4. Conclusion

Our results shed light on how sensory information, derived from the kinematics of the observed action, interacts with prior expectations to enable one’s understanding of other people’s intentions. We first showed that the contribution of participants’ prior knowledge was sensitive to the reliability of the sensory information available from the action scene. A greater contribution of this knowledge was observed in condition of sparse visual information, suggesting the engagement of a mechanism of data completion operating through the default use of prior expectations.

Second, we found that the priors’ contribution also depended on the type of intention that was inferred. An increased reliance on priors was indeed observed in conditions where the agent’s intention could not be predicted by the sole visible, current motor act, but further required estimating the superordinate goal this act contributed to achieve. In this case, participants’ expectations – being progressively acquired from observation – were found to most frequently supersede the visual information conveyed by the current motor kinematics. Thus, the more participants responded towards the biased (e.g. expected) intentions, the more the visual information tended to play a confirmatory, rather than a predictive role. Such a shift in the contribution of visual evidence is likely to account for why participants, in this condition, mostly over-relied on their priors to make their decision, even though it ran counter to the perceptual evidence.

Crucially, an over-reliance on priors was also massively observed in social conditions. We suggested that the early influence of social-specific expectations (e.g. expectations on how agents are the most likely to behave in a context of reciprocal interaction) may account for this important shift in the response toward participants’ priors. Contexts of social interaction are indeed prone to elicit modular, high-level expectations, which may contribute to giving priority to some intentional causes (e.g. cooperation if previous cooperation, defection if previous defection) at the expense of other competing causes. These a priori expectations, being acquired from experience (probabilistic bias) or derived from domain-specific knowledge (TFT reciprocation), were found to favour some action representations so that less sensory evidence was needed for the participants to be confident about their decision, i.e. about which kind of intention was most likely the cause of the observed action.