Using action understanding to understand the left inferior parietal cortex in the human brain

• Homology of parietal area involved in understanding action in humans to area PFG in the macaque.
• Observers viewed pantomimed actions and had to decide what object was being used.
• Activation in regions in left anterior inferior parietal cortex may have differentiated from PFG.
• Evidence of automatic engagement of dominant premotor areas and their feedback to parietal cortex.
• The data may account for mirror neuronal activity in the parietal cortex.

Humans have a sophisticated knowledge of the actions that can be performed with objects. In an fMRI study we tried to establish whether this depends on areas that are homologous with the inferior parietal cortex (area PFG) in macaque monkeys. Cells have been described in area PFG that discharge differentially depending upon whether the observer sees an object being brought to the mouth or put in a container. In our study the observers saw videos in which the use of different objects was demonstrated in pantomime; and after viewing the videos, the subject had to pick the object that was appropriate to the pantomime. We found a cluster of activated voxels in parietal areas PFop and PFt and this cluster was greater in the left hemisphere than in the right. We suggest a mechanism that could account for this asymmetry, relate our results to handedness and suggest that they shed light on the human syndrome of apraxia. Finally, we suggest that during the evolution of the hominids, this same pantomime mechanism could have been used to ‘name’ or request objects.