Neural bases and behavioural responses of vitality forms in human-human and human-robot interactions

The observation of actions performed by others allows us to understand what they are doing and why. Such ability is related to the existence of a set of neurons, "mirror neurons", that discharge both when individuals perform a goal-directed action and when they observe another person performing the same action. However, besides action goal and intention, there is a third fundamental component that an observer may capture when viewing an action: its form. Indeed, actions can be executed with different forms, such as gently or rudely, vigorously or hesitantly, communicating the affective state of the agent as well as positive and negative attitudes towards the receiver. Daniel Stern defined these aspects of social communication “vitality forms”. The recognition and expression of vitality forms is already present in infants during mother-child interactions, suggesting that they represent a primordial way to relate to and understand others. Despite their pervasiveness in human life, research has always focused on the action goal, neglecting the role of vitality forms in the study of human behaviour and social interactions. In this view, my PhD project involves an interdisciplinary approach that spans the fields of neuroscience and robotics aiming to contribute to the enrichment of vitality forms research. Particularly, it deepens the study of neural correlates and behavioural responses underlying vitality forms processing in humans and proposes vitality forms as a novel and valuable tool to promote human—robot interactions. The present thesis aimed to address the following objectives: 1) to deeply investigate in humans the activity of brain areas involved in vitality forms processing, 2) investigate the influence of vitality forms on action perception and expression during human—human interactions and 3) human—robot interactions. These challenges are outlined through three main studies, whose findings represent the contribution of this work. Results of the first study show that both the dorso-central insula (DCI) and middle cingulate cortex (MCC) are endowed with a mirror mechanism specific for vitality forms processing. Indeed, a voxel based analysis revealed a significant correlation of the BOLD signal during observation and execution of actions endowed with vitality forms. Moreover, a multifiber tractography analysis showed that these two areas are anatomically connected, forming a circuit selective for the processing of hand-arm actions conveying vitality forms. This circuit may allow individuals to understand vitality forms expressed by others and to prepare an adequate motor response. This suggestion is supported by results provided in the second study, which show an influence of vitality forms conveyed by an agent, through different modalities, on the action perception (estimated action duration) and execution (kinematic parameters) of a receiver. Results of the third study assess that vitality forms expressed by a humanoid agent (the iCub robot), can induce the same influence on actions performed by participants. Most importantly, the iCub robot was used to demonstrate how positive (happy) and negative (angry) facial expressions can modify the perception of its action vitality forms and consequently modulate the motor response of the human partner. Summing up, this thesis provides new neurophysiological and behavioural insights of vitality forms, highlighting their essential role in human—human interactions and proposing them as constitutive feature also for robots to reach an effective communication with humans. Results and methodology of this PhD project lay important foundations for future studies aiming to investigate vitality forms processing in clinical populations with social and motor impairments.