Positron emission tomography (PET) was used to localize brain regions that are active during the observation of grasping movements. Normal, right-hand- ed subjects were tested under three conditions. In the first, they observed grasping movements of common ob- jects performed by the experimenter. In the second, they reached and grasped the same objects. These two condi- tions were compared with a third condition consisting of object observation. On the basis of monkey data, it was hypothesized that during grasping observation, activa- tions should be present in the region of the superior tem- poral sulcus (STS) and in inferior area 6. The findings in humans demonstrated that grasp observation significant- ly activates the cortex of the middle temporal gyrus in- cluding that of the adjacent superior temporal sulcus (Brodmann's area 21) and the caudal part of the left infe- rior frontal gyrus (Brodmann's area 45). The possible functional homologies between these areas and the mon- key STS region and frontal area F5 are discussed.
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