Activation differences in observation of hand movements for imitation or velocity judgement.

We aimed to investigate the brain areas engaged in observation of hand movements with the intention of imitation or judging movement velocity. Both processes reflect different analytic approaches in movement observation. We were interested if these two processes can be distinguished or share common activation foci. Twelve healthy, right-handed volunteers were required to observe video clips of hand gestures and of object related grasping movements while the regional cerebral blood flow was measured using positron emission tomography. The subjects were instructed either to imitate the actions or to judge the velocity of the observed movements after scanning. Action observation with the instruction to judge movement velocity engaged bilaterally the temporo-occipital junction and adjacent visual cortical areas. In contrast, observation with the instruction to imitate them afterwards, yielded large activation clusters covering the left parietal and premotor cortex. Both contrasts demonstrated activation in the inferior frontal cortex, however, on opposite sides. Results suggest that movement observation with the goal of imitation activated specific areas of the parietal cortex in the dominant hemisphere probably related to programming of the movement kinematics. In contrast, observation with the goal to characterize the velocity of the finger movements activated the ventral visual pathways. Thus, movement observation recruits non-overlapping cortical networks, depending on the information attended to which are characterised by a dorsal ventral dissociation.