Two forms of apraxia in Alzheimer's disease

Apraxia was tested in 12 DAT patients with mild to moderate dementia using two assessment procedures: conceptual tasks requiring object use and pantomime knowledge, and tasks based on the imitation of meaningless oral and hand movements. Both object use and pantomime performance were markedly impaired in the DAT group; imitation of hand postures, hand movements and multiple oral movements was also defective while single oral movements could be well imitated. Error analysis revealed several underlying deficits: on conceptual tasks, deficits in object and action knowledge and poor action planning were frequent, whereas impaired recall and coordination of spatial and temporal movement features were found on imitation tasks. A correlation analysis between both types of apraxia showed low and nonsignificant results. These and previous observations suggest that motor control is organized by two functionally separate systems: a conceptual system for purposeful and symbolic motor acts, and a system controlling sensorimotor and spatiotemporal movement features. Both systems may be subject to early damage in Alzheimer's disease.