Spatial and kinematic features of apraxic movement depend on the mode of execution

Knowledge about the dependency of apraxic movements on the mode of execution may enhance the understanding of apraxia and of the cerebral representation of actions. We examined a common tool-use action in nine patients with left hemisphere damage and apraxia. Arm movements during the use of a handsaw were measured and tested in three different modes of execution: pantomime, pantomime with a bar shaped like the handle of the saw, and actual sawing. Analysis concentrated on spatial and kinematic features of the wrist trajectories during these repetitive movements. In healthy control subjects, both pantomime conditions differed from actual use mainly by larger amplitudes during miming. Apraxic patients executed large proportions of their pantomiming movements in an incorrect direction away from the appropriate anteroposterior direction, while other movement features were normal. The availability of the handle-like bar did not improve performance. During actual use, movement direction was constraint by mechanical demands. In this condition patients moved with moderately decreased velocity. However, this deficit was not related to the errors in movement direction characteristic of pantomiming. These data suggest that pantomiming and actual execution of an action are dictated by different external requirements and constraints, pantomiming serves to convey information, while actual use has to obey the mechanical demands of the task. Due to these differences, spatial and kinematic movement features in healthy subjects vary between the modes of execution, movements are differently vulnerable to apraxia, and deficits in patients may dissociate.