offers a solution to the problems of degrees of freedom and context for motor systems. Evidence is reviewed from apraxia of speech and limbs in terms of a coalitional style of control and an experimental approach, consonant with coalitional organization, is developed. We promote the claim that an understanding of apractic behavior—and perhaps motor systems in general—will benefit when clinicians and experimenters embrace a theory of context and constraints rather than a theory of commands as is currently in vogue.