Impaired imitation of meaningless gestures in ideomotor apraxia: a conceptual problem not a disorder of action control? A single case investigation.

A defining characteristic of ideomotor apraxia is an inability to imitate meaningless gestures. This is widely interpreted as being due to difficulties in the formulation or execution of motor programs for complex action, but an alternative view is that there is a higher level cognitive problem in conceptualisation of the target posture. In a single case with inferior left parietal and temporal damage, severely impaired imitation was accompanied by preserved motor skill and spatial awareness but inability to make a conceptual match between the fingers of his own hand and an observed hand. Also, he was able to match pictures of visually similar gestures but not cartoon drawings of gestures which were conceptually the same but visually dissimilar. Knowledge of body structure seemed largely intact as he was only slightly inaccurate in showing correspondences between locations on drawings of a human figure and his own body, or a visually dissimilar figure. This indicated that difficulty on matching gestures was specific to representation of body posture rather than body structure, or that gesture imitation tasks place higher demand on a structural representation of the body. These data imply that for at least some cases of ideomotor apraxia, impaired gesture imitation is due to a deficit in representing the observed posture and is not a deficit in memory for action or of motor control.