Functional imaging insights into the pathophysiology of apraxia

Apraxias are disorders of motor cognition that cannot be explained by basic sensorimotor deficits or aphasia. The relatively high frequency of apraxia (approximately half of all patients with I eft-hemi spheric stroke suffer from apraxia during the acute phase) as well as its prognostic value for determining the outcome of rehabilitative therapy clearly convey the necessity of more comprehensive research into the pathophysiology of apraxia in order to develop new therapeutic strategies. In recent years, functional imaging (PET and fMRI) has helped to provide important new insights into the pathophysiology of ideomotor apraxia (defective movement plan) and ideational apraxia (defective action concept). In this review, the neural bases for the clinically observed dissociations between the imitation of abstract and symbolic movements (as in ideomotor apraxia) and for the object-trigger system (which is disturbed in ideational apraxia) will be exemplified. Furthermore, we will recapitulate recent studies that provide evidence for the complementary functions of the right and left parietal cortices in the spatial and temporal organization of complex, object-related actions. The particular importance of the left parietal cortex for motor cognition is further supported by studies examining the integration of spatial and temporal movement information during the generation of a movement plan as well as by the generation of such movement plans in the left parietal cortex independent from the hand that executes the movement.