Objects Mediate Goal Integration in Ventrolateral Prefrontal Cortex during Action Observation

Abstract

Actions performed by others are mostly not observed in isolation, but embedded in sequences of actions tied together by an overarching goal. Therefore, preceding actions can modulate the observer's expectations in relation to the currently perceived action. Ventrolateral prefrontal cortex (vlPFC), and inferior frontal gyrus (IFG) in particular, is suggested to subserve the integration of episodic as well as semantic information and memory, including action scripts. The present fMRI study investigated if activation in IFG varies with the effort to integrate expected and observed action, even when not required by the task. During an fMRI session, participants were instructed to attend to short videos of single actions and to deliver a judgment about the actor's current goal. We manipulated the strength of goal expectation induced by the preceding action, implementing the parameter "goal-relatedness" between the preceding and the currently observed action. Moreover, since objects point to the probability of certain actions, we also manipulated whether the current and the preceding action shared at least one object or not. We found an interaction between the two factors goal-relatedness and shared object: IFG activation increased the weaker the goal-relatedness between the preceding and the current action was, but only when they shared at least one object. Here, integration of successive action steps was triggered by the re-appearing (shared) object but hampered by a weak goal-relatedness between the actually observed manipulation. These findings foster the recently emerging view that IFG is enhanced by goal-related conflicts during action observation.

Fig 1. Snapshots of exemplary action videos.

Action video pairs were assigned a parametric value of goal-relatedness as assessed in subjects in a post fMRI session survey. The parametric value of goal-relatedness of consecutively presented actions ranged from low (right column) to high (left column). Independent of this factor, videos shared a common object (upper row) or not (lower row). Examples show the following actions (in reading direction, from upper left): opening a bottle of water; pouring in a glass of water; grasping a telephone receiver; cleaning a telephone receiver; folding laundry (t-shirt); turning socks inside out; drying the dishes (spoon); unlocking a petty cash.

Fig 2. Event-related design (extract from the experimental time course).

Action videos were presented intermixed with verbal action descriptions (question trials, 20% of action trials). Participants were instructed to attend to the videos and recognize the actions. In question trials participants had to indicate per button press whether an action description matched the preceding action (50%) or not (50%).

Fig 3. Areas that correlate negatively (blue) or positively (red) with the goal-relatedness of consecutively presented action videos.

Upper row shows the parametric effect only for videos that shared at least one object with the preceding action. Middle row shows the same effect selectively for actions that did not share an object with the preceding action. The lower row depicts the interaction of these two parametric effects. Activation increased in bilateral IFG for weak goal-relatedness, but only when video clips contained a shared object. Z-thresholded at 2.33, corrected cluster threshold p = .05. IFG inferior frontal gyrus, AG Angular gyrus, aSFS/G anterior superior frontal sulcus/gyrus, PRG precentral gyrus, TPJ temporo-parietal junction.