Instruction-induced modulation of the visual stream during gesture observation.

Although gesture observation tasks are believed to invariably activate the action-observation network (AON), we investigated whether the activation of different cognitive mechanisms when processing identical stimuli with different explicit instructions modulates AON activations. Accordingly, 24 healthy right-handed individuals observed gestures and they processed both the actor's moved hand (hand laterality judgment task, HT) and the meaning of the actor's gesture (meaning task, MT). The main brain-level result was that the HT (vs MT) differentially activated the left and right precuneus, the left inferior parietal lobe, the left and right superior parietal lobe, the middle frontal gyri bilaterally and the left precentral gyrus. MT (vs HT) differentially activated the left and right calcarine cortex, the fusiform gyrus bilaterally, the left inferior temporal gyrus, the left and right hippocampus and parahippocampal gyri, and the temporal pole bilaterally. Processing the actor's moving hand modulates the dorsal action observation network (while processing gesture meaning modulates the ventral object recognition stream). The present results suggest instruction-induced modulation on the visual stream during gesture observation.