From the perception of action to the understanding of intention

Humans have an inherent tendency to infer other people's intentions from their actions. Here we review psychophysical and functional neuroimaging evidence that biological motion is processed as a special category, from which we automatically infer mental states such as intention. The mechanism underlying the attribution of intentions to actions might rely on simulating the observed action and mapping it onto representations of our own intentions. There is accumulating neurophysiological evidence to support a role for action simulation in the brain.Key Points Humans have an inherent ability to understand other people's minds. This process is a component of a 'theory of mind'. This review focuses on a low level of theory of mind — the ability to understand other people's intentions by observing their actions. This ability might be a prerequisite for the higher-level understanding of others' minds. Psychophysical studies show that biological motion is processed as a special category, to which humans from an early age attribute mental states such as intention. The posterior superior temporal sulcus (STS) is involved in the detection of biological motion. This area receives convergent information from both dorsal and ventral visual streams, making it an interface between perception for identification and perception for action. Humans attribute intentions to basic stimuli, including dynamic point-light displays and simple animations, as long as their movement is 'animate' — that is, it is self-propelled, its path may be nonlinear and it may undergo sudden changes of velocity. The STS and the medial prefrontal cortex are activated when mental states are attributed to moving shape stimuli. These areas are also consistently activated by higher-level theory-of-mind tasks in which subjects think about their own or others' mental states. A mechanism for inferring intentions from observed actions is proposed that depends on the same 'forward model' system that labels the consequences of one's own actions as being produced by one's own intentions. Forward models store predictions of the consequences of one's own actions. These stored representations could be recruited when observing someone else's movements to estimate their intentions. This mechanism may be based on simulating the observed action and estimating the actor's intentions based on a representation of one's own intentions, an idea that is reminiscent of simulation theory. There is evidence that simulation and imitation — a case of overt action simulation — facilitate intentional attribution. In particular, neurophysiological evidence supports the existence of a matching system between perception and action, which is recruited during imitation. Psychophysical and neurophysiological studies support the idea that the brain is a powerful simulating machine, designed to detect biological motion in order to extract intentions from the motion and to predict the future actions of other animate beings.