Humanoid facial expressions: a tool to study human behaviour

The action form (vitality form, VF) is a fundamental aspect of social communication which enables to infer affective states of others. For example, an agent can grasp a cup vigorously or delicately and move it rudely or gently. However, it is noteworthy that actions are nested in social contexts and additional cues such as facial expressions represent fundamental tools to understand the state of an agent. We proposed the use of the humanoid iCub robot as interactive and controllable agent to investigate whether and how different facial expressions could modulate participants motor behavior. To this purpose, we conducted a kinematic experiment in which participants observed videoclips of the iCub robot performing a rude or gentle action (giving request) with a happy or angry facial expression. After the request, participants were asked to grasp a little ball and pass it towards iCub, positioning it on a target. Results showed that iCub VFs significantly influenced participants motor response. Most importantly, we demonstrated that iCub face significantly modulated most of the kinematic parameters characterizing the passing phase and participants reaction time (time elapsing between iCub request and motor response), meaning that positive or negative facial expressions influenced the interaction of participants towards the robot. An additional behavioral study confirmed these data, showing that the perception of positive (gentle) and negative (rude) action VFs is modified by positive (happy) or negative (angry) facial expressions. Our findings are promising for future studies aiming to investigate affective communication in population with face and action recognition impairments.