The apraxias in movement disorders

Introduction In 1900 Hugo Liepmann first described apraxia as the inability to perform purposeful skilled movements in the absence of elementary motor deficits, abnormality of sensation, or impaired comprehension or memory [1]. He noted that a patient may substitute the requested action with an incorrect action, fragment and spatially displace the movements, or perseverate. Liepmann and Maas (1907) postulated that the left hemisphere incorporates not only language but also motor engrams that control purposeful, skilled movement [2]. Liepmann later distinguished two stages of praxis [3]. First, the plan or ideation of movement must be evoked, and then the appropriate motor engrams must be decoded for action execution. Since those first descriptions, much work has been done to try to further our understanding of praxis and apraxia. The current models of limb praxis, similar to that suggested by Liepmann, involve both conception and production areas [4]. The dominant parietal lobe stores motor engrams for praxis conception, and the dorsal, or parietofrontal, system serves as the production system [5, 6]. The latter can be further divided into a medial system that plays a role in movement preparation and the selection of necessary parameters of movement (e.g., amplitude and velocity) and a lateral system that contributes to temporal perceptions through time-dependent attention and working memory.