Neural Representations of Perceived Engagement During Action Observation.

Interpersonal motor interactions are central to social life, yet it remains unclear how social cues relevant to detecting engagement are encoded in the brain. Recent evidence suggests that regions traditionally associated with mentalizing, such as the dorsomedial prefrontal cortex (dmPFC) and temporo-parietal junction (TPJ), co-activate with nodes of the Action Observation Network (AON) during motor engagement with others, pointing to a synergistic role in the processing of action features during interaction. Using fMRI and Representational Similarity Analysis (RSA), we examined brain responses to reach-to-grasp actions varying in Goal (passing vs. placing), Perspective (2nd vs. 3rd person), and Gaze visibility, creating a gradient of perceived engagement. Our results show TPJ-AON convergent representational geometry during action observation, with temporo-parietal and premotor regions, but not dmPFC, showing correlated dynamics of pattern modulation. Model-based analyses showed a graded organisation within the right temporo-parietal cortex, with perspective encoded in the rTPJ, goal-related information in superior temporal regions, and perceived engagement uniquely encoded in the rIPL. Moreover, the rTPJ and left premotor cortex shared representational geometry for action direction, linking mentalizing and sensorimotor systems in the encoding of first-person-relevant cues. In contrast, the dmPFC showed an isolated representational geometry and did not encode action features or perceived engagement, consistent with its recruitment during richer interactive contexts. Together, these findings support a distributed, sensorimotor account of engagement encoding and reveal new functional links among key social cognition areas.