The use of a tool requires its incorporation into the movement: evidence from stick-pointing in apraxia

When using a tool, the agent has to control the movement of this implement to perform an efficient action. While perceptual assimilation of a tool has been shown to occur during its use, the incorporation of an implement into the control of movements has received little attention. Here, we investigated the relation between the ability to integrate an object into motor coordination, and tool use. Apraxic patients impaired for familiar tool use were asked to point at targets located at various distances using either their finger, or two sticks of different lengths (23 and 53 cm). Left-brain damaged patients without apraxia participated as controls. Movement kinematics were analysed, with a particular focus on the working point (WP) of the action, defined as the tip of the finger or stick. Control patients produced movements with similar kinematic characteristics in all conditions, and were slightly less accurate with the sticks than with their finger. In contrast, more than half of the apraxics (6/10) showed various degrees of impairment when pointing with the sticks, manifested by deviant and inefficient motor strategies and abnormally large errors. In light of previous work in motor control and kinematics, it is argued that this reflects a deficit in anticipatively controlling a WP located in the stick, that is, in integrating the stick into the movement. We propose that this impairment might be related to dysfunction of the processes involved in dynamic body representation, which needs to be extended to include the tool to be used. Future directions are discussed to further characterize the relation between object incorporation and tool use, and its relevance for understanding apraxia and the neural bases of tool use.