Understanding object-directed actions involves analyzing how body parts interact with objects and how these interactions lead to changes in the objects. However, whether, and if so, how interaction (as cause) and object change (as effect) are distinctively processed in the brain remains unexplored. Based on previous findings, we hypothesized that interaction and change are represented in a lateralized manner in left vs. right nodes of the action observation network, respectively.
In four fMRI sessions, 30 right-handed participants observed videos of object-directed actions (e.g., breaking a stick), corresponding abstract animations (e.g., a triangle hitting a rectangle, causing it to break in half) and, importantly, animations of interaction (triangle hits rectangle) and object change (rectangle breaks in half) in isolation. Using cross-decoding, we isolated either the interaction or change in the actions (train classifier on actions, test on interaction-only or change-only animations, respectively).
As hypothesized, we found that interaction and change are represented in a lateralized manner in anterior inferior parietal lobule (aIPL) and lateral occipitotemporal cortex (LOTC). In addition, in bilateral aIPL, cross-decoding between actions and animations depicting both interaction and change was stronger than the sum of cross-decoding between actions and interaction-only or change-only animations. This super-additive effect points toward a higher-level representation of cause-effect structures beyond representations of interaction and change as isolated components.
These findings demonstrate that left and right hemispheres have distinct roles in representing the interaction between entities and the induced change, respectively, and that interaction and change are integrated to cause-effect structures in aIPL.