The variability of contact position errors in apraxic imitation

Apraxia is a poorly understood disorder of voluntary movement that is not due to basic sensory, motor or cognitive deficits. We conducted a study on the imitation of meaningless gestures in apraxic patients with left fronto-temporo-parietal lesions. In a first condition, the patients were instructed to imitate the orientation and position of a hand touching the face (complex task). In a second condition, the patients were instructed to solely imitate the position of contact between the face and hand, using any orientation of the hand (simple task). We did not observe contact position errors (CPEs) in the simple task, which argues in favor of preserved spatial and motor representations of the body. However, the patients made a substantial amount of CPEs in the complex task, indicating a misuse of these spatial and motor representations of the body when the task involved several imitation goals (e.g. the thumb touches the nose and the hand is oriented vertically). The error variability was remarkably low across trials. Most of the CPEs could be explained by an incorrect coordination between the reproductions of multiple imitation goals. If simple movements are easier to imitate and less prone to errors, an efficient rehabilitation therapy may use movements of increasing complexity.