What apraxia tells us about the brain hemispheres’ contributions to human imitation

Praxic functions are frequently altered following brain lesion, giving rise to apraxia - a complex pattern of impairments that has proven difficult to assess or interpret. Here we revisit a study on imitation of meaningless gestures in a patient with disconnected brain hemispheres. We performed extensive statistical analyses over several objective geometrical variables (e.g. hand orientation) that revealed a clear dissociation in the performance of imitating different goals. Our results show that the right hemisphere is perfectly capable of imitating a hand posture with the contralateral hand, but not its position of contact with the face. We propose that the underlying deficit is incorrect coordination of the reproduction of multiple imitative goals. This accords well with a left hemisphere dominance for the body part coding of gestures. In addition, our results provide neuropsychological evidence in favor of the hypothesis of goal-directed imitation. Finally, we found that only the stimuli with unnatural hand postures were informative as they were selectively impaired.