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Humans can recombine thousands of different facial expressions. This variability is due to the ability to voluntarily or involuntarily modulate emotional expressions, which, in turn, depends on the existence of two anatomically separate pathways. The Voluntary (VP) and Involuntary (IP) pathways mediate the production of posed and spontaneous facial expressions, respectively, and might also affect the left and right sides of the face differently. This is a neglected aspect in the literature on emotion, where posed expressions instead of genuine expressions are often used as stimuli. Two experiments with different induction methods were specifically designed to investigate the unfolding of spontaneous and posed facial expressions of happiness along the facial vertical axis (left, right) with a high-definition 3-D optoelectronic system. The results showed that spontaneous expressions were distinguished from posed facial movements as revealed by reliable spatial and speed key kinematic patterns in both experiments. Moreover, VP activation produced a lateralization effect: compared with the felt smile, the posed smile involved an initial acceleration of the left corner of the mouth, while an early deceleration of the right corner occurred in the second phase of the movement, after the velocity peak.

Published: 23 July 2021, In everyday-life scenarios, prior expectations provided by the context in which actions are embedded support action prediction. However, it is still unclear how newly learned action–context associations can drive our perception and motor responses. To fill this gap, we measured behavioral (Experiment 1) and motor responses (Experiment 2) during two tasks requiring the prediction of occluded actions or geometrical shapes. Each task consisted of an implicit probabilistic learning and a test phase. During learning, we exposed participants to videos showing specific associations between a contextual cue and a particular action or shape. During the test phase, videos were earlier occluded to reduce the amount of sensorial information and induce participants to use the implicitly learned action/shape-context associations for disambiguation. Results showed that reliable contextual cues made participants more accurate in identifying the unfolding action or shape. Importantly, motor responses were modulated by contextual probability during action, but not shape prediction. Particularly, in conditions of perceptual uncertainty the motor system coded for the most probable action based on contextual informativeness, regardless of action kinematics. These findings suggest that contextual priors can shape motor responses to action observation beyond mere kinematics mapping.

Social interaction is an essential part of the human experience and actions are the primary means by which humans interact with the surrounding world. The ability to recognize and understand other people’s actions is necessary for an efficient interaction with other agents. Our motor system can promptly and accurately coordinate these forms of interactions in our daily activity. However, how the brain is able to produce such appropriate output has yet to be fully understood. The present work aims at unveiling the processes taking place in the motor system while observing actions of interactive agents and action calling for an (interactive) involvement of the observer. The core argument is to explore the flexibility of the motor system when preparing identical and non-identical responses in complex realistic situations, and to test the automaticity of these processes by directly investigating the role played by visuospatial attention during action observation. The introductory section of this thesis will first provide an overview of the state of the art regarding the mechanism that could be at the basis of the comprehension of other’s actions, that is the ‘mirror mechanism’ (Chapter 1). Mirror neurons are neural cells which activate both during the execution of an action and during the observation of the same action performed by another individual. From their first discovery in the premotor cortex of the macaque brain in the early nineties (Di Pellegrino, Fadiga, Fogassi, Gallese and Rizzolatti, 1992), these visuomotor neurons have been extensively studied both in primates and in humans. Chapter 1 will review evidence on the existence of such mirror mechanism and on its basic properties and anatomy, with a particular focus on the human literature concerned with the Action Observation System (AOS). Convergent evidence suggests that actions are coded in the observer’s brain in such a way that resembles the actual execution of the action. As a result, an embodied simula