Coding observed motor acts: different organizational principles in the parietal and premotor cortex of humans.

Understanding actions of conspecifics is a fundamental social ability depending largely on the activation of a parieto-frontal network. Using functional MRI (fMRI), we studied how goal-directed movements (i.e., motor acts) performed by others are coded within this network. In the first experiment, we presented volunteers with video clips showing four different motor acts (dragging, dropping, grasping, and pushing) performed with different effectors (foot, hand, and mouth). We found that the coding of observed motor acts differed between premotor and parietal cortex. In the premotor cortex, they clustered according to the effector used, and in the inferior parietal lobule (IPL), they clustered according to the type of the observed motor act, regardless of the effector. Two subsequent experiments in which we directly contrasted these four motor acts indicated that, in IPL, the observed motor acts are coded according to the relationship between agent and object: Movements bringing the object toward the agent (grasping and dragging) activate a site corresponding approximately to the ventral part of the putative human AIP (phAIP), whereas movements moving the object away from the agent (pushing and dropping) are clustered dorsally within this area. These data provide indications that the phAIP region plays a role in categorizing motor acts according to their behavioral significance. In addition, our results suggest that in the case of motor acts typically done with the hand, the representations of such acts in phAIP are used as templates for coding motor acts executed with other effectors.