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, l earned movements, At present, limb aprazia is primarily classified by the n ature of the errors made by the patient and the pathways through which thes e errors are elicited, based on a two-system model for the organization of action: a conceptual system and a production system, Dysfunction of the for mer would cause ideational (or conceptual) apraxia, whereas impairment of t he 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 fo r reaching; transformation of information about the location of body parts necessary for the control of movements; somatosensory transformation for po sture; visual transformation for grasping; and internal representation of a ctions. Evidence from anatomical and functional brain imaging studies sugge sts that the organization of the cortical motor system in humans is based o n the same principles. Imitation of postures and movements also seems to be subserved by dedicated neural systems, according to the content of the ges ture (meaningful versus meaningless) to be imitated. Damage to these system s would produce different types of ideomotor and limb-kinetic praxic defici ts depending on the contest in which the movement is performed and the cogn itive demands of the action. On the other hand, ideational (or conceptual) apraxia would reflect an inability to select and use objects due to the dis ruption of normal integration between systems subserving the functional kno wledge of actions and those involved in object knowledge.