STs/STG region responds parametrically to goal-directedness during observation of abstract agents

Goal-directed behaviour is an important cue for the attribution of animacy to elements of the environment, as has been repeatedly shown in behavioural studies of children and adults. In monkey and human, the superior temporal sulcus (STS) and gyrus (STG) are known to respond to stimuli displaying biological motion, and could thus participate in the detection of living entities. As goal-directedness is important for the attribution of animacy, we expected STS and STG to respond when objects appearing animate try to reach a goal. As the STS also appears to be involved in tasks involving mentalizing, it might also be sensitive to the way the agents reach their goal. In two fMRI experiments, we presented healthy adult volunteers with two agents (interacting, round shapes moving in a seemingly animate way) and varied the goal-directedness in their behaviour. In exp. 1, we parametrically increased the goal-directedness in the interaction of the agents, and in exp. 2 we varied the strategy used to reach the goal: agents either seemed to rely on mentalizing or only on physical cues. Stimuli were controlled for speed and quantity of movement. Increase in goal-directed behaviour parametrically increased activation in STS and STG, even when subjects performed an incidental task. In exp. 2, watching agents using a mentalizing strategy increased activation in the STS and STG; this was reduced when subjects performed an incidental task. We conclude that 1) the STS / STG region responds to goal-directed behaviour independently of the task performed by the subject, and 2) it reponds more when the goal-directed behaviour is apparently relying on mentalizing. This second activation increase seems to be only significant when subjects explicitly look for mental states in the observed behaviour