You still intend to move: Inferred mental states imply motion to static body postures

Motion can be implied in static images through the explicit depiction of kinematic and dynamic cues. An unresolved question is whether the perception of implied motion can emerge solely from observers' prior expectations when explicit visual motion cues are ambiguous. We employed human actions to resolve this as, while static depictions of biological motion often imply movement through articulated joint positions, the causes of such motion are the hidden mental states of the agent. Across two experiments participants heard an agent state either "I'll take it" or "I'll leave it" and observed them either reach for or withdraw from an object, or remain motionless. The hand disappeared and participants touched the screen coordinates of the last seen position. In Experiment 1, static hands were perceived nearer to the object if reaches were more likely than if withdrawals were more likely. In Experiment 2, static hands were perceived nearer the object if the agent had stated, "I'll take it" than if they had stated, "I'll leave it". Crucially, the static stimulus was the same, and directionally ambiguous in all conditions, suggesting that prior expectations based on the likelihood of the agent's behaviour, and their intentions, imply motion when explicit visual cues are uninformative.