3.2. Motor vs. superordinate intentions

Both non-social motor and superordinate experiments required recognising one motor act, the superordinate condition also requiring the final goal of the sequence (i.e. the shape being constructed) to be taken into account. Yet, across both experiments, prior expectations were found to differentially contribute to the participants’ response. In the motor non-social experiment, a bias response towards the preferred intention was only observed in the condition where the amount of visual information was low. When participants were exposed to a moderate amount of visual information, these expectations no longer exerted an influence on performance, which then substantially depended upon the processing of the visual information alone. As previously suggested, this result may be explained by the fact that motor intention stands to action in a one-to-one relation: motor intentions like ’transport’, ‘rotate’ or ‘lift’ a cube are directly accessible to the viewer from mere observation of the motor acts. On the other hand, a heightened contribution of prior knowledge is observed when shifting from a relation of strict correspondence (one-to-one relation) to a relation where the current action only represents one step, among others, of the final intention. In the superordinate experiments, prior knowledge was indeed found to significantly influence participants’ performance in conditions of both low and moderate amounts of visual information.

One explanation for this increase in response bias might reside in the differences in complexity between both tasks. Indeed, participants were required to attend to a single motor act in the motor conditions while the superordinate conditions involved paying attention to a sequence of three successive motor acts. In superordinate conditions, this supplementary difficulty could have led subjects to adopt a strategy of the least effort that would have consisted in disregarding the viewed sequence and relying primarily on their prior knowledge. However, if that was the case, poorer performances should have been observed in these conditions. Yet, participants performed at comparable levels across motor and superordinate experiments showing that the differences in terms of contribution of prior information between both experiments are accounted for by the type of intention being considered rather than by the difficulty of the task.

Contrary to motor intention, superordinate intention stands to action in a many-to-one relation, meaning that the very same intention can be achieved by several distinct (commutative) sequences of actions. Because of this unpredictability, participants were thus unable to infer the underlying intention on the basis of the sole visual information arising from the first two actions. However, the present results suggest that, despite the unpredictability of the sequence, participants still initiated a response, before observing the last action, by appealing massively to their prior expectations.

This early use of prior expectations may be accounted for by the existence of a system that would pre-process the current action chain depending on the sequences previously encountered. Observing the beginning of an action, or a sequence of actions, would automatically activate a representation of the preferred intention that would be progressively suppressed or reinforced as the amount of visual information increases. Such a pre-processing would be particularly salient in superordinate conditions, where the beginning of the act chain proved to be of little importance for inferring the final intention it achieved. As such, it would explain why selecting a non-preferred intention in bias sessions induced a significant cost on participants’ performance. In these sessions, selecting a non-preferred intention would indeed imply disengaging from the early activation of a preferred and better represented intention. Finally, such pre-processing may account for why prior expectations are favoured over visual information in conditions of high perceptual uncertainty, as it would account for the role that priors continue to play when the amount of perceptual information increases. In superordinate conditions, the current sequence of actions would pre-activate the representation of the preferred intention (i.e. the intention with the highest probability) to such an extent that a greater amount of visual information would be required to counteract it.