Ideomotor apraxia, visuomotor control and the explicit representation of posture

Ideomotor apraxia is normally viewed as a disorder of the representation or execution of action. However, the evidence from gesture imitation is that apraxic patients are unable to reproduce the final posture of a gesture but may not show abnormality in movement kinematics. This supports a hypothesi s that impaired gesture imitation is due to a deficit in a conceptual repre sentation of body posture rather than indicating a disorder specific to act ion control. The present study evaluated whether other aspects of apraxic b ehaviour might also be most consistent with a deficit in the representation of posture. Patients with left hemisphere damage and apraxia on gesture im itation had difficulty in reproducing a series of actions due to failure to adopt the required configuration of the hand whereas there was no consiste nt reduction in speed of response once the actions had been learned. They a lso tended to use an abnormal, clumsy grasp when using a spoon but this dis sociated from accurate performance on other tests of manual dexterity where there was less scope for variation in configuration of the hand. These dat a suggest that the most common deficit in ideomotor apraxia is impaired abi lity to perform tasks where the goal is to reproduce or adopt complex hand configurations and that impaired execution of some types of action is secon dary to this problem. This is consistent with a role for the human left par ietal lobe in providing an explicit representation of posture which is used to guide some actions and to supplement a more primitive system of direct visuomotor control. (C) 2000 Elsevier Science Ltd. All rights reserved.