6. Conclusion

Inferring other people’s intentions requires properly dealing with both the visual information from the action scene and prior knoweldge about the most likely causes of what is observed. Schizophrenic patients exhibited some abnormal dependence on both these types of information. In particular, patients’ prior knowledge was found to abnormally modulate their decision about other’s action depending on the type of intention considered and their prevalent symptoms.

We first showed that patients with severe positive symptoms overly relied on their priors, but also on disconfirming visual evidence from the action scene. This pattern of response, resulting in overgenerating internal hypotheses and dismissing potentially disconfirmatory evidence, may favour in these patients a paranoid interpretation of other people’s goals. It may also result in abnormal distinguishing between one’s own and other’s intention, a confusion that patients with passivity symptoms frequently experience, such as in the well-documented delusion of control.

In social conditions, on the other hand, schizophrenic patients exhibited an inappropriate use of social domain-knowledge. This biased knowledge was found to induce patients with inappropriate expectations of how agents were the most likely to behave in a context of social interaction. Patients with negative symptoms compensated for this by giving priority to visual evidence in making their decision. We suggested that this profile of performance parallels the stimulus-induced nature of negative behaviours in schizophrenia and could further account for action initiation disorders commonly observed in this dimension of the disease.