Cortical regions involved in the observation of bimanual actions.

Although we are beginning to understand how observed actions performed by conspecifics with a single hand are processed and how bimanual actions are controlled by the motor system, we know very little about the processing of observed bimanual actions. We used fMRI to compare the observation of bimanual manipulative actions with their unimanual components, relative to visual control conditions equalized for visual motion. Bimanual action observation did not activate any region specialized for processing visual signals related to this more elaborated action. On the contrary, observation of bimanual and unimanual actions activated similar occipito-temporal, parietal and premotor networks. However, whole-brain as well as region of interest (ROI) analyses revealed that this network functions differently under bimanual and unimanual conditions. Indeed, in bimanual conditions, activity in the network was overall more bilateral, especially in parietal cortex. In addition, ROI analyses indicated bilateral parietal activation patterns across hand conditions distinctly different from those at other levels of the action-observation network. These activation patterns suggest that while occipito-temporal and premotor levels are involved with processing the kinematics of the observed actions, the parietal cortex is more involved in the processing of static, postural aspects of the observed action. This study adds bimanual cooperation to the growing list of distinctions between parietal and premotor cortex regarding factors affecting visual processing of observed actions.