Tool use kinematics across different modes of execution. Implications for action representation and apraxia.

Studying the characteristics of movements performed under different action conditions may foster the understanding of disturbed tool use in apraxia and may enhance the knowledge about the links between states of action representations. We registered hand and arm movements during a hammering action executed under three task conditions: pantomime, demonstration with the hammer only, and actual execution with hammer and nail. Various movement parameters were calculated to characterize the kinematic aspects of the hammering movements. An apraxia score that reflects conceptual errors was derived from video-evaluation of pantomiming. Twenty-three patients with left brain damage (LBD), 10 patients with right brain damage (RBD), and 19 control subjects were tested. Patients performed with the non-paretic ipsilesional hand. Four apraxic LBD patients failed to perform the task due to severe conceptual errors. The remaining LBD patients frequently produced movements that were slower, shorter, and less vertical than those of control subjects in all task conditions. Lesion analyses for the LBD patients suggested that inferior frontal areas were particularly responsible for impaired performance. RBD patients performed normally in most kinematic task aspects. Although the conditions differed characteristically in geometry and kinematics, correlations of performance measures indicated that individual patterns in patients as well as in control subjects were stable across the conditions. Performance stability across conditions and the overlapping neural network both support the concept of a general action prototype that is adapted flexibly to environmental constraints. Findings in patients show that LBD can affect the execution of an actual hammering action also in the absence of conceptual errors. It remains to be shown however whether conceptual errors and abnormalities of movement kinematics have a common cause or are two independent manifestations of damage to the motor-dominant brain.