Fronto-parietal activationduring observation of impossible grasping actions : an fMRI study

Neuroimaging experiments have shown that observation of others’ actions activates a fronto-parietal network in the brain of observers (Broadmann areas 6, 9, 2, 3 and 40). This activation is assumed to be a sensory-motor representation of observed actions, generating an embodied simulation used to understand observed actions and their intentions. Utilizing natural (‘palmar’ finger flexion) and impossible (‘dorsal’ finger flexion) actions with identical grasping intention, neurophysiological experiments have also shown that motor resonant responses (TMS-evoked motor potentials) in primary motor cortex reflect the specific motor program encoding the movements composing an action rather than its intention. To test the hypothesis that different features of observed actions may be encoded at different levels in the CNS, here we compare brain activation during observation of the same natural and impossible grasping actions performed by an avatar right hand. Twenty right-handed subjects participated in a fMRI experiment; group analyses were performed, identifying areas active in both conditions (“natural” and “impossible”; p<0.05FWE-corrected). Results show that observation of the avatar’s natural grasping actions activates the expected fronto-parietal network exclusively in the left hemisphere, while observation of impossible grasping produces a bilateral activation of these same areas, plus a stronger, as well as right-lateralized, activation of area BA3. These results suggest that while impossible and natural actions are represented by the same fronto-parietal network, perhaps reflecting their common intention, impossible actions show a stronger and broader recruitment of parietal areas, possibly involved in processing and encoding their unpredictable somatosensory consequences.