3. General discussion

The present study aimed at investigating how two distinct sources of information – perceptual (bottom-up) evidence and prior (top-down) expectations – interact to enable one to make an intentional inference. To do so, we manipulated the participants' prior expectations about the probability of the underlying intention while varying the amount of visual information of the action scene. Our second purpose was to determine whether the contribution of these two sources of information would vary depending on the scope (motor vs. superordinate intention) and the target of the intention (social vs. non-social intention) that had to be inferred. To test this second hypothesis, we therefore manipulated the type of intention underlying the observed action using four distinct tasks (motor non-social, superordinate non-social, motor social and superordinate social tasks).

Two main results emerged. First, we observed that the intentional judgment indeed rests on an interplay ofthe participants’ prior expectations (their probability of being true varied across the blocks) and of the reliability of sensory information available from the action scene. When this reliability decreased, the bias effect (i.e. the contribution of prior expectations) on performance increased, with participants responding more towards intentions they estimated as being the most likely cause of the observed behaviour. Second, this interaction was found to vary according to the type of intention, defined here by its scope (motor vs. superordinate) or its target (social vs. non-social). Indeed, directly comparing performance between intentions of different scopes but identical targets, and between intentions with the same scope but distinct targets, revealed an increase in the bias effect for both superordinate and social intentions. While this effect was only observed when the amount of visual information was low in the motor task, it was found to be significant for both low and moderate amounts of information in the superordinate task, and for any amounts of visual information in the social conditions.

Taken together, these results indicate that the degree of participants’ prior knowledge contribution is sensitive to the type of intention that is focused on. From motor to superordinate, and from non-social to social intentions, participants’ prior expectations exerted an increasing influence on their responses, at the expense of the sensory information available from the action scene.