Touching-untouching patterns organize action representation in the inferior parietal cortex

At an abstract temporospatial level, object-directed actions can be described as sequences of touchings and untouchings of objects, hands, and the ground. These sparse action codes can effectively guide automated systems like robots in recognizing and responding to human actions without the need for object identification. The aim of the current study was to investigate whether the neural processing of actions and their behavioral classification relies on the action categorization derived from the touching-untouching structure. Here we show, using a representational similarity analysis of functional MRI data from two experiments, that action representations in left anterior intraparietal sulcus (aIPS) are particularly associated with this categorization of touching-untouching structures. Within the examined action observation network, only the touching-untouching category model selectively correlated with the representational profile of the left aIPS. The behavioral results showed a significant relation between the touching-untouching structure and the observers' judgments on the similarity of actions with weakly-informative objects. Extending prior research on touchings and untouchings as meaningful anchor points for explicit action segmentation, our findings suggest that touching-untouching sequences serve as an organizing principle in inferior parietal action representation.