Neural correlates of human action perception: motor, semantic and social aspects

In the last decades, human action processing has been the research focus of a series of studies aimed at investigating the brain mechanisms underlying this complex process. Converging neurophysiological and functional neuroimaging literature suggested that human actions processing is associated with a large scale network involving areas within the temporal, parietal and frontal cortices. Against the amount of literature available on the localization of these areas, the temporal course of their activations is poorly understood. The purpose of the present project was to explore the temporal dynamics associated with human action perception investigating the neural responses to different aspects of human behavior by means of a series of high-density electrophysiological recording experiments combined with source localization methods. Specifically, the motor (Chapter 1), the semantic (Chapter 2) and the social (Chapter 3) aspects were investigated. Results highlighted a crucial role of the social/affective content, revealing a very early recognition (at 170 ms) operated by the temporal and limbic areas, of this aspect of the human behavior. Starting from 250 ms the processing of the different aspects occurs temporally aligned, involving firstly the mid-superior temporal sulcus (STS) and subsequently the fronto-parietal mirror (hMNS) circuit. Evidence from the source localization analysis suggested also a later involvement of the medial prefrontal cortex (mPFC), associated with mentalizing process. A deepening of the social content of actions was then performed (Chapter 4). Specifically, the modulation of the neural response to particular social actions, that is the communicative actions, caused by the different role held by the observer was investigated using fMRI. Activations in the brain circuits associated with action perception, namely the STS, the hMNS and the mPFC, were recorded only when participants were the addressees of the communicative actions, clearly indicating the importance of social involvement in processing human actions. All in all, the present results point toward a complex interplay of different brain networks to process in parallel distinct aspects of the human behavior in order to ensure a rapid and effective comprehension of the surrounding social environment. The prominent role of the social aspect in human action perception is also supported by the clear result of the prevalence of the affective/social content on the others.