Activation in superior temporal sulcus parallels a parameter inducing the percept of animacy

An essential, evolutionarily stable feature of brain function is the detection of animate entities, and one of the main cues to identify them is their movement. We developed a model of a simple interaction between two objects, in which we could control the percept of animacy by varying one parameter. The two disk-like objects moved along separate random trajectories but were also influenced by each other's positions, such that one object followed the other, in a parametrically controlled fashion. An increase of the correlation between the object's movements varied the amount of interactivity and animacy observers attributed to them. Control animations were only different from the experimental in terms of the interactivity level, but not in terms of object speed and separation. Twelve observers lying in a magnetic-resonance-imaging scanner had to rate the amount of interactivity and the overall speed of the objects in separate, subsequent tasks. Behavioural results showed a significant difference in interactivity ratings between experimental and control stimuli, but no difference in speed ratings, as expected. There was no response-time difference between the tasks. The fMRI data revealed that activation in the posterior superior temporal sulcus and gyrus (pSTS/pSTG) increased in relation to the degree of correlated motion between the two objects. This activation increase was not different when subjects performed an explicit or implicit task while observing these interacting objects. These data suggest that the pSTS and pSTG play a role in the automatic identification of animate entities, by responding directly to an objective movement characteristic inducing the percept of animacy, such as the amount of interactivity between two moving objects. These findings are consistent with literature showing that, in monkey and human, pSTS and pSTG respond to stimuli displaying biological motion