Limb apraxias: higher-order disorders of sensorimotor integration.

Limb apraxia comprises a wide spectrum of higher-order motor disorders that result from acquired brain disease affecting the performance of skilled, learned movements. At present, limb apraxia is primarily classified by the nature of the errors made by the patient and the pathways through which these errors are elicited, based on a two-system model for the organization of action: a conceptual system and a production system. Dysfunction of the former would cause ideational (or conceptual) apraxia, whereas impairment of the latter would induce ideomotor and limb-kinetic apraxia. Currently, it is possible to approach several types of limb apraxia within the framework of our knowledge of the modular organization of the brain. Multiple parallel parietofrontal circuits, devoted to specific sensorimotor transformations, have been described in monkeys: visual and somatosensory transformations for reaching; transformation of information about the location of body parts necessary for the control of movements; somatosensory transformation for posture; visual transformation for grasping; and internal representation of actions. Evidence from anatomical and functional brain imaging studies suggests that the organization of the cortical motor system in humans is based on the same principles. Imitation of postures and movements also seems to be subserved by dedicated neural systems, according to the content of the gesture (meaningful versus meaningless) to be imitated. Damage to these systems would produce different types of ideomotor and limb-kinetic praxic deficits depending on the context in which the movement is performed and the cognitive demands of the action. On the other hand, ideational (or conceptual) apraxia would reflect an inability to select and use objects due to the disruption of normal integration between systems subserving the functional knowledge of actions and those involved in object knowledge.